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Nipponoclava gigantea is restricted to the tropical Western Pacific waters of southern Japan at depths of between 5 and 70 m and lives in coarse sand and gravel. In many respects, N. gigantea appears to be similar to other endobenthic penicillids in that there is a true shell, the margins of which, via the intermediary of a narrow ‘saddle’, are united with an adventitious tube. This has a large watering pot anteriorly and pleated ruffles, representing either growth or repair events, posteriorly. Nipponoclava gigantea differs from other penicillids, however, in that its true shell is large, i.e. ˜20 mm high as compared to 2–3 mm, and the angle of valve splay is ˜80° as opposed to 180°. Internally the shell has a wide pallial line and shallow pallial sinus and there are anterior adductor muscle scars. There are possibly posterior adductor muscle scars also. These are not present in the adult animal, however, although the anterior adductor is. Also present are paired anterior and (possibly) posterior pedal retractor muscles, the latter having a nervous union with the visceral ganglia to create simple stretch or tonus proprioreceptors. It is argued that N. gigantea provides clues about how the tube-dwelling lifestyle has evolved in the various taxa of the Penicillidae. It is suggested that through a process of heterochrony, in which the animal produces its adventitious tube at progressively earlier and earlier developmental stages, the true shell has become actually and relatively smaller and smaller and, hence, more splayed in more derived penicillids. Nipponoclava gigantea is thus an important link between an hypothetical lyonsiid ancestor and the more specialized penicillids and helps us to understand the adaptive radiation of this group of tube-dwelling bivalves.

Bentholyonsia teramachii has hitherto been described from Japan, but a conspecific specimen is herein recorded from Western Australian shelf waters at a depth of ∼100 m. Hitherto placed in the Lyonsiidae, B. teramachii is herein relocated in the Lyonsiellidae, alongside a second genus –Lyonsiella. Features of the anatomy of Bentholyonsiateramachii, including highly sensory siphons, reduced ctenidia unable to collect and transport particulate food, non-sorting labial palps, and a simplified Type II stomach and muscular gut all suggest that the species is a predator like Lyonsiella formosa. Other features of the anatomy, such as pallial taenioid muscles, radial mantle glands, and the structure of the ctenidia all point to a link between B. teramachii and the Lyonsiellidae and thus with the Verticordiidae and Parilimyidae and, therefore, with some of the oldest anomalodesmatans, the Pholadomyidae. In the possession by B. teramachii of a unique pair of antero-dorsal suspensory muscles, the link with fossil pholadomyoideans is reinforced. Bentholyonsia thus represents another genus of predatory bivalve, not only expanding our view of the prevalence of this feeding mode in the deep sea by this class but also helping us show how it evolved, in anatomical terms, from a suspension- (or deposit-) feeding ancestor.

The anomalodesmatan Clavagellidae can be separated from the Penicillidae on the basis of ligament and adventitious tube or crypt structure. The Clavagellidae includes species of Clavagella (and Dacosta), Bryopa and Stirpulina. Two species, first described as Clavagella torresi Smith, 1885 and Aspergillum multangularis Tate, 1887, have consistently been placed in the genus Clavagella, but both are herein located in the new genus Dianadema. Because more material is available, this is primarily a study of Dianadema multangularis (Tate) which is designated as the type species of the new genus. Dianadema multangularis is unlike Clavagella, Bryopa and Stirpulina which are, respectively, recessing, boring and endobenthic representatives of the Clavagellidae, in that it is cemented and possesses a crown of dorsal tubules, in addition to anterior and ventral ones. As in all clavagellids (sensu stricto) the left valve is cemented into the fabric of an adventitious crypt while the right valve is free inside it. There is also a posterior multi-angular siphonal tube. The anatomy of D. multangularis is described and it is, in most respects, a typical clavagellid. The species differs from other clavagellids in the inferred aeration of the crypt via patterns of water flow, the crown of crypt tubules, and its unique cemented lifestyle. There are three (known) genera of clavagelloids endemic to southern and eastern Australia, i.e. Dacosta and, now, Dianadema (Clavagellidae) and to which the third, Humphreyia (Penicillidae), is similar in that the latter two are both cemented, albeit convergently. They also provide a surprising picture of convergent evolution of the cemented lifestyle even to the extent that both possess a unique pair of pericardial proprioreceptors, each derived differently, to monitor body tonus. Moreover, two other anomalodesmatan genera are also cemented, i.e. Cleidothaerus (Cleidothaeridae) and Myochama (Myochamidae), and these too are restricted to eastern Australia (and New Zealand). What forces promoted cementation in these four antipodean anomalodesmatan families are unknown.

Amongst the watering pot shells of the Clavagellidae (Anomalodesmata), the least well known is Humphreyia, with a single species H. strangei. Unlike representatives of the other extant clavagellid genera, i.e. Clavagella and Brechites, however, where juveniles are unknown, there is a single juvenile (plus adult specimens) of H. strangei available for study. The Clavagellidae contains the weirdest of all bivalves, encased in a huge (in proportion to the tiny juvenile shell) adventitious tube with an anterior ‘watering pot’. The juvenile H. strangei is dimyarian with a large foot and byssal groove, and a large pedal gape; the entire body is enclosed within a bag-like periostracum-covered mantle cavity. Upon permanent residence, however, the anterior watering pot component of the adventitious tube is cemented to the chosen substratum, the adductor muscles are lost and the foot and pedal gape greatly reduced; the fourth pallial aperture closes and the pedal gape and the tubules of the watering pot are similarly occluded. Once cementation is achieved, further growth anteriorly is impossible and the adventitious tube is secreted posteriorly, to house the long siphons, and subsequently can be added to as growth increments. Anatomically, H. strangei has a typical anomalodesmatan arrangement of mantle cavity and digestive organs so that it is, probably, a suspension feeder. Inside the pericardium, however, is a pair of unique proprioreceptors which probably serve to monitor rectal tonus and thus control defecation, co-ordinated with siphonal retraction and extension. They probably also prevent over-filling of the capacious rectum. It is believed that cementation and adventitious tube formation occur at the time of sexual maturity and this change in lifestyle between juvenile and adult represents a form of metamorphosis – quite distinct from that which occurs in all bivalves between the pediveliger and juvenile stages – and seems to be unique to Humphreyia and probably Brechites. This family of anomalodesmatan bivalves is thus actually stranger than the already aberrant watering pot would suggest.

Living individuals of the bivalve ‘watering pot’ superfamily Clavagelloidea were collected from Withnell Bay, Burrup Peninsula, Dampier Archipelago, Western Australia. Details of the anatomy suggest that, in most respects, Brechites vaginiferus is a typical filter-feeding bivalve, albeit encased in a tube of its own secretion. The true shell is tiny (∼ 4 mm long) and surrounded by a ‘saddle’ of initially-secreted adult shell. To this is fused, by an internally produced layer of calcium carbonate, the adventitious tube which comprises a single layer, confirming that it is produced only once. Posteriorly, however, it can be added to internally as ‘plaited ruffles’ creating a multi-layered structure. The anterior watering pot comprises a perforated plate fringed by tubules, with a small pedal aperture. Brechites vaginiferus pumps interstitial water into the mantle cavity via the watering pot, at least principally, not out of it as previously thought. In this way, the animal is either tapping an interstitial source of accessory food, or aerating the anterior end of its burrow (or both). It has also been demonstrated, however, that pumping interstitial water into the mantle cavity via the pedal gape results in hydraulic pressures being built up that extend the siphons. The pedal disc pump, therefore, acts in this amyarian bivalve in the same way as the adductor muscles do in more typical species. The ctenidia collect suspended material in the usual way from seawater and from interstitial water: they do not, however, possess chemoautotrophic sulphur-oxidizing bacteria, and the species is thus not using sulphur as an energy source. Pedal gape feeding is considered to be the ancestral condition not only in the earliest anomalodesmatans, e.g. the Pholadomyidae, but also the autobranchs in general. This has been refined in the Clavagelloidea to create the unique form of the adventitious tube, its watering pot and a wholly surprising lifestyle. Tube form in B. vaginiferus varies in relation to habitat occupied. Its major habitat on Withnell Bay, however, is the riffles of a high-salinity stream draining down the mangrove-fringed shore from the semi-arid land behind. Although much more is now known about B. vaginiferus and ‘watering pot’ shells, no pre-tube juvenile is known and awaits discovery to confirm hypothesized life-history details.

The shallow, offshore, hypersaline (∼ 60‰), benthic marine environment of Shark Bay, Western Australia, is dominated by the cockle Fragum erugatum. This small (< ∼ 11 mm long) bivalve occurs in large numbers (4000/m2) and probably completes its life cycle in 1 year. Cyclones with a periodicity of ∼ 50 years have, over the last 5000 years, deposited huge numbers of the shells of F. erugatum onto the shore as windrows, creating the unique Shell Beach, or Hamelin Coquina. In most anatomical respects, F. erugatum is a typical cockle, with a powerful digging foot, siphons formed by fusion of the inner mantle folds only, plicate ctenidia with a ciliation of type C(1) and powerful rejectory currents on the visceral mass and mantle. It is thus able to collect filterable material and digest it. The labial palps are, however, reduced, allowing the ingestion of only the finest particles of food via the acceptance tracts in the ctenidial axes and distal oral grooves. The waters of Shark Bay are hypersaline and oligotrophic, so what is the food source that supports such large numbers of filter-feeding bivalves? Fragum erugatum possesses symbiotic zooxanthellae, as do its close relatives the giant clams (Tridacnidae), and probably obtains the same nutritional benefits from the association, which may facilitate survival in the hyperosmotic environment of Shark Bay. The association between giant clams and their zooxanthellae enhances shell growth and results in a long life expectancy and gigantism. Conversely, in F. erugatum the association facilitates a large population size (but short life expectancy), allowing it to dominate a unique, hypersaline, oligotrophic environment. Like giant clams, F. erugatum possesses a zooxanthella tube system linking the digestive diverticula of the stomach with the kidneys. Either accidentally or periodically, ingested zooxanthellae are transferred from the gut, via this tube, along with senescent zooxanthellae from the blood haemocoel, to the kidney tubules for lysosomal hydrolysis and eventual expulsion. In the light of this discovery, the functioning of the recently confirmed zooxanthella tube system in the Tridacnidae needs re-examination and re-evaluation.

The cave-dwelling Congeria kusceri is described and aspects of its unique ancestry and biology elucidated in the context of its, for a bivalve, similarly unique habitat. The species lives in subterranean waters of the Dinaric karst of the former Yugoslavia. C. kusceri is acutely heteromyarian in form commensurate upon its lifestyle of byssal attachment to the walls of either permanently or periodically inundated karst caves. Shells are sometimes covered by travertine, precipitated calcium carbonate, and the tubes of the, similarly unique, serpulid Marifugia cavatica. Neither species occurs outside such caves and, as a consequence, the shell and tissues of C. kusceri are colourless and there are few sense organs, i.e. no statocysts and light receptors, although the pallial papillae possess tiny sense cells and there is a supra-branchial osphradium. Mantle fusions are extensive and involve the inner folds posteriorly to create the siphons, but also the inner surfaces of the middle folds ventrally so that here the mantle is largely covered by a thin (4–5 μm), fibrous, periostracum and there are, on the separated middle folds around the pedal gape, external ciliary cleansing tracts. There are internal cleansing tracts too, but the former have been identified, hitherto, only in representatives of the Pectinidae. In terms of general anatomy, Congeria kusceri has large ctenidia and tiny labial palps as an adaptation to the mineral waters of its habitat containing little food and, thus, that little is rejected when collected. There are few sorting currents in the stomach either and the intestine is short. The aragonitic shell comprises two layers, outer and inner, both with a homogeneous crossed-lamellar structure. Each shell valve, however, contains many growth checks, represented by thin prismatic layers. Assuming that a growth check is formed once each year, possibly in late summer when water levels fall in the caves, making them accessible for investigation, an individual of, for example 12.7 mm shell length, was possibly 25 years of age. If so, an individual of 25 mm would be very much older. Because, with age, the species lays down growth checks at progressively reduced increments, a lifespan of many decades may be possible. Extreme reduction in shell growth causes marginal thickening and incurving. The species is dioecious, i.e. separate sexes, with females brooding lecithotrophic (70 mm) eggs in the ctenidia, the filament bases of which become highly glandular. Mature and brooding individuals occur when water levels are low in the caves, suggesting that internal fertilization takes place at this time too, sperm transfer being in the AElm of surrounding water to prevent loss. Fertilized eggs probably hatch either as larvae with a short planktotrophic life or as crawl-away juveniles. Analysis of large samples suggests that recruitment is probably low, and there is evidence that the species is threatened by pollution of its habitat.

The egg capsules and larvae of Engina armillata (Reeve) (Gastropoda: Buccinidae) are described for the first time. Its general ecology in the Cape d'Aguilar Marine Reserve, Hong Kong, is also described, the species occurring under stable boulders with a community of encrusting species, notably sedentary tubicolous polychaetes. The diet of E. armillata at Cape d'Aguilar consists of serpulid and spirobid polychaetes. Of the four commonest such species, Hydroides elegans was preferred to Pomatoleios kraussi, Simplicaria pseudomilitaris and Spirorbis foraminosus. Consumption rates by E. armillata varied considerably amongst individuals, but generally ranged between 1.2 and 6.5% of predator dry weight day-1 during conditions wherein prey quantity was maintained. Such a figure is low when compared to other boring naticid and muricid predatory gastropods which spend much time and energy accessing their prey. The ready availability of easily accessible prey may account for the low consumption rate of E. armillata (as also seen in other non-boring predatory gastropods) and explain its apparent specialist feeding habits.

The shallow subtidal mactrid bivalve Mactrinula reevesii is a deposit-feeder in the southern and south-eastern oceanic waters of Hong Kong. Buried obliquely, large quantities of fine sediment are taken into the mantle cavity and sorted on enormous labial palps. The small ctenidia probably have little value in collecting material, amounts taken in being too large. The mid gut is long and complexly folded inside the visceral mass. It is also capable of distension, although superficial visceral muscles maintain internal tonus. The rectum is narrow and creates compact faecal pellets. Most interest is in the ventral mantle margin which is, posterior to the pedal gape and the base of the inhalant siphon, united by a sheet of cuticle. There is no fourth pallial aperture. There are, however, two pairs of flaps extending along the posterior third of the internal ventral mantle surface. These arch over left and right mantle rejection tracts which transfer unwanted material to the base of the inhalant siphon for final expulsion. The mantle flaps prevent such material from being returned to the anterior end of the mantle cavity, for palp reprocessing, when new material arrives. They, thus, maximize sorting efficiency by separating unsorted from sorted and rejected material. Other mactrids have similar mantle flaps which they use in different ways, including the channelling of unwanted material to a fourth pallial aperture for expulsion, as in Lutraria lutraria. The Mactridae have thus evolved a unique method of increasing the efficiency of pseudofaecal waste rejection which has thereby facilitated the deposit-feeding lifestyle by the diverse representatives of this family.

The shallow subtidal scallop Minnivola pyxidutus occurs in the relatively clean oceanic, southern, and south-eastern waters of Hong Kong and elsewhere in the Western Pacific and is highly polymorphic in terms of shell patterning of the left, upper valve. Locally, because of sea-bed perturbations, numbers are declining. The species can probably live for ∼ 3 years, with reproduction occurring in winter. M. pyxidatus is monoecious. Shell form and anatomy are described. Normally, the species lives attached by a fragile byssus to sand grains or stones, but can swim, albeit weakly, in response to predation risks. In most respects, Minnivola pyxidatus is a typical scallop, with a bowl-shaped, right lower valve and a concave, disc-like, left upper valve. The mantle has pallial eyes and a complex array of tentacles which possess small, ciliary-based sensory cells. There is also a cleansing tract on the outer surface of the middle mantle fold and a muscular velum formed from the inner fold. The latter is developed into a simple exhalant aperture posteriorly. Both mantle lobes possess extensive posterior pallial glands and an abdominal sense organ on the right lobe only, anterior to the single posterior adductor muscle. Ciliary currents of the mantle cavity are for cleansing and are powerful, particularly on the visceral mass and right mantle lobe but not the left. The ctenidia and their ciliary currents are typical of the Pectinidae except that the ascending lamellae of all demibranchs are greatly reduced and, it is suggested, previse the situation seen in the Propeamussiidae where they are lost and the ventral margins of the inner and outer descending lamellae fuse to create an ‘inside out’ gill. Unusually, the labial palps of Minnivola pyxidatus are fused to the mantle and visceral mass, preventing their collapse in the mantle cavity and thus maintaining particle selection efficiency. The gut is simple. Left and right statocysts are unusually equal and the pericardium is located dorsal to the adductor muscle. Minnivola pyxidatus is adapted to an epibenthic life on subtidal sands and muds. Powerful cleansing currents and an ability to swim have allowed this species to survive in dredged and trawled waters with high levels of settling and resuspended silt.

All anomalodesmatans are ‘rare’ but Trigonothrucia jinxingae is relatively common in Xiamen Harbour, Fujian Province, China. This is because the species has a life span of approximately one year and is a simultaneous hermaphrodite, probably with either a short or absent planktonic larval stage. That is, success results from rapid maturation, self-fertilization, direct development and within-habitat recruitment over an extended period in early summer. Trigonothracia jinxingae is interesting in another way, however. The Thraciidae is the Mesozoic stem group of the Thracioidea which also contains the more modern (Caenozoic) Laternulidae and Periplomatidae. Features of the anatomy of T. jinxingae, such as the method of hydraulically moving the foot by the pumping of blood into a capacious pallial haemocoel, and the structure of the stomach, are reminiscent of the earliest (Palaeozoic) anomalodesmatans, i.e. the Pholadomyoidea, represented today by Pholadomya candida. The thraciid Asthenothaerus sp. (Pelseneer, 1911) even has, like P. cundida, an opisthopodium on its visceral mass. P. candida, however, fed on sub-surface deposits using its foot. T. jinxingae is also a deposit feeder, but on surface deposits using the inhalant siphon. Modern periplomatids resemble thraciids in their separate siphons, but both representatives of this family and the Laternulidae are suspension feeders with extensive sorting areas on the wall of the stomach to process such material. The Thraciidae thus form a link between the oldest, pedal feeding, pholadomyoidean anomalodesmatan and the most advanced, suspension feeding, laternulids and periplomatids.

Pteria brevialata characteristically attaches to Hicksonella princeps (Cnidaria: Gorgonoidea) at depths > 10 m in Hong Kong and the South China Sea. Attachment is by a stout byssus to the basal regions of the gorgonian colonies. The shell appears typical of the Pteriidae, but closer inspection shows that the antero-ventral margin, in particular, moulds itself to the form of the cylindrical gorgonian, which also changes its growth form, so that securer attachment is achieved. The margin of the right valve is not nacred so that the flexible outer prismatic layer adpresses firmly against the more extensively nacred left valve when they close; mantle and ctenidia also being withdrawn deeply. This is probably an anti-predation device. There are no mantle fusions, even between posterior inhalant and exhalant streams. The mantle margin is, however, complexly tentaculate. Pteria brevialata is monomyarian with reduced anterior byssal retractor muscles. The anterior face of the shell is thus greatly reduced whereas the posterior is inflated and postero-dorsally ’winged’. Other features of the anatomy are described, including the ctenidial eyespot and simple photoreceptors in the inner component of the duplicated outer mantle folds, under the periostracum. Similar structures in other pterioideans and arcoideans suggest a close relationship between their respective orders. It is concluded that gorgonian-associated pterioids can be derived from an epibyssate, crevice-dwelling ancestor, exploiting, as solitary individuals, the currents and clean oceanic waters above the substratum. This has three important advantages: (a) removal from surface-roving predators (the gorgonian providing additional protection); (b) exploitation of a niche hitherto unoccupied by bivalves and (c) removal of the animal from the sea-bed and exclusive exploitation of potential food held in suspension. Current stresses to attachment are avoided by modifications to the shell.

Exposed rocky shores in Hong Kong are dominated at mid-tidal levels by a broad band of the mytilid Septifer virgatus. A 48-month study of a population inhabiting the shores at Cape d'Aguilar has been undertaken. The species matures at a shell length of ≅ 15 mm, about one year after recruitment. Septifer virgatus is dioecious with a slight, but insignificant, female bias overall. There is, however, a significant juvenile male bias. Generally, most adult individuals remain mature year round, although spawning is limited to two periods in spring (February to March) and autumn (September to December) with subsequent recruitment into the adult population. The species lives for ≅ 4–5 years, although older individuals, possibly up to 12 years of age and with a maximum recorded shell length of 65 mm, occur as solitary individuals lower down the shore. Mortality in winter mainly affects newly recruited juveniles. A heavy mortality of adults in summer is thought to be related to high rock temperatures at midday (≅ 50°C), concurrent with low spring tides. The life-history tactics and sexual strategy of Septifer virgatus can be correlated with seasonal changes in hydrography. There are a number of similarities with other local mytilids, notably the dioecious condition and the bimodal pattern of spawning and recruitment. Such similarities are discussed in relation to the habitats occupied by these species.

Mid-tidal levels on Hong Kong's exposed shores are dominated by sessile barnacles (Tetraclita squamosa) and mussels (Septifer virgatus). Such shores experience dramatic elevations in air and rock temperatures in mid summer that have been causally linked to mortalities of these species. Tetraclita squamosa and S. virgatus had lower body temperatures relative to their rock substratum. Mussel inhabitants of high- and low-zoned pools were in temperature equilibrium with contained water. On the rocks, body temperatures of T. squamosa were higher than those of S. virgatus. Middle shore individuals had higher body temperatures than lower shore ones. Individuals in the sun had higher body temperatures than those in the shade. Factors possibly affecting body temperature are orientation to the incident sunlight, height on the shore and the contact area between the individual and the substratum. Mean heat coma temperature (HCT) values for submerged Septifer virgatus were 45.0dC and 44.5dC for individuals of between 10-20mm and 30-50mm shell length, respectively. Mean median lethal temperature (LT50) values for submerged S. virgatus were the same for both size classes (47.0dC). A longer exposure in air resulted in a lower LT50 value; a shorter exposure in air resulted in a higher LT50 value. Water content of Septifer virgatus was between 89.21–90.26%, i.e. including the water from the mantle cavity. In air temperatures of between 30–32dC and after long periods of exposure, Septifer virgatus showed significant water loss, i.e. of between 13.87-15.52% body water content. In air temperatures of between 33–36dC and after relatively short exposures, Septifer virgatus showed small water losses, i.e. between 1.92-5.74% body water content. On Hong Kong's exposed rocky shores, sessile components of the middle shore environment are thermally stressed in summer when low tides occur in the afternoon in combination with periods of cloudless sunny days and when rock temperatures may climb to ≅ 50dC through solar insolation. Lower-zoned individuals and high-zoned pool residents are buffered from the adverse effects of such environmental extremes. Mortality occurred mainly in the sun-exposed shore populations of both Tetraclita squamosa and Septifer virgatus and is particularly evident when the above combination of climatic and tidal factors first occurs in July.

The calcareous substratum borer, Lithopaga aristata Dillwyn, 1817, secretes posterior incrustations that take the form of interlocking ‘forceps’. These are secreted, initially, as asymmetrical ridges by similarly asymmetrical dorsal and ventral glands in the left and right middle folds of the posterior mantle lobes. In the adult, the secretions are more uniformly spread over the posterior shell margins, concealing the juvenile ridges. Opening and closing of the valves smooths the outer surfaces of the ‘forceps’ against the burrow wall, which also comes to be lined with calcium carbonate that is reciprocally smoothed, so that they occlude the borehole more effectively. Extension and retraction of the siphons probably smooths the inner surfaces of the ‘forceps’ which are sharpened by abrasion, one against the other, during valve opening and closing. The pair of inequilateral spikes so produced, project from the burrow aperture, occluding it, but probably, more importantly, distancing aperturally attacking predators from the true posterior shell margin. Interlocking of the ‘forceps’ and their sharpness may further deter would-be predators.

The expression of sexuality, overall sex ratio and variation in sex ratio with age of 14 southern Chinese freshwater, brackish and marine bivalves have been investigated. Freshwater species are often hermaphroditic and brooding, e.g. Musculium and Pisidium (Pisidiidae), while Corbicula fluminea (Corbiculidae) demonstrates wide intraspecific variability in sexual expression, but also broods. Anodontu woodiunu (Unionidae) and Limnoperna fortunei (Mytilidae) are dioecious, with low incidence of hermaphrodites in the former, but both have strong female-biased overall sex ratio which does not change with age. Juvenile age classes of Corbicula cf. fluminalis have female-biased sex ratio. A pronounced female bias is means to optimize reproductive success, by maximizing resource allocation into more energy-demanding oogenesis, in temporally and spatially heterogeneous lacustrine habitats. In such habitats, moreover, there are probably diminishing returns to producing sons. Mangrove and brackish water species are all dioecious (except Succostrea cuculluta) with slight male-biased sex ratio overall, but are strongly male-biased as juveniles. This strategy similarly optimizes success in fluctuating intertidal environment, where high rates of juvenile mortality may be expected possibly, for example, through enhanced predation. In such case, therefore, the greater investment in sons is prerequisite to an overall balanced sex ratio. The marine intertidal species, Perna ciridis (Mytilidae) and Donax semigranosus (Donacidae) have stable I:I sex ratio, notvvarying with age, and are representative of the ‘typical’ dioecious condition recorded for most marine bivalves. Thus, although intraspecific variations in life history tactics and sexual strategies equip all species for variable environment, it is postulated that permutations of the ‘dioecious’ condition, mediated through variations in the sex ratio either overall or with age, broadly equip the Bivalvia for the total range of aquatic habitats.

Hitherto, the unique pallial organ of the Pinnidae has been considered to fulfil a cleansing role, removing sediment and shell fragments from the mantle cavity. This study of the pallial organ of Atrina pectinata describes structure in greater detail than before but was unable to demonstrate a cleansing function. Rather, the organ is thigmotropic and massively secretory. Study of the pallial organ head demonstrates it to have a pH of between 2 and 4 and that tall, apparently empty cells in the head epithelium contain sulphuric acid which can be precipitated as barium sulphate crystals following irrigation with barium chloride. Acid-secreting cells are recorded from a number of animal groups, notably opisthobranch gastropods, where they are concluded to possess a defensive function. Such a structure has not hitherto been recorded from the Bivalvia.

The biology and functional morphology of the unusual boring anomalodesmatan bivalve Clavagella (Dacosta) australis (Clavagellacea) is described. The animal occupies a borehole in calcareous (including coral) rocks to which it cements itself by the left shell valve. Radial mantle glands developed in the middle folds of the siphonal tip produce the glue but also stick sand grains to the periostracum to camouflage the siphons. The right valve is free. The shell is somewhat inequilateral in form, the adductors large and slightly heteromyarian. The pallial retractor muscles are also large so that the ventral mantle is very thick. Because mantle fusion involves inner, middle and inner surfaces of the outer folds (type C), the periostracum is continuous so that shell and mantle are almost completely, except for the siphonal orifices and the pedal gape, enclosed in periostracum. The simple sunken primary ligament comprising inner ligament layer only and located between chondrophores is flanked anteriorly and posteriorly by fused periostracum, assisting, in the absence of teeth, in hinge alignment. Boring is probably a chemical process, a secretion from the inner surface of the anterior mantle being discharged onto the burrow wall from the pedal gape. The calcium carbonate eroded in this way is possibly used to partially line the burrow. The distal siphonal tube, however, is secreted from siphonal glands discharging from the tip of the siphons. In most other anatomical respects Clavagella is typical of the Anomalodesmata, the ctenidia and labial palps for example being of characteristic form. The statocysts, however, are unusual in that the statolith comprises a large number of small, diamond-shaped crystals that stimulate statocyst cilia. C. australis appears to be dioecious, whereas most other anomalodesmatans are simultaneous hermaphrodites. Clavagella may be linked to a pholadomyacean ancestor and most closely resembles members of the infaunal Latemulidae. It represents an evolutionary line that has adopted first a nestling then a boring mode of life. It seems probable that the more specialized infaunally buried clavagellids with both valves incorporated into the adventitious tube are derived from a nestling/boring ancestor.

The abyssal propeamussiid Propeamussium is well known from studies of shell structure and is believed to be representative of a relict lineage, sister to the group which gave rise to all shallow-water pectinaceans. From analyses of stomach contents, species of Propeamussium have been reported to be predators. This study confirms this and shows that small meiobenthos or epibenthic planktonic larvae and crustaceans constitute the diet. Prey is thought to be caught by an inrush of water to the mantle cavity and pushed into the mouth with the foot. The stomach is small, secondarily simplified and adapted for the digestion of small creatures. It is convergent with the stomachs of similarly predatory, abyssal anomalodesmatans. Palps are vestigial. The ctenidia are of a new type (B(4)), perhaps derivable from the dimyid ctenidium by the loss of the ascending lamellae of all four demibranchs and the ventral fusion of inner and outer descending lamellae on each side of the body to create a gill where the apices of the filaments are internal. Filter feeding is thus impossible, the ctenidia now wholly subserving a respiratory function. Aspects of shell structure, adductor muscle arrangement and form of the pallial margin suggest that P. lucidum is a proficient swimmer. It is concluded, however, that prey is not caught during swimming, and that the two activities are mutually incompatible. Probably swimming is used only for escape. Ctenidial structure forges links with the Plicatulacea (Dimyidae, Plicatulidae), shell structure with the Pectinacea (Pectinidae, Spondylidae) and it is concluded that the Propeamussiidae is indeed a relict group with links back to more primitive plicatulaceans and with the modern shallow-water Pectinacea, but evolving, in the isolation of the abyss, remarkable adaptations for a predatory mode of life. The Propeamussiidae also demonstrate striking convergence with the abyssal predatory Anomalodesmata.

The leptonacean genus Montacutona is represented by a number of species, mostly recorded from Japan and known only from type specimens. An Australian species, M. ceriantha, has been more adequately described but still in insufficient detail. This study reports upon the Japanese species, M. olivacea. Both M. ceriantha from Australia and M. olivacea from Hong Kong and Japan inhabit the tubes of species of the burrowing anemone Cerianthus. M. compacta inhabits the surface apertures and crevices of coral heads in Hong Kong, apparently not in association with any of the other inhabitants, but probably with the coral itself.

Claudiconcha japonica Dunker 1882 is a member of the Petricolidae in which family there is a clear trend towards a tunnelling mode of life culminating in the evolution of species such as Petricola pholadiformis–a borer of soft rocks and shales–with appropriate anatomical and morphological adaptations. Such convergent adaptations recall those seen in another unrelated group of rock-borers–the Pholadidae. The tunnelling habit in modern petricolids undoubtedly arose from a nestling mode of life which is commonly encountered in other members of the Veneracea. Some, possibly more primitive, petricolids only have the ability to enlarge the crevice they inhabit. C. japonica is different, however, in that for the first part of its life it is a motile (though usually found byssally attached), equivaive bivalve. With site selection, however, the margin of the right valve undergoes considerable enlargement to curl around and partially enclose the unmodified left valve. This development, associated with the selection of a crevice for permanent habitation, has no effect upon the disposition of the organs of the mantle and body cavities as in other inequivalve bivalves and they remain typically bilaterally equal. It is impossible for C. japonica to tunnel since the secondary development of the right valve fills the chosen site and in so doing comparatively immobilizes the left valve and closes the pedal gape. Thus C. japonica and its close relatives must be considered to be on a separate line of evolution from other petricolids and it is doubtful that the present sub-generic status of Claudiconcha is sufficient to highlight this difference. It seems more appropriate to genericize this name thereby amplifying this trend. In other morphological respects, which are described, C. japonica is clearly a member of the Petricolidae and functional aspects of these adaptations suit the animal to a life in relatively clear waters but where nevertheless the niche tends to accumulate detritus which must be kept out of or removed from the mantle cavity. The secondary enlargement of the right valve, associated with the selection of a suitable site for permanent habitation also appears to be associated with the onset of sexual maturity –a feature again reminiscent of the tunnelling Pholadidae.

A 12-month investigation of the population dynamics, growth, ageing, reproduction and sexuality of Anodonta woodiana (Lea, 1834) in Plover Cove Reservoir, Hong Kong, has been undertaken. Recruitment occurs in the summer, May through July being the period of peak incidence of glochidia on captive fish hosts. Juveniles grow rapidly to a shell length of 30 mm and are sexually mature by the following spring. A study of the population structure of A. woodiana indicated that most animals were less than 8 years old. Results of a Walford plot for this species indicated a maximum theoretical shell length (in Plover Cove) of 162·5 mm and a life span of approximately 12 years; the latter is in good agreement with age estimates for European unionids. Anodonta woodiana is dioecious with females predominating in a ratio of 60:40. A very small number of individuals (0·3%) are hermaphrodite. Males possess mature gonads throughout the year whereas females come into reproductive condition during the spring. There was no evidence of shell form sexual dimorphism, as has been recorded for other unionaceans. The reproductive strategy and time of gonad maturation of A. woodiana is compared with that of unionids from elsewhere and with other fresh water bivalves from southern China.

The structure of the pallial glands in limestone and coral boring species of Lithophaga (Bivalvia: Lithophaginae) is examined and related to evolutionary trends in habitat specialization. Boring glands of all species occur in the middle mantle fold but display a progressive degree of complexity from simple epithelial to ducted sub-epithelial structures coinciding with increasing specialization of habitat from natural limestone to species specific live coral dwellers. The size of this gland is relatively reduced anteriorly and along the entire length of the mantle in live coral borers.

Sinonovacula constricta is representative of two genera of deep burrowing bivalves hitherto placed in the Novaculininae of the Tellinacea. This species, widely cultured in China, has not previously been critically examined. Aspects of its anatomy show that it should more properly be placed in the Solenacea. The presently accepted classification of this superfamily suggests that it should be placed close to Pharella in the Cultellidae but it is clear that the group requires more detailed re-examination. Sinonovacula is somewhat posteriorly elongate with long separate siphons. The anterior pedal gape acts as a valve to the piston-like foot enabling rapid reburrowing to occur. Sensory tentacles around the pedal gape respond to mechanical disturbance and water can be squirted from the gape to, presumably, flush away the source of irritation. Modifications of this kind are rare in the Bivalvia. The anatomy of Sinonovacula is described and related to its life style as a suspension feeder in soft intertidal muds. The ciliary currents of the mantle cavity are powerful, the labial palps in particular being large as compared with the relatively small ctenidia. The stomach is adapted for dealing with large amounts of food, though the sorting areas are not extensive; principal sorting is clearly the function of the labial palps. The style sac and mid-gut are separate (unlike the conjoined style sac and mid gut of the Tellinacea), and ensure separation of the style from the sediment loaded mid gut. In many ways Sinonovacula foreshadows the highly modified true razor shells and conditions in these bivalves can be accounted for by further posterior elongation of a bivalve essentially similar to Sinonovacula.

Hitherto uninvestigated in Hong Kong are the population dynamics, reproductive strategies and life history tactics of the Pisidiidae of which three species are known to occur locally. This study is of Musculium lacustre inhabiting a drainage ditch of the Lam Tsuen River in the New Territories of Hong Kong and extended over 15 months during 1983–84. In Hong Kong, M. lacustre is a protandric, simultaneous hermaphrodite that matures at a length of 2 mm; the majority of each generation is, however, not mature until a length of between 4–6 mm is attained. Fertilized eggs are brooded in the inner demibranchs until they reach a length of 1.5 mm when they are released as post-foetal larvae. Musculium lacustre is univoltine. Reproduction occurs twice a year but, since M. lacustve is semelparous, the population contains, at any one time, two overstepping generations. Thus, a spring generation is recruited and grows rapidly to generate a second, smaller, autumn generation. This in turn grows rapidly and matures to produce the spring generation of the succeeding year. Adult mortality occurs in late summer and winter. A few of the late-born spring generation may overwinter to contribute to the spring generation of the succeeding year. This is not so with the autumn generation. Such sexual and life history tactics can be related to the climate and hydrology of Hong Kong and its freshwater habitats. Reproduction takes place in spring and autumn when temperatures are moderate. Mortality is high in summer and winter when the habitat is either, respectively, hot and flushed out by summer rains or cold and dry. As M. larustre is able to overwinter but not aestivate at these latitudes (22° N), it is postulated that this holarctic species is approaching the southern limit of its asiatic range. Musculium larustre is compared with the other bivalves inhabiting fresh and brackish-water habitats in southern China.

Eufistulana mumia Spengler 1783 (Gastrochaenidae) is one of the rarest of all bivalves, a preserved specimen only once having been previously reported upon (Fischer, 1866). Eufistulana lies buried in subtidal sediments, entombed, except for a siphonal orifice, in an adventitious tube of its own construction. The tube comprises four layers secreted in stages by an equal number of pallial glands, housed in the reflected middle folds anteriorly, the reflected inner folds further posteriorly and the siphons. The process of tube formation is discussed. The larva probably settles on a calcareous substrate, possibly an empty shell, and then proceeds to construct a vertical tube when the initial site of settlement has been penetrated. Periodic dissolution of the original tube basally, possibly by a gland in the foot, is followed by growth downwards with a concomitant secretion of new tube material. At the end of a phase of growth a new base to the tube is secreted. The true shell of Eufistulana is modified with the anterior face very reduced, laterally expanded and, in life. covered by reflected middle folds. The posterior face is elongate and covered by reflected inner folds. The ventral margin is widely emarginate. The pedal gape is small and the foot reduced, the sole acting as a sucker attaching the animal to the tube and leaving a characteristic impression. There is a tiny, papilla-like anterior pedal projection to the foot. A septum in the tube separates tissues contained within the shell from the siphonal region, possibly restricting entry into the basal region of sediment and small, shelter-seeking, invertebrates. Similarly, the tips of the siphons possess glands that produce a secretion which probably deters would be inhabitants of the siphonal region of the tube. The major specializations of Eufistulana to its mode of life are pallial. In other respects it is a typical bivalve with little reduction of the pedal apparatus and few modifications to the organs of the mantle cavity. The labial palps are small implying little food selection in the mantle cavity. The intestine is adapted for handling large amounts of sediment. Soft deposits have been colonized in a number of ways by a number of bivalve lineages. Eufistulana is an example of another way in which this habitat has been exploited, albeit from an ancestor that was probably a borer.

Statocyst structure in representatives of each, as currently recognized, family of the bivalve sub-class Anomalodesmata has been investigated. Three types are recognized—A, B and C. Type A is possessed only by Pholadomya Candida, a representative of the ancient Pholadomyacea. Type C is found only in the Cuspidariacea. Type B, possessed by all other anomalodesmatans is divided, on the basis of statolith structure, into three sub-types—B1, B2 and B3. Type A statocyst is extraordinarily complex with a capsule penetrated by nerve endings and a multicellular statolith. Such a sophisticated structure is adjudged essential for a bivalve that is postulated to lie angled on its back in the sediment and to collect subsurface deposits via the pedal gape, using the foot as a suction piston. Type B statocyst comprises a large, multicellular capsule of ciliated cells as the sensory receptors. Variations in structure relate to whether or not there is a single, large, statolith (B1), a large statolith and other statoconia (B2), or numerous crystal-like statoconia, one of which may be enlarged into a statolith (B3). Type C statocyst comprises a small capsule of but four or five cells in transverse section, each of which is swollen and lined by microvilli. A large, oval statolith is not freely mobile within the lumen and probably confers little fine orientation discrimination. It is concluded that statocyst structure has some phylogenetic significance, widely separating the Pholadomyidae from other anomalodesmatans and indicating that the Cuspidariacea may have a separate origin from the other deep-water ‘septibranch’ predators. Type B3 statocyst (crystal-like statoconia) appears typical of the Clavagellacea, but is also possessed by Myadora striata. All other anomalodesmatans possess either Type B1 or B2 statocysts, and with so much variation between and within constituent genera, families and superfamilies, little adaptive significance is attached to form. All other bivalves investigated to date have a statocyst structure of Type B, this probably representing ‘primitive’ conditions. Earlier conclusions from studies of other bivalves that type B2 or B3 is more primitive than B1 cannot be substantiated, because more comprehensive studies of these phylogenies may reveal a wider diversity of form than is at present realized.

Certain families in the subclass Anomalodesmata possess highly characteristic ligaments exhibited in simplest form in the Lyonsiidae (Pandoracea) and in the Verticordiidae (Poromyacea). A ventral ly displaced primary ligament consisting of inner (fibrous) and outer (lamellar) layers lies beneath a dorsal secondary ligament of fused periostracum. The latter, which occupies the pivotal axis, is responsible for alignment of the edentulous valves, the former for production of the opening thrust. A calcareous lithodesma which extends down the centre of the inner layer (and so also formed by the mantle isthmus) ensures the continued effectiveness of the widened ligament. Conditions in the laterally compressed Pandoridae and in Myadora (Myochamidae) resemble those in the Lyonsiidae with the addition of secondary teeth. Cementation by the right valve causes bilateral asymmetry in Myochama but to a far greater extent in the Cleidothaeridae where the added effect of a tangential component in shell growth causes separation of primary and secondary ligaments and a twisting of the lithodesma. In the Verticordiidae and Cuspidari-idae (Poromyacea) ligamental structure is the same although modified in the latter. In the Poromyidae the primary ligament remains external and there is no lithodesma while fused periostracum forms anterior and posterior ligamental extensions. Despite these resemblences in ligamental structure between Poromyacea (pre-septi-branchs and septibranchs) and Pandoracea, palaeontological evidence, as well as the modified ctenidia in the former, supports the continued separation of these superfamilies. On the other hand the Thraciidae, Periplomatidae and Laternulidae, hitherto included in the Pandoracea (but originating earlier, in the Mesozoic instead of the Cenozoic) should be removed from that superfamily. In them the primary ligament is not ventrally displaced and an anterior lithodesma is solely concerned with alignment of the edentulous valves to which it is closely attached.

Tridacna crocea Lamarck 1819 possesses a diurnal rhythm of activity. During the daylight hours the hypertrophied siphonal tissues, containing zooxanthellae, are expanded and phasic adductions occur that pump fresh sea water into and filtered water and pseudofaeces out of the mantle cavity. During the night the shell valves become more apposed, the mantle folds are withdrawn and few phasic adductions occur. Such a pattern of activity is similar to that described for other sublittoral bivalves. With this cycle of activity and inactivity is correlated the process of extra-cellular digestion of filtered food in the stomach. This is accomplished by the alternate, but incomplete, dissolution and subsequent reformation of the crystalline style. The style partially dissolves at night and reforms during the day. The digestive diverticula undergo a sequence of cytological changes related to the intra-cellular digestion of filtered food, again related to the cycle of night and day and thus to feeding. During the night there is a transport and intra-cellular digestion of zooxanthellae by amoebocytes from the mantle to the haemocoel surrounding the visceral mass. During the day digested zooxanthellae are transported to the digestive diverticula where they are discharged (together with the waste products of the intra-cellular digestion of filtered food) into the tubule lumina and thus back into the stomach to be eventually voided via the anus. Recognizable zooxanthellae are also transported to the kidney, where they are taken up by the cells of the distal limb to form cellular globular concretions; these too are probably also eventually voided. The discharge of digested zooxanthellae via the digestive diverticula has resulted in modifications to the processes of tubule breakdown and reformation involving the digestive cells only. These unique modifications are described and compared with the structural changes seen in the digestive diverticula of other bivalves. It is concluded that the unusual functioning of the digestive diverticula is a direct consequence of the utilization by Tridacna of a secondary source of food without the concomitant evolution of special organs of digestion, as for example there has been in the Teredinidae feeding on filtered food and wood. Specific comparisons are made between the Tridacnidae and the Teredinidae and suggestions are made as to how zooxanthellae came to be used as food in these unusual bivalves.

Philobrya (Philobrya) munita was originally described as P. (Hochstetteria) munita Finlay 1930, but the features of the shell and ligament clearly ally it with the former subgenus. P. munita, unlike the majority of its closest relatives which are benthic, lives intertidally attached by a stout byssus to the holdfasts of algae on moderately exposed shores in New Zealand. P. munita is monomyarian though it possesses a shell which has the basic heteromyarian form with an inflated posterior slope (and correspondingly enlarged posterior adductor and posterior byssal retractor muscles) and a reduced concave anterior slope (with a reduced anterior byssal tetractor inserted on the posterior slope of each valve). The umbones are beak-like and there is a distinct byssal notch. Internally a septum or modified hinge plate is located under the opisthodetic alivincular ligament. There are no hinge teeth or denticles. The significance of the shell and ligament of P. munita is discussed in relation to the basic duplivincular ligament of other arcoids and the ligament of other bivalves. P. munita possesses a posterior inhalant aperture only. This is unusual since some other members of the Arcoida have both anterior and posterior inhalant streams. The posterior inhalant stream in P. munita results from the very great reduction of the anterior slope of the shell. The ciliary currents of the mantle cavity are described; the ctenidia are of type B(la) and there is a single cephalic eye located just anterior to the anterior filament of each ctenidium. The roof of the supra-branchial chamber above the inner demibranch possesses a hypobranchial gland. The labial palps are small and there is a labial caecum located within the inner lip of the mouth. The mantle comprises three folds, with the outer fold possessing a secondary fold. The outer mantle fold bears a row of pallial eyes; light reception is thus through the periostracum. The structure of the stomach has been reconstructed from serial sections and the basic histological features of the alimentary canal are described. The pericardium possesses a heart with a single ventricle and a pair of auricles which communicate with each other posteriorly. The kidney and gonads discharge via a common urino-genital aperture. P. munita incubates fertilised eggs within the infra-branchial chamber of the mantle cavity; incubation within the supra-branchial chamber being impossible with a filibranch ctenidium. P. munita is compared with other philobryids, other members of the Arcoida and with other bivalves. It is concluded that P. munita is a very unusual bivalve–the morphological eccentricities it possesses resulting from its primitive arcoid origins, the evolution of the heteromyarian form, and specialisations not seen in other bivalves.

The relationship between parasitic glochidia of Anodonia woodiana (Unionidae: Anodonti-nae) and potential fish hosts was investigated in the laboratory. Intensity of parasitism was highest on the exotic fish Gambusia affinis, lower on the native species Puntius semijascio-latus and Metzia takakii, and least on Rhodeus sinensis. Glochidia generally attached at fin margins, particularly the pectorals and caudal. In Gambusia affinis, the incidence of glochidia on the pectorals was higher than would be expected on the basis of ratios between fin margin lengths. Apparently, the role of the pectoral fins in locomotion makes them more liable to glochidial contact, thereby increasing their susceptibility to attachment. The duration of glochidial attachment was shortened as temperatures increased. Mean values ranged from 14-4 days at 15°C to 6 days at 27°C. At 33°C glochidia rapidly detached and metamorphosis was unsuccessful. Significantly, water temperatures in A. woodiana habitats in Hong Kong rarely exceed 30°C.

Enigmonia aenigmatica (Holten 1803) is an anomiid bivalved mollusc highly adapted for life attached to the trunks, projecting prop roots and leaves of mangrove trees. It is a characteristic member of the S.E. Asian mangrove community. It exists in four growth forms, or morphs determined by the nature of the surface on which settlement takes place. It is also orientated and positioned on the tree trunks of Avicennia and the prop roots of Rhizophora, in relation to the long axis of the tree and the direction of the current. Further aspects of the biology and ecology of Enigmonia are discussed. Enigmonia is monomyarian and lies on its right shell valve. This has resulted in an asymmetry which is of especial significance with regard to an understanding of the functional morphology of the organs of feeding and digestion in particular. The eccentricities seen in Enigmonia have been compared with the adaptations found in other anomiids in particular and other bivalves in general. The feeding and digestive cycle of Enigmonia has been described and found to be correlated with the rhythm of the tide; it is essentially similar to that described for some other littoral bivalves. The adaptations and specializations of Enigmonia are discussed in relation to its life in the mangal.

The coral-boring bivalve Jouannetia cumingii (Pholadacea) has been investigated from Phuket, Thailand. It only bores dead heads of Porites lutea and the siphons open, not on to the general coral surface, but into galleries created by other borers and inhabited by cryptobion. The bivalve may thus have a relationship not with the coral, except as a substratum, but with these other gallery occupants. The juvenile shell is anteroposteriorly reduced, the anterior face represented only by dorsally reflected shields between which extends the anterior adductor muscle. A considerable area of the anterior mantle is thus exposed to the burrow heading and there is a small pedal gape through which can be extended a large foot. This is absent, however, in the adult. The large posterior adductor is located between shell buttresses, one on the posterior face of each valve to reduce shear. Also attached here are small posterior pedal retractors. Anterior retractors are located on condyles projecting downwards from the hinge plate which is sunken and covered by the umbones and reflected anterior shell shields. The ligament is small, inequilateral, and covered by periostracum. A normal complement of mantle cavity organs is present but is reduced and posteroventrally-anterodorsally aligned. Gill ciliation is of type C(1) and the labial palps are large. Mechanical boring, despite anterior shell teeth, is impossible and a chemical boring process is proposed from a large gland located internal to, and discharging via, the pedal gape. Secretion cannot be from the outer surface of the exposed mantle as this, because of extensive mantle fusion, is covered by periostracum. In the adult, a callum is formed over the exposed anterior regions of the mantle. Callum formation and sexual maturity occur over a wide size range and is related, as in other pholads, to an inability to bore further. The callum is not an extension of the true shell and is thought to be secreted from pallial glands located on the internal surface of the mantle and discharging via the pedal gape. As with boring, secretion cannot be from the exposed general mantle surface. The callum of the left valve is much larger and external to that of the right. Left callum formation must take place first, against the heading of the boring. Right callum formation must take place subsequently, largely against the left. Dorsally, the callum covers the anterior shields and all areas of exposed mantle. The unequal callum is a consequence of the mode of shell adduction in which the valves rock about dorsal and ventral fulcra: an equal callum would not permit this, the unequal form does, the right callum sliding beneath that of the left. Adult J. cumingii are almost perfectly spherical except for a spiked siphonoplax developed on the posterior face of the right valve only, by enhanced growth of shell lamellae. The spherical shell form in Jouannetia may be a consequence of this animal's more intimate association with the coral community which, in providing protection, obviates the need for deeper burrowing and thus an elongate shell form.

Aspects of functional design in the Pectinidae have led scientists to speculate that scallops of the Amusium group (Hertlein, 1969) are among the best swimmers. This study of Amusium pleuronectes confirms this view, with average swimming speeds ranging from 37–45 cm/second. A speed of 73 cm/second was recorded from one scallop. Swimming times varied, on average, between 8–10 seconds; one scallop, however, swam for 18 seconds, covering a distance of 10 metres. This study has also investigated pertinent aspects of the functional design of A. pleuronectes, including the adaptations to the shell, mantle and musculature that increase swimming efficiency: These include: (1) A thin, very smooth, biconvex, rounded shell; (2) Internal strengthening ribs; (3) A small valve convexity; (4) A more central point of maximum valve convexity; (5) Upturned antero- and postero-lateral upper valve margins; (6) Emarginated anterior and posterior shell margins; (7) A highly elastic ligament; (8) A single large, centrally located, adductor muscle; (9) Small perpendicularly oriented “slow” and large obliquely oriented “quick” adductor muscle components; (10) Expulsion of water both antero-and postero-dorsally; (11) Highly muscular middle and inner mantle folds acting as a (mechanical) “valve” to direct and regulate water flow. It has occasionally been reported in the literature that small scallops swim “better” than large specimens of the same species; this has been investigated for A. pleuronectes and the general conclusion is that large scallops can swim just as well as small specimens (indeed distances covered are greater in animals of the former category), but that the threshold stimulus necessary for swimming to occur is raised. This is true also of the escape reaction; small scallops are much more likely to perform an escape response of between one to three adductions than large specimens which seem to rely more on sustained adduciion for protection. Earlier authors have advocated ontogenetic allometric growth changes in the position and obliquity of the “quick” component of the adductor muscle to explain how larger scallops overcome the increasingly negative forces of gravity and drag. No evidence of such allometric changes have been recorded for A. pleuronectes and the present author believes that the increasingly negative forces of drag and gravity are easily overcome by scallops of any size. Increasing weight with age and physiological ageing of the adductor muscle is probably sufficient to account for differences in swimming ability between small and large specimens of A. pleuronectes. It is finally concluded that swimming in A. pleuronectes is not a response to predation. Circumstantial evidence is presented which, when considered with the efficiency of swimming and aspects of functional design in this species, suggests that swimming is concerned with active and seasonal migration, possibly linked to reproduction.

The infaunal Clavagellacea (Anomalodesmata) are among the weirdest bivalves, and include the “watering pot” shells, with a minute true shell embedded in an adventitious tube. This paper describes the formation and function of the tube of Brechites vaginiferus and is correlated with an investigation of shell structure and anatomy. Because of extensive mantle fusion, the body is entirely covered by periostracum. The only apertures to the exterior are the pedal gape and the siphonal orifices (a small fourth pallial aperture progressively closes up). Glands lining the pedal gape secrete the basal plate of the tube. Perforations in it (the plate tubules and lateral tubules) are caused by papillae locally inhibiting the laying down of secretion. The siphonal part of the tube is secreted by glands discharging from the siphonal orifices (middle folds). The secretion presumably pours down the maximally expanded siphons, which compact the burrow wall and against which the tube is laid down, sand grains adhering to it. Fusion with the concave basal plate is at the margin of the pedal disc forming a fringe of fused tubules around it. Radial mantle glands in the siphonal orifices (again middle fold) produce a secretion that glues sand grains to the periostracum around the siphonal tips, for camouflage. The tube is secreted when an otherwise naked, infaunal juvenile reaches a mature size. The tube is produced only once so that further enlargement is impossible, except by addition of “plaited ruffles” at the siphonal end, as the siphons elongate and as the tube sink's into the sand. New tube material is laid down inside and beyond the existing tube opening. Similarly, the basal plate is internally strengthened by further secretion from the pedal gape. Progressively, lateral tubules and the orifice in the plate corresponding to the pedal gape are occluded; other disc tubules remain open. A juvenile clavagellid, possibly that of Humphreyia strangei but more likely a species of Brechites, has been examined. In most respects it is a typical bivalve. The true shell has an external ligament with a lithodesma and both adductor muscles are present. Visceral mass dimensions are typical, with a digging foot. The ctenidia are greatly enlarged within the expanded mantle cavity, mostly accounted for by long siphons, with a fourth pallial aperture mid-ventrally. The adult Brechites vaginiferus is functionally “amyarian” with only minute anterior pedal retractor muscles. The foot is reduced and strap-like. The body is minute in relation to the tube, with only the enormously elongated ctenidia extending to the base of the siphonal orifices. Most modifications concern the mantle (as noted above). In nearly all other respects the animal is simplified in recognition of its aberrant mode of life. The function of the tube of Brechites is discussed. A protective role is self-evident but burrowing can be achieved by water being expelled from the basal plate. The same action may be used to take in nutrients from the subsurface deposits. The relationships of Brechites and the Clavagellacea are discussed. Affinity is with the passive burrowing Laternulidae and origins lie in the Pholadomyacea.

Gaimardia (Neogaimardia) finlayi Powell 1933 is a member of the little known family Gaimardiidae which collectively have an Australian, New Zealand and sub-Antarctic distribution. N. finlayi nestles amongst algal holdfasts at the bottom of wave-exposed beaches in New Zealand. It is a small, weakly heteromyarian dioecious bivalve attached by a relatively stout byssus. The mantle and the siphons are exceedingly simple; mantle fusions, other than anterior and posterior to the internal ligament, are of the inner folds only. The siphons are non-extensible wide openings with few sensory papillae. Each ctenidium comprises an inner demibranch only with no inter-lamellar and but a few inter-filamentar junctions. Within the ctenidium large telolecithal eggs are incubated to a crawling, shelled stage of approximately 0–75 mm in length. Sperm were found in the ctenidium of two mature males. The possible storage of sperm and eggs and the incubation of fertilized eggs have survival value for a small bivalve living in a difficult environment. The modifications which suit N. finlayi to its mode of life are discussed, but major interest centres around the taxonomic position of the Gaimardiidae. It is concluded that the Cyamiidae and Gaimardiidae are very closely related, but more comprehensive studies on representatives of both families are needed.

First recorded from Hong Kong in 1980, Mytilopsis sallei now dominates the intertidal pier community within the Government Dockyard, Victoria Harbour. Analysis of monthly samples of M. sallei has demonstrated aspects of the reproductive strategy and life-history characteristics correlated with seasonal changes in local water quality. Mytilopsis sallei is dioecious, has a high fecundity, matures early (at 8–10 mm shell length), grows fast and the majority of the population is probably semelparous. These are characteristic traits of opportunistic r-strategists. Comparable characteristics are typical of other introduced bivalves. Maximum life span of M. sallei in the Dockyard is probably less than 20 months with, however, most of the population dying before that. Heavy recruitment, particularly during spring-summer, suffocates older individuals. The shells of dead animals similarly last for little more than one year, dissolution and crushing effecting this. A mat of byssal threads serves to bind the mass and anchor it to the substratum. Mytilopsis sallei exhibits two periods of reproductive activity and settlement per year. These are possibly two elements of a single recruitment divided into minor (autumn-winter) and major (spring-summer) components by either high summer temperatures or low salinities. Summer and winter recruits of M. sallei, respectively, mature to produce the summer and winter juveniles of the following year. Over-stepping generations plus survival of a few individuals into their second year thus ensure continued success of M. sallei, and account for its spread and dominance in its introduced range. Such a bimodal pattern appears typical of many Hong Kong intertidal, estuarine and freshwater bivalves.

This study is of sympatric species of Pisidium inhabiting an artificially managed flooded vegetable furrow in the New Territories of Hong Kong. It extended over 16 months during 1983–84. Because of Hong Kong's subtropical climate, the habitat is strongly seasonally influenced with regard to temperature, and is flushed by summer rains. Normally, such a habitat would be ephemeral, drying in winter; flushed in summer. The water is acidic, soft and would typically be slow-moving and nutrient deficient, but because of agricultural practices it is periodically culturally eutrophicated. The pisidiids inhabiting the furrow show life history tactics which favour the habitation of a normally ephemeral subtropical habitat. Three birth periods occur each year, typically in winter, spring and in either summer (P. clarkeanum) or autumn (P. annandalei). Each generation is largely the product of the previous one. Pisidium clarkeanum attains a maximum shell length of 7.0 mm, sexual maturity at 2.5 mm and shelled larvae are present in the ctenidia at 3.5 mm. Average larval brood size is 9.49. Pisidium clarkeanum is generally univoltine and semelparous, though there is the probability that some individuals may produce one litter, continue to grow and contibute to the succeeding generation. These individuals are thus iteroparous and bivoltine. Pisidium annandalei, on the other hand, is a small species, with a recorded maximum shell length of 4.0 mm. It is born at 0.8 mm shell length and attains sexual maturity at 1.5 mm. Larvae are brooded from a parental length of 2.0 mm, average brood size being 7.17. Pisidium annandalei is typically univoltine and strongly semelparous, with the possibility only (as compared with the probability of this occurring in P. clarkeanum) of one or two individuals of the winter and spring recruits only surviving to be iteroparous and bivoltine. Autumn recruits are always univoltine as the following winter results in total adult mortality. The life history tactics of these two pisidiids are therefore similar and represent adaptations to ephemeral habitats in subtropical and tropical Asia. Hong Kong's other freshwater bivalves are discussed: all are occupants either of other ephemeral lotie or more predictable lentic and lotie systems and show adaptations appropriate to these habitats.

The discovery, around Galapagos Rift hydrothermal vents, of an unique community of animals dependent upon the chemoautotrophic oxidation of hydrogen sulphide by bacteria, has aroused wide interest. In the gutless pogonophoran Riftia pachyptila. trophosomal symbiotic bacteria are thought to be principally responsible for this unique form of nutrition. Similar symbiotic bacteria have been postulated for the ctenidia of the Rift clam Calyptogena magnifica. Such a mode of nutrition was deemed necessary since Calyptogena was thought not to possess ctenidial food grooves, thereby making normal filter-feeding impossible. This study reports upon the anatomy of a specimen of C. magnifica and demonstrates the presence of narrow ctenidial food grooves and the normal bivalve complement of feeding and digestive organs. Using a variety of general bacterial and DNA specific stains, no evidence of symbiotic intracellular bacteria has been found in the ctenidia (or any other tissues). It is concluded that C. magnifica is principally a filter feeder, albeit with modification for collecting and processing a diet of bacterial cells, with the possibility (as in all bivalves) of direct absorption. High chemoautotrophic activity levels in the ctenidia probably result from entrapment of vent water bacteria collected during filter feeding.

Deroceras caruanae (Pollonera 1891) possesses an endogenous cycle of activity and inactivity entrained by the cycle of night and day. D. caruanae remains inactive on the undersurface of experimental feeding panels during daylight hours and as dusk approaches undertakes a phase of crawling bringing it into contact with food on the upper surface of the feeding panel at night. As dawn approaches, the slugs retire to the undersurface of the panel. In total darkness, the slugs lose their normal rhythm of light and dark regulated phases of activity and inactivity. The rhythm is retained (albeit out of phase with slugs experiencing a normal light-dark sequence) in constant illumination. The relative feeding rates of D. caruanae have been correlated with this cycle of activity and inactivity. Structural and functional changes in the organs of digestion, notably the digestive diverticula, have been correlated with the cycle of night and day, the cycle of activity and inactivity and feeding. The digestive tubules undergo a sequence of cytological changes over the course of 24 hours that primarily involves the digestive cells. These cells absorb and intra-cellularly digest food in the hours of daylight and in late evening commence–and at night complete–a process of breakdown releasing fragmentation spherules and excretory concretions into the stomach. The fragmentation spherules, by the release of unused intra-cellular enzymes, possibly assist in the primary extra-cellular digestion of food in the crop on the next feeding cycle. The digestive diverticula comprise three cell types; digestive cells, calcium cells and thin cells. The function of these cell types is discussed and the structure of the digestive diverticula of D. caruanae compared with that of other pulmonates. Finally a co-ordinated scheme of the processes of activity and inactivity, and feeding and digestion is constructed showing how such processes are inter-related and affected by environmental variables; most notable among these is the cycle of night and day.

Various aspects of the biology, population dynamics, growth, sexuality, reproduction, settlement and mortality of populations of Corbicula fluminea (Muller, 1774) inhabiting Plover Cove reservoir and the Shum Chuen river, Hong Kong are described. These data are correlated with the hydrological picture of the reservoir to obtain a better understanding of the conditions under which the processes of growth, reproduction and settlement occur. An analysis of the shell dimensions of the component age groups comprising the population is obtained and, using the growth formula of Bertalanffy (1938), the maximum theoretical length of C. fluminea for this locality is calculated, which when considered in conjunction with other data relevant to the biology provides a more meaningful definition of this species. Such information, if obtained for other species, would provide a better taxonomic picture of the Corbiculacea as a whole and permit a greater understanding of those corbiculids introduced into the U.S.A. which are a pest of water supply systems. A preliminary comparison with such information as is available from the North American populations of Corbicula, indicates that at least two species, namely C. fluminea (Muller 1774) and C. fluminalis (Muller 1774) have been possibly introduced. Other species may have been introduced, but until a more thorough examination of the species both within their home and introduced ranges has been undertaken, the taxonomic picture will remain obscure. This paper demonstrates that in Hong Kong C. fluminea is a protandric consecutive hermaphrodite, which incubates its young in a ctenidial marsupium. This refutes the conclusion reached by earlier researchers and emphasizes the need for more detailed studies.

The extremely rare Periploma (Offadesma) angasai Crosse & Fisher 1864 is a member of the widely distributed Periplomatidae (Anomalodesmata) occurring on surf-protected beaches in New Zealand and Australia. The species is low intertidal-sublittoral in distribution, living 7–8 cm below the sand surface on its left valve with separate siphons projecting to the sand-water interface above. The shell is sharply inequivalve (the right deeply, the left shallowly convex) and inequilateral, the posterior rostrate and truncate and covered by thick periostracum densely studded with sand grains. These are attached to the periostracum by a sticky secretion produced from radial mantle glands in the middle mantle fold. Similar glands have been reported for members of the Lyonsiidae and Verticordiidae. Offadesma angasai is adapted for a life in which large amounts of fine sediment may enter the mantle cavity. Thus the ciliary rejection currents are powerful, the labial palps are extremely long and the sediments are bound up in mucus produced by an extensive gland, particularly in the right mantle lobe, in the posterior region of the mantle cavity. A gland with a similar structure but different staining reactions also lines the visceral mass and the mantle of the supra-branchial chamber and probably consolidates particles penetrating the ctenidial filter. Such glands are similar in structure and function to the primitive molluscan hypo-branchial gland. In many respects, especially with regard to the secondary ligament of fused periostracum, a ctenidial structure with type E ciliation, the presence of a 4th pallial aperture, extensive mantle fusions and simultaneous hermaphroditism, Offadesma is clearly similar to the majority of the Anomalodesmata. As in many other anomalodesmatans also, large, yolky, encapsulated eggs are produced; these are thought to be an adaptation facilitating survival by a sparsely distributed species occupying a narrow niche. Many other features of Offadesma, notably the structure of the ligament and shell clearlyally it and the Periplomatidae with the Laternulidae and the Thraciidae in a separate line of evolution from the other families of the Pandoracea, and probably deserving of super-family status—the Thraciacea Stoliczka 1870.

The ecology, habits and functional morphology of Galeomma (Paralepida) takii (Kuroda) are described, and comparisons made with other free living and more specialized commensal galeommatids, other Leptonacea and other bivalve phylogenies. Galeomma takii lives under stones at mid tide level and possesses both primitive and specialized characters which make it successful in such a restricted habitat. One of the most important specializations is numerous pallial tentacles that can autotomize and, it is thought, release a noxious substance to deter would be predators. Such a mechanism is advantageous for an animal living in a well defined ecological niche shared by other species, some of which may be predatory. Post settlement protection of the young also probably occurs.

The structure of the pallial eyes of Laternula truncata (Lamarck 1818) has been studied using the light and electron microscopes. The eye is complex and can be- considered to be- the most advanced yet described for a bivalve mollusc. The cornea consists of modified flattened epithelial cells with an external border of microvilli. The cornea covers a large, circular, multinucleate lens. The lens comprises (1) centrally located translucent lens cells, (2) laterally located supporting cells from which cell processes interdigitate with processes from the lens cells. The retina is two layered and inverted. The proximal and distal retinae are made up of concentrically arranged laminae derived from the membranes of ciliary basal bodies. The cilia comprise a base and feet, but no root system and have a 9+0 arrangement of filaments. The pigment cup or tapetum is bounded by a sclerotic coat and is three layered, each layer possessing characteristic pigment granules. From the base of the eye arises a large optic nerve. The eye possesses an eye appendage, the epithelium of which is invaginated on its internal border to form a groove within which are found some 28 cilia. The cilia, it is thought, make contact with the microvilli of the epithelium when the appendage is touched. Such an action serves to protect the delicate eye from damage. The structure of the eye is compared with that of other molluscs, particularly members of the Bivalvia.

Arthritica crassiformis Powell is found in New Zealand waters commensally associated with the pholad Anchomasa similis (Gray). Its location on the host is highly specific and is occasioned by the complex needs of the commensal and the facilities and limitations imposed by the tunnelling activity of the host. Other species of pholad because of different morphological adaptations do not serve as hosts. With increasing size of the host, the number of commensals/host increases which is due to (a) retention of young individuals within the parental burrow and (b) by colonization from the external environment. A. crassiformis typically, however, occurs in pairs and is probably a protandric consecutive hermaphrodite. The young are incubated in the interlamellar spaces of the ctenidia of the parental female. This anatomical adaptation together with others including strong rejectory tracts suit the commensal for a life with an active tunnelling host. The ciliary currents of the mantle cavity of A. similis are described.

Pseudopythina subsinuata (Lischke) has been found in Hong Kong waters to be commensally associated with the following stomatopod crustaceans: Oratosquilla perpensa (Kemp), Oratosquilla interrupta (Kemp), Oratosquilla oratorio (De Haan), Harpiosquilla japonica Manning. The location on the host is highly specific. Studies on the functional morphology and biology of this bivalve have shown that it is relatively unspecialized except with regard to its sexual cycle. It has been shown to be a protandric hermaphrodite in which a smaller male normally occurs in close association with a larger female. Fertilization occurs in the supra-branchial chamber of the female and the young are subsequently incubated in the same position. Solitary females also occur. It is suggested that a complex sexual cycle was a prerequisite for the successful exploitation of the commensal mode of life by the Leptonacea in general and that subsequent morphological specializations have occurred in only some. Summary: Pseudopythina subsinuata has been found in Hong Kong waters living commensally on the following species of stomatopod crustaceans: Oratosquilla perpensa (Kemp), Oratosquilla interrupta (Kemp), Oratosquilla oratorio(DeHaan), HarpiosquiliajaponicaManning. [ts location on the host is extremely specific and is occasioned by the requirements of the commensal and the limitations imposed by the behaviour of the host. Pseudopythina subsinuata possesses many characters which suggest that it has only comparatively recently adopted a commensal mode of life: (1) It is not host specific. (2) It possesses a large shell with distinct hinge teeth. (3) The mantle edges are not reflected over the shell, and there is no inhalant siphon. (4) There is a well developed byssal apparatus. (5) The stomach is large, not reduced and non-specialized. (6) The style sac still retains a connection with the mid-gut along half its length. Some of these primitive characters, e.g. the large byssus, have been retained because they possess an obvious survival value. Other characters, e.g. the large labial palps, and the absence of interlamellar junctions in the ctenidia are probably specializations. The sexual cycle of Pseudopythina subsinuata is complex, the animal being a protandrous consecutive hermaphrodite. Solitary females also occur in the population. Fertilization occurs in the supra-branchial chamber and the young are incubated there at least until the late veliger stage. It is suggested that a highly specialized sexual cycle was developed early in the evolution of the Leptonacea and was a prerequisite for the successful adoption of a commensal mode of life. Anatomical specializations evolved later, and to different degrees in different phylogenies.