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International audience; he dynamics determining the establishment and maintenance of epibiotic relationships are fundamental to understand the role of biological interactions in the functioning of marine coastal ecosystems. Epibiosis of barnacles on mussels is a common feature of many intertidal rocky shores. There are several possible explanations for this relationship, but it is unclear whether the association is commensal, antagonistic, or mutualistic. We investigated the diets of barnacles and mussels in an epibiotic relationship (epibiont barnacle and basibiont mussel), and compared them to the diets of conspecifics living on the same rocky shore but independently (free barnacle and free mussel), using stable isotope (SI) and fatty acid (FA) techniques. The SI results indicated that the diet of mussels did not change as a function of epibiosis, while free barnacles were characterized by higher δ15N values than epibiotic conspecifics, indicating that they were feeding on more recycled food sources. This result was, however, supported by data from only one site. Results from FA were partially consistent with those of SI analysis, indicating that mussel FA profiles did not change between basibiont and free mussels, but that epibiotic barnacles were characterized by a lower amount of polyunsaturated FA than free barnacles. This suggests that the diet of epibiotic barnacles is of lower quality. This could be due to competition for food with mussels, driven by the different mechanisms of food assimilation between epibiotic barnacle and basibiont mussel; or by the different sizes of particles ingested by the two species under epibiosis vs free-living conditions. The discrepancy between the FA and SI results observed could reflect the different integration time of the analytical techniques, with FA reflecting shorter term variability than SI. We conclude that epibiosis represents an unfavourable relationship for barnacles due to the lower quality of food available when colonizing mussel shells, whereas mussels neither benefit nor were harmed by this association. Thus, this relationship is amensalistic: negative for barnacles but neutral for mussels.

The availability of suitable microhabitats has emerged as a key requirement for maximising species diversity gains from ecological engineering of coastal habitats. This includes intertidal boulder field habitat, which is threatened by increasing urbanisation. We examined faunal use of microhabitats offered by natural intertidal boulders on two continents, South Africa and Australia, and then used artificial boulders to test possible mechanisms driving the observed patterns. Gaps often occur between the undersides of boulders and the substratum on which they lie. Substrata underneath boulders are generally uneven, so gaps can occur anywhere along the boulder undersurface, but gaps are generally larger near the edges of naturally-occurring boulders. Boulder edges were found to provide a microhabitat that had greater densities of almost all macrofauna than closer to the centre of boulder undersides. To test the model that microhabitat use of boulder edges reflects their larger gap sizes (relative to boulder centres), artificial boulders were constructed with the underside surface either flat, or with a gap underneath mimicking the mean size of the gap normally found only under boulder edges (when their average shape is considered among numerous naturally-occurring boulders). Artificial boulders were deployed intertidally for seven weeks, and macrofaunal colonisation was compared (a) between flat boulders and those with gaps, and (b) between edges and centres. Five times more macrofauna colonised boulders with artificial gaps, while no effect of proximity to edges was found when the gap-width was controlled for. The size of gaps under boulders appears to be an important microhabitat feature that can explain a widespread distributional pattern in the diverse assemblages of boulder fields. Such gaps are clearly used by numerous species, including some that are rare or commercially important. Provision and augmentation of this microhabitat should be considered for any ecological engineering project that involves intertidal or subtidal boulders.

Understanding the dynamics of epibiosis is fundamental to understanding the role of biological interactions in the functioning of marine ecosystems. At many coastal sites, barnacles are abundant as epibionts on the shells of mussels. As they approach the shore, cypris larvae explore the environment, actively seeking suitable settlement sites and rejecting unsuitable substrata. Suitability involves micro-features of the surface, including its texture and contours, chemical signals and the presence of conspecifics. To investigate whether epibiosis on mussel shells indicates a preference for this substratum on the part of the barnacle Chthamalus dentatus, artificial plates were deployed at two rocky shore sites on the south coast of South Africa. These plates offered settling cyprids four habitat choices: a live mussel, the shell of a recently dead mussel, a fine resolution resin replica of a mussel shell to mimic architectural micro-surface, without the chemical characteristics of natural shells and a rock mimic, a surface covered in a film of hard plastic that resembled natural rock surface as closely as possible. Settling surfaces were photographed monthly, and new barnacles were counted and allocated to size classes, so that survival could be estimated over a period of several months. Barnacles showed a clear preference for the rock mimic surfaces, which on average supported double the number of settlers found on mussel replicas. These in turn were significantly higher than the numbers on live and dead mussel shell surfaces, implying that there are features of both dead and live mussel shells that deter barnacle settlement. In contrast, final abundances of adults showed no significant effect of settling surface. The results suggest that during settlement, cyprids avoid chemical cues from mussel shells that persist even after the death of the mussel. This suggests that the high numbers of barnacles often found on the surface of live mussels could be due to saturation of substratum.

The temporary ability of larvae to select optimal substrata during settlement can strongly influence the success of sedentary benthic invertebrates. Larval responses to cues from algae, epilithic microbial assemblages and conspecific species are well recognised. Comparatively, little is known about how the threat of predation or of competition influences settlement. We used a manipulative field experiment to test if the presence of a predator or spatial competitor affects settlement patterns of two intertidal mussel species, Perna perna and Mytilus galloprovincialis . By deploying treated artificial units of settlement habitat (larval collectors) in ‘no choice’ and ‘choice’ combinations, we determined whether the observed settlement patterns resulted from active larval choices (preference/avoidance). Among treatments where no choice was offered, settlement was generally greater on the predator treatment, lower on treatments with epilithic biofilm or competitor presence, and least on collectors without biofilm. In all but one case, the observed proportions of settlement in the ‘choice’ tests were significantly different from that expected by random association based on the ‘no choice’ tests. Choices related primarily to the absence of biofilm and to the threat of predation, with little influence of competitor presence. Although active preferential behaviour was confirmed, patterns of selectivity differed between the two study sites. Settlers of both species preferred the predator treatment at one site, possibly suggesting a predator swamping strategy. In contrast, settlers at the second site avoided the presence of predators. This suggests that settlement behaviour may be contingent on site-specific factors and demonstrates a greater degree of flexibility in settlement behaviour than expected.

When a range of life-history groups within a population exploit similar niches, hierarchies are often established whereby optimal resources are capitalized by dominant individuals. The present study investigates the feeding and movement of a size structured population of gobies residing in crab burrow micro-habitats, within this context. All life-history stages of the goby Redigobius dewaali were found to utilize burrow-pools formed by the crab Scylla serrata in the mangrove intertidal zone of an estuary at low tide. Gut content analysis was therefore conducted to test for feeding overlap and potential intraspecific competition among various sizes of fish residing in these habitats. In addition, an exclosure experiment was conducted to determine whether these fish leave the burrow-pools at high tide to forage over the intertidal flats. While ontogenetic dietary shifts across size-classes occurred, copepods and amphipods comprised the most important components of the diet for most size-classes, resulting in a large degree of dietary overlap. Of the adult R. dewaali sampled from pools, a single large mature male was sampled from each burrow, suggesting territoriality, a trait well observed in males of many goby species. The experimental component of the study highlighted the movement of smaller individuals, but not larger individuals, from the burrow-pools at high tide. As all life-history stages occur in the same microhabitats, and forage on much of the same prey, we suggest that the existence of a competitive hierarchy resulted in the need for the smaller individuals to find alternate feeding areas, consistent with dominance hierarchy theory.

article i nfo Opportunistic Preference Prey Upwelling The relationship between the distributions of predators and their prey is particularly strong for predators with specialised diets. Predicting the diet of omnivores is, however, more complex. We investigated the diet of the omnivorous nereidid polychaete, Pseudonereis variegata with respect to spatial scale, upwelling and small-scale behaviour. To investigate the diet of P. variegata across a range of spatial scales, shores in the Eastern and Western Regions, within the Warm Temperate South Coast (Agulhas) biogeographic prov- ince of South Africa were sampled. Two shores, kilometres apart within an area of upwelling and two within a non-upwelling area were sampled within each region. Gut-contents were composed of plants and animals. Neither region nor upwelling was found to affect the overall diet of P. variegata, with significant differences occurring only between shores in the Eastern Region. When the species were separated into plants and ani- mals, in the Eastern Region, significantly more animals were consumed by P. variegata in the upwelling area compared to the non-upwelling area. Carefully designed laboratory experiments for preference showed that P. variegata consumed amphipods (various species of Gammaridae) and macroalgae (Hypnea spicifera), but rarely ate gastropods (Afrolittorina knysnaensis). It was clear that P. variegata rarely chose to eat gastropods, but there was no experimental evidence to support that they preferred amphipods and macroalgae over gas- tropods. Findings from the experiment based on gut-contents required knowledge of whether P. variegata were making active choices among the range prey that were available in the field. Similarly, the laboratory experiments in which individuals were offered different types of food required background knowledge of the entire suite of prey consumed and also an understanding of the broader-scale environmental factors influencing the diet of P. variegata. This study, therefore, highlights the usefulness of combining large-scale studies with carefully planned laboratory experiments that consider preferences for specific types of food.

article i nfo Using a multi-disciplinary approach, we evaluated the potential consequences of long-term contrasting prey availability on the condition Cape gannets Morus capensis. We compared breeding adults from a decreasing colony on Malgas Island off the west coast of South Africa, where the abundance of small pelagic fish has de- creased, with an increasing colony on Bird Island off the south coast, where pelagic fish are more abundant. We investigated (1) pelagic fish density using data from a hydro-acoustic survey, (2) gannet diet (stomach content analyses, fatty acid and stable isotope analyses), (3) gannet foraging effort and at-sea feeding areas (nest attendance patterns, GPS-tracking and blood haematocrit), (4) chick growth rates, and (5) adult body condition (morphometric measurements and breast muscle thickness). Our data confirmed contrasting prey availability between colonies, although pelagic fish were more abundant on the west coast than in pre- vious years. Gannets exhibit dietary plasticity, feeding on pelagic fish and trawler discards, but favour natural prey when available. Stomach content samples showed that gannets from both islands mainly ate natural prey in 2009, but there were differences in their stable isotope and fatty acid signatures, supporting evidence of long-term diet differences. Nevertheless, chick growth rates and adult body condition were similar at both colonies, which was surprising for breeding adults from the west coast colony that had been feeding exten- sively on energy-poor fishery waste for several years. Breeding gannets' behavioural flexibility seems to have succeeded in maintaining body condition. However, this might have long-term costs, as adult survival at Mal- gas Island has decreased in recent years. Population decreases at this colony are exacerbated by low juvenile survival and perhaps differential recruitment to Bird Island. Higher foraging effort by adult breeding on the large colony of Bird Island compared to birds from the west coast, despite greater pelagic fish abundance on the south coast, probably suggests a greater intra-specific competition there. Monitoring of population trends, population health indices and foraging behaviour, as well as prey availability is necessary to better understand the mechanisms underlying the population trends.

Few studies have examined the nature of spatial structure in intertidal systems, especially that of macroalgae and grazers, and it is not clear how consistent spatial heterogeneity is among sites and levels on the shore. Here we studied the consistency of spatial heterogeneity in several algal functional groups and in grazer biomass between two sites and across the shore using a combination of semivariograms and fractal analysis of data collected from transects. Additionally, we tested for consistency in spatial co-variation between algae and grazers using cross-semivariograms. The study was conducted at two sites in South Africa and across a gradient of physiological stress at three levels on the shore (low, mid and high shore). We observed spatial heterogeneity in macroalgal functional groups and in the biomass of grazers at one of the sites and at different levels of environmental harshness (i.e. heights on the shore), while the other site showed consistently random patterns. In terms of alga–grazer co-variation, most transects showed significant co-variation, but with more differences in the sign and the number of lags between sites than among levels on the same shore, suggesting stronger differences in spatial patterns between sites. Also we observed differences in co-variation between macro and mesograzers on the midshore. Variability in spatial patterns among levels on the shore and between sites is not explained by topographic complexity, but may be a product of several biotic factors interacting at small scales (cm–m). Differences in spatial patterns among shore levels and between shores also appear to reflect the indirect effects of an upshore reduction in algal biomass (due to heat and desiccation stress) and human harvesting of grazers at one of the sites, respectively. The results show that spatial heterogeneity of particular organisms (algal groups and grazers) and co-variation between them can emerge at small spatial lags, and reduction in spatial heterogeneity in the intertidal can be induced by a cascading effect driven by disturbance.

Top-down effects on intertidal communities concern species interactions such as competition and predation, and occur on small-scales. In contrast, bottom-up factors concern the supply of resources such as food and recruits. These are generally understood to be large-scale effects that are oceanographically controlled. Here we examined the effects of small-scale bottom-up factors on the growth of intertidal mussels to test the hypothesis that such factors can also operate on small (cm m) spatial scales. L-shaped aluminium baffles were used to reduce flow around mussels on very small (cm) scales in before/after experiments run at different sites in 2004 and 2006. Each experiment had two phases, each phase lasting approximately 100 days. At the start of phase one, mussels were individually marked at the growing edge of the shell in 10 control and 10 treatment patches before applying the treatment. At the start of phase 2, the same individuals were marked again at the new growing edge and baffles were placed around treatment patches. After phase one, there was no significant difference in growth rates between treatments in either year. In both years, applying the treatment reduced growth rates by approximately 30% compared to control patches. Condition index at the end of phase two showed the reverse and was lower for control mussels. Untreated control patches also showed a significant temporal effect (i.e. differences in growth between the two phases) during 2004 but not 2006. The results clearly show that bottom-up factors associated with food-supply operate at very small, within-shore scales, strongly influencing growth rates and possibly competitive abilities. They also indicate that bottom-up and top-down effects can interact across multiple spatial scales.

Scales of spatial and temporal variation in the settlement of early and late plantigrades of the mussel Perna perna to artificial substrata were measured on the south coast of South Africa. One study examined daily settlement over one month at three sites of similar tidal height and aspect. The sites were separated by 100′s of m with 5 replicated artificial settlement substrata approximately 1 m apart within each site. Early (

The mussel Mytilus galloprovincialis is invasive worldwide, has displaced indigenous species on the west coast of South Africa and now threatens Perna perna on the south coast. We tested the hypothesis that Mytilus will replace Perna by examining changes in their distribution on shores where they co-exist. Total cover, adult density, recruit density, recruit/adult correlations and mean maximum lengths of both species were measured in 2001 at two contrasting sites (Plettenberg Bay and Tsitsikamma) 70 km apart, each including two locations 100 m apart. Cover and density were measured again in 2004. Total mussel abundance was significantly lower in Tsitsikamma, and recruit density was only 17% that of Plettenberg Bay. Abundance and cover increased upshore for Mytilus, but decreased for Perna, giving Mytilus higher adult and recruit density and total cover than Perna in the upper zones. Low shore densities of recruits and adults were similar between species but cover was lower for Mytilus, reflecting its smaller size, and presumably slower growth or higher mortality there. Thus, mechanisms excluding species differed among zones. Recruitment limitation delays invasion at Tsitsikamma and excludes Perna from the high shore, while Mytilus is excluded from the low shore by post-recruitment effects. Recruitment limitation also shapes population structure. Recruit/adult correlations were significant only where adult densities were low, and this effect was species-specific. Thus, at low densities, larvae settle or survive better near adult conspecifics. After 3 years, these patterns remained strongly evident, suggesting Mytilus will not eliminate Perna and that co-existence is possible through partial habitat segregation driven by recruitment limitation of Perna on the high shore and post-settlement effects on Mytilus on the low shore.

Population dynamics of many intertidal organisms are strongly affected by the abundance and distribution of larvae arriving on the shore. In particular, not only absolute numbers of settlers but also the degree of synchronisation of settlement can have a strong influence on whether density-dependent or density-independent processes shape adult shape populations. Temporal variation in rates of settlement and recruitment of the mussel Perna perna on the south coast of South Africa was investigated using a nested spatial design at different temporal scales. Variability in settlement at spring tides was examined at two temporal scales: lunar (to investigate the effect of state of the moon on settlement) and tidal (to investigate the influence of state of the tide on mussel settlement). Recruitment over neap tides was examined at one temporal scale, fortnight (to investigate the effect of date on mussel recruitment). Strong temporal variation was evident for both settlement and recruitment, but not at all time scales. Distinct peaks of settler/recruit abundance were observed during the lunar and neap tide studies. Recruitment intensity differed over the course of the year, and pulsing of recruitment was generally synchronised among locations. However, the strength of pulsing differed dramatically among locations, giving a significant interaction between fortnight and location. The finest temporal scale, investigated in the tidal study, did not reveal a significant effect of the state of the tide on settlement. The state of the moon (new or full) was not significant as a main factor (p = 0.052), although generally more settlers arrived on the shore during new moon. Phase of the moon appeared to have an effect on settler abundances, but only when and where densities were high.

We examined the spatial heterogeneity in three sessile rocky shore organisms, the mussel Perna perna, the barnacle Octomeris angulosa (Sowerby) and the red alga Gelidium pristoides (Turn.) at a range of continuous local scales along horizontal transects within mid- and upper mussel beds of South African shores. We also examined the relationships between variability of organisms and topographic features (rock depressions, slope, aspect), and between mussel, barnacle and algal variability over the same scales. To estimate spatial heterogeneity, we analyzed scaling properties of semivariograms using a fractal approach. Relationships between different variables at the different scales were examined by cross-semivariograms. Spatial dependence of P. perna variability increased with spatial dependence of topographic variability, so that scaling regions of mussel and topographic distributions corresponded well. This relationship often improved with larger local scales (mussel cover increased with depressions, steeper slope and aspect towards waves), while at smaller spatial scales, variability in mussel cover was less well explained by variability in topography. The variability of the barnacle O. angulosa exhibited spatial dependence, even on topographically unstructured shores. In contrast, the distribution of the alga G. pristoides revealed high fractal dimensions, showing spatial independence on topographically unstructured shores. Algae also showed a very strong negative relationship with mussels at most local scales, and a negative relationship with barnacles in upper zones, especially at larger local scales. Barnacles may show clear spatial dependence because of hydrodynamics (at larger local scales) and the need to find a future mate in close proximity (at smaller local scales), while algae may show a strong negative relationship with mussels because of competition for space. D 2004 Elsevier B.V. All rights reserved.

Hierarchically scaled surveys were carried out on beds of the brown mussel Perna perna (Linnaeus) on the South coast of South Africa. The object was to assess spatial and temporal variations in the complexity of mussel beds and to investigate relationships between mussel bed complexity and mussel recruitment. Complexity was divided into three components: physical complexity; demographic complexity; associated biota. A series of variables within each component were recorded at two different scales (10 and 50 cm) within nested quadrats on three separate occasions. The nested ANOVA design explicitly incorporated spatial scale as levels of the ANOVA. These scales were: shores (areas 1 km in length separated by 25 km); transects (areas 20 m in length separated by 100s of meters); 50×50-cm quadrats separated by meters and 10×10-cm quadrats separated by cm) This approach was intended to generate hypotheses concerning direct associations between recruitment and complexity versus co-variation due external processes. Three main questions were addressed: (1) At what scale does each variable of complexity exhibit greatest significant variation? (2) At these scales is there similar ranking of variables of complexity and recruitment? (3) Within this/these scales, is there any significant relationship between the variables measured and mussel recruitment? On two occasions (Nov. 97 and Mar. 98) the majority of variables showed greatest significant variation at the transect-scale. On a third occasion (Oct. 97) most variables showed greatest significant variation at the quadrat-scale and the site-scale. On all occasions a markedly high percentage of the variation encountered also occurred at the smallest scale of the study, i.e., the residual scale of the ANOVA analyses. Some similarity in the ranking of variables occurred at the transect scale. Within the transect-scale, there was little indication of any relationship between variables of complexity and recruitment. Relationships were inconsistent either among transects or among sampling occasions. Overall, the results suggest that a high degree of variation in mussel bed complexity consistently occurs at very small scales. High components of variance generally also occur at one or more larger scales; however, these scales vary with season. Mussel recruitment does not appear to be directly affected by complexity of mussel beds. Instead it appears external factors may influence both complexity and recruitment independently. In addition recruitment may influence complexity rather than vice versa.

The limpets Patella granularis Linn. and Siphonaria capensis commonly co-exist on the coast of South Africa where S. capensis often reaches very high densities. Patella occurs in the diets of various predators, but Siphonaria is rarely eaten. The possibility of chemical defence in Siphonaria was tested using co-occurring predators. Live limpets were used to establish that predators differentiate between the two species; limpet tissue to establish that this was not due to behavioural defence; and chemical extracts from Siphonaria to eliminate the possibility of physical effects such as tissue toughness. The predators capable of taking live limpets (clingfish, starfish and two species of whelks) took Patella in preference to Siphonaria (χ2 tests, P

The style enzymes and enteric bacteria of two intertidal mussels, Perna perna (L.) and Choromytilus meridionalis (Kr), were tested for activity on ten structural and storage carbohydrates. Both mussels digested the storage carboyhydrates amylose, glycogen and laminarin, and the structural carbohydrate carboxymethyl cellulose (CMC). C. meridionalis broke down the digested storage carbohydrates with equal efficiency, while P. perna digested laminarin and glycogen more efficiently than amylose. P. perna produced large volumes of enzymes with low dissociation rates, while the opposite was true for C. meridionalis. As a result, P. perna released significantly more glucose (ANOVA: P

The systematic relationships among 12 species of the intertidal pulmonate limpet, Siphonaria were assessed using the technique of Random Amplified DNA fingerprinting. Previous systematic relationships proposed on the basis of morphology were confirmed for nine of the species considered. A strong systematic basis to mode of larval development (planktonic or direct development) was revealed. The subgenus Patellopsis includes both direct-developing and planktonic-developing species but these nested separately in the classification analysis, suggesting that the subgenus may be polyphyletic. The status of two species in the subgenus Sacculosiphonaria was unclear, but one, the direct-developer Siphonaria compressa, nested with the direct-developing species of the Patellopsis clade. Re-assigning S. compressa to the subgenus Patellopsis would resolve the anomaly of having both developmental modes within Sacculosiphonaria and would be consistent with the DNA findings. In contrast to analyses based on total cellular proteins, DNA analysis indicates that within-population variation was greatest among direct-developing species. This was unexpected and difficult to explain as one anticipates easier distribution of planktotrophic than direct-developing larvae and, thus, greater genetic diversity within populations of planktonic-developing species. One possible explanation is that low levels of gene flow between direct-developers cause small-scale founder effects in sub-populations, thus increasing the overall genetic diversity of the whole population. Within-population variation was highest for the direct-developer, S. compressa, which is not only the smallest and presumably is the least dispersive species examined, but is the species with the most restricted distribution. It is believed to be endemic to a single location in South Africa.

The Mediterranean mussel Mytilus galloprovincialis was accidentally introduced onto the South African west coast in the late 1970s and has since become invasive. It is spreading onto the south and east coasts where it has the potential to outcompete the indigenous mussel Perna perna . One possible explanation for this phenomenon is that it is not affected by the natural parasites of the area where it was introduced. A survey of trematodes was done at different spatial scales for populations of the indigenous mussel Perna perna and the invasive Mytilus galloprovincialis in South Africa, as well as for indigenous populations of M. galloprovincialis in Spain. Both South African and Spanish populations of M. galloprovincialis were found to be free of trematodes, whilst P. perna was commonly infected by trematodes. The most important trematodes found in this mussel were metacercariae of Proctoeces and bucephalid sporocysts, with infection rates up to 62 and 49%, respectively. Both Proctoeces and the bucephalid sporocysts were randomly distributed within a mussel bed. Bucephalid sporocysts were found in very low prevalences at most sites along the South African coast. Prevalences were highly dependent on the size of the mussel, and there were no seasonal variations in infection rates with this parasite. Proctoeces was only found between Cape Agulhas on the south coast and Sodwana Bay on the east coast, and infected significantly more females than males. Prevalence of infection was highly dependent on the size of the mussel in the case of females but not in the case of males. There were no seasonal variations in prevalence of infection with Proctoeces in either females or males. Mytilus galloprovincialis has proved to be a competitively superior species which has replaced the indigenous Aulacomya ater as the dominant mussel on the west coast. The fact that it is free of trematodes, whilst Perna perna on the south and east coasts is commonly infected by trematodes, may be one of the reasons for its success in South Africa as a whole and its incipient invasive behaviour on the south coast.

Using polyacrylamide gel electrophoresis of total soluble proteins, estimates of genetic variability were obtained from populations of seven species of southern African limpets. Among these sampled populations, greater levels of genetic variability were associated with the planktonic developing species, Siphonaria concinna, S. capensis and S. oculus with clear protein polymorphisms associated with S. concinna . The direct developing species, S. anneae, S. nigerrima, S. serrata and S. tenuicostulata were characterised by lower levels of genetic variability within a species and their mean value of genetic variability was lower than that of the planktonic developing species. All species had similar protein banding patterns and on a dendrogram produced from a similarity matrix they largely nested apart from each other and the three outgroups. The systematic relationships within Siphonaria were unclear. Six of the seven species fall into the subgenus Patellopsis but PAGE did not discriminate these from S. serrata of the subgenus Siphonaria . However, the earlier branching of the planktonic developing species with direct developing species nesting among them on the dendrogram suggests that the planktonic species may be primitive within the Patellopsis subgenus.

In situ diel recordings of the heart beat of the upper mid-shore, intertidal pulmonate limpet Siphonaria oculus Kr., were made during summer and winter. The characteristics of the impedance traces are described. With the exception of occasional temperature-independent bradycardia ( p > 0.05), but were different at a low level of significance ( p = 0.0438) in winter. Heart rate was significantly lower at night than during the day in both summer and winter ( p p > 0.05 in both cases), which suggests the absence of seasonal metabolic compensation. Non-acclimation would reduce metabolic cost in winter, and, in combination with previously recorded facultative metabolic rate depression in S. oculus , may represent an energetically conservative adaptation for life on the upper shore.

Octopus vulgaris in South Africa preys primarily on mussels. Small mussels are simply pulled open, while large ones are drilled first and then pulled open. Medium mussels fall into an overlap size range and may be opened by either method. Actively pulling a mussel open was estimated to have an energetic cost 4.5 times that of resting metabolism, but active pulling lasts for only seconds and forms a very small proportion of handling time. The energetic cost per minute of handling prey while using force is only 1.29 times greater than when they are drilled. Curves of rate of energetic return ( E / H ) against mussel size were derived from laboratory measurements. Curves varied for individual octopuses and the technique used to open mussels. They were monotonie with sharp discontinuities between the two techniques in the overlap size range. Drilling takes much longer than using force alone and produces lower E / H values. Mussels just smaller than the length at which 50% are drilled (L drill 50) are “optimal” in terms of maximum E / H values. L drill 50 increases with octopus weight. In the laboratory octopuses selectively took mussels with maximum E / H . The force required to remove mussels from rocks in the laboratoryis less than that needed to open them, but increases with mussel size. Previously published data on prey selection in the field indicate that octopuses of > 500 g take the largest mussel size classes. Below 500 g, mean prey size in the field is related to octopus weight. Small octopuses may take smaller mussels because of they are unable to remove large ones from the rocks.

Two abundant South African mussels show intrashore habitat segregation and marked differences in their responses to physiological stress. Choromytilus meridionalis (Kr.) occurs mainly low on the shore and on rocks associated with sand, while Perna perna L. occurs higher on the shore on rocks which arc not usually influenced by sand. The sand tolerant C. meridionaus withstands prolonged exposure to hypoxia ( 2 : LT 50 > 30 days) and shows significantly better survivorship in air (LT 50 ≈ 7 days) than the sand intolerant species P. perna . with LT 50 values of ≈6.5 days and ≈ 1.5 days for hypoxia and air exposure, respectively. C. meridionalis appears to activate anaerobic pathways more readily than P. perna when exposed to air and hypoxia. In cither medium it shows a much stronger tendency to close the shell valves and a more rapid induction of reduced heart rate (bradycardia). Also, some individuals of P. perna showed an absence of oxygen debt when returned to seawatcr after exposure to air. With declining oxygen tension C. meridionalis regulates oxygen uptake down to a lower level (2 ppm O 2 ) than P. perna (3 ppm O 2 ). Mortality of P. perna in hypoxic seawatcr is suggested as being linked to a decline in cellular energy level, while in air it is more likely to he caused by an acid-base disturbance. The above behavioural and physiological responses can explain the exclusion of P. perna from sand-influenced rocks on the lower shore, where oxygen availability is likely to he reduced. However, the greater tolerance of air exposure by C. meridionalis docs not correlate with its exclusion from the higher regions of the shore where P. perna lives.

Littorina africana knysnaensis (Philippi) exhibits a size gradient of increasing shell length up the shore on the Cape Peninsula caused by separate size dependent effects on juveniles and adults. There is no apparent correlation between mean shell length and density, and animals of separate size classes which are transferred to zones where they do not normally occur rapidly migrate to their original zones. Juvenile settlement occurs almost exclusively on the upper shore to avoid the heavy wave action of the lower Littorina zone. If tethered in the lower zones juveniles show a loss of body weight or may even be torn free of their tethers and lost. Juveniles do, however, occur in the lower zones where shelter from the effects of wave action is available in the form of narrow crevices. Adult animals have a much greater tenacity and when tethered, they develop a lower length/body weight ratio in the lower Littorina zone than in the upper where microphytic productivity decreases considerably. The greater tenacity of adults allows a gradual downshore migration of growing animals in response to gradients of food availability. The resulting size gradient of L.a. knysnaensis is the reverse of that predicted for such a supralittoral fringe species.

Littorina africana knysnaensis (Philippi) increased markedly in mean shell length up the shore with preferential settlement in the higher zones. Density was greatest at the top of the shore (around 348 animals · m−2) but there was no apparent relationship between mean size or biomass · m−2 and density. Reproduction appears to involve pelagic larvae and recruitment is continuous throughout the year. Settlement success, however, was extremely variable and was probably influenced by offshore conditions. The total population declined slowly from November 1976 to March 1977 due to low rates of settlement and high mortality. From May 1977 to January 1978 recruitment rates were higher and mortality decreased resulting in a 231% rise in population over 8 months. Peak periods of settlement were in winter (May–June) and Summer (December–January). Juvenile cohorts appear at 2–3 mm and attain sexual maturity at ≈5 mm, growing to 6–7 mm in their first year. Maximum size of ≈ 10 mm is attained after 3 yr. Growth for animals in all zones was minimal in winter. Growth was maximal in spring for large animals (lowest zone), in early summer for medium animals (middle zone) and in late summer for small animals (top zone). This pattern in growth is thought to reflect the seasonal variability of the microphytic standing crop in the different zones.

Nucella dubia (Kr.) occurs in the middle and upper balanoid zone of rocky shores in the Cape of Good Hope. Ten prey species, all cirripedes or molluscs, were recorded under field conditions, diet being partially size dependent. Gastropods and barnacles were usually attacked through the opercular opening and rarely drilled. At Dalebrook the main prey were Tetraclita serrata Darwin in the mid-balanoid zone and Littorina africana knysnaensis (Phillipi) on the upper shore. Predator-prey relationships and the impact of predation contrasted strongly for these two prey species. Predation in the Littorina zone was cyclical. Nucella dubia migrated upshore from the upper balanoid as spring tides approached and downshore during neap tides. Both the number of whelks present and the percentage of them feeding peaked 1–2 days before spring tides. Whelk and Littorina size were positively correlated in predator-prey couples. Despite high rates of consumption (mean of 0.47 Littorina · whelk −1 ·day −1 over a 2-wk cycle) only ≈12% of the Littorina population are lost to predation per year. Caging experiments in the balanoid zone revealed consistently low predation rates (0.02 barnacles ·whelk −1 ·day −1 ) for both large and small whelks. Due to high whelk density (86·m −2 ) potential predation was estimated at 45.66% of the barnacle standing crop per year. This may be adequate to control barnacle density and maintain free space in barnacle beds. This study thus indicates that predation by Nucella dubia on sessile prey such as Tetraclita is greater than on the mobile Littorina and supports the suggestion of others that predators have a greater influence on sessile than mobile intertidal invertebrates.