Search

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 During the night the shell valves become more apposed, the mantle folds are withdrawn and few phasic adductions occur.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.

Larval trematode (Family Bucephalidae) infection of Tridacna crocea (Lamarck 1819) was investigated to determine pathological effects on the host. Of 108 T. crocea gonads examined, 13 (12%) were found to be parasitised. The bucephalid caused castration in heavily infected clams, gonadal tissue being completely replaced by sporocysts. The parasite was also found in the kidney, digestive gland and ctenidia. This is the first record of a bucephalid trematode in a member of the Tridacnidae. The parasite is of potential importance to tridacnid shellfisheries currently being developed.

The ultrastructure of the adductor muscle of the boring clam (Tridacna crocea) was investigated. The adductor was composed of opaque and translucent portions. The opaque portion contained smooth muscle cells; the translucent portion contained obliquely striated cells. Smooth muscle cells were classified, according to the statistically analyzed diameters of their thick myofilaments, into two types, S-1 and S-2. S-1 cells had thick myofilaments, 50-60 nm in diameter. S-2 cells had thick myofilaments of two sizes, about 55-65 nm and 85-100 nm in diameter, respectively. Obliquely striated muscle cells in the translucent portion were also classified into two types: O-1 cells, with thick myofilaments 30-35 nm in diameter, and O-2 cells, with myofilaments of 50-60 nm.

The morphology and the growth rate of three populations of Penitella penita from substrata of different hardness were studied. Morphology differed significantly, the valves being delicate and elongate in the soft rock and heavy and tumid in he hard rock. The rate of growth decreased as the hardness of the substratum increased. Evidence is presented which suggests that these differences are not genetically controlled but are induced by the relative hardness of the substratum. Growth rate is dependent both on the width of growth bands and on the frequency with which these are deposited. The width of the growth band deposited during the shell deposition period is determined by the amount that the animal was able to enlarge its burrow during the preceding period of active boring. Growth band width increases with age and decreases with harder substrata. The duration of the growth interval increases with age and probably also with decreased availability of food. See full-text article at JSTOR

Ground sections, acetate peels, and scanning electron microscopy are used to visualize the microstructure and fine growth lines of the shells of Penitella penita, collected from different substrates. Growth increments of approximately daily duration were found in the area of the umbonal ventral sulcus. Two morphologically different types of growth increments are described, those deposited during the period of quiescence and growth (7 to 10 days) and those deposited during the succeeding period of active boring (5 to 8 days). The duration of a full cycle of growth and boring varies considerably, but averages 15 days.The width of growth increments and the microstructure of the shell is controlled by the amount of space within the burrow, and this in turn depends primarily on the substrate hardness. The effect of other factors, such as disturbance, and tidal and seasonal fluctuations, is minor by comparison.The microstructure of juveniles differs from that of older animals and the number of lines per group is reduced.

The byssal apparatus appears during post-larval life when its secretion permits brief attachment during metamorphosis when the animal assumes the form which fits it for life in the adult habitat.The byssus persists for continued temporary attachment in animals which may lose it in adult life, e.g. Mya arenaria, or in which it becomes an organ of permanent fixation, e.g. Mytilus edulis, Pododesmus macroschisma and Tridacna crocea

Penitella penita , a west coast rock-boring clam, forms a conical burrow in the rock the shape of which is controlled by a number of environmental variables. The most important of these being substrate hardness. A method for determining the relative friability hardness of sandstone is described. The possibility that this information could be used by the paleontologist to establish the hardness of sedimentary rock at known times in the past is discussed.

We report the development and characterization of nine microsatellite markers from the Tridacna crocea genome. In a wild population of 20 individuals from Ulugan Bay in the Republic of the Philippines, we observed an average of 9 alleles per locus and expected heterozygosity ranging from 0.41 to 0.92. Cross-species priming was tested in three additional Tridacna species with moderate success. These markers will be used in studies of fine-scale population genetic structure and gene flow in T. crocea. The markers may also be useful in a forensic context by identifying source populations of giant clams collected illegally for the live aquarium trade, while protecting legal exports from aquaculture facilities.

Penitella penita (Conrad) is a dioecious and oviparous clam. It differs from most marine invertebrates in that its growth is determinate and full sexual maturity is postponed until cessation of growth.

TRIDACNID clams can be divided into two distinct groups on the basis of whether they are physically fastened to the substrate. Of the six species comprising the family Tridacnidae1, Tridacna crocea, T. maxima and T. squamosa live byssally attached to dead coral, coral rubble or limestone during the whole of their adult lives. The other species, T. gigas, T. derasa and Hippopus hippopus, live byssally anchored as juveniles, but when approaching maturity their byssus glands atrophy, and the adult clams take up unattached existence on coral rubble or hard packed sand. This habitat difference is also expressed in the weight and morphology of their shells. The byssally attached species possess shell valves which are in most respects larger editions of those found in other bivalves (Fig. 1 A). The unattached clams, on the other hand, possess bizzare shells which are unusually massive resulting from a unique deposition of solid shell material within the cavities of the umbones (Fig. 1 B).

B. truncata larvae are nearly round in appearance with a dark band around the margin of the shell. The shell is faintly pink near the umbo and at the ventral margin of large larvae. Two interlocking hinge teeth are present in each valve of larvae 70 μ in length or larger. One tooth is about 25 μ long while the other three are 5–10 μ. Straight hinge larvae measure 55–95 μ in length with a straight hinge line about 45 μ long. Height is 0–10 μ less than length, and thickness 20–25 μ less. The umbo is a rounded knob giving larvae a circular appearance and is present in larvae from 85–315 μ in length. In umbo larvae, height is 0–20 μ less than length, and thickness 25 to 80 μ less. Gills are formed before the loss of the velum, and metamorphosis occurs when larvae reach a length of 250–315 μ.