Donax Serra Research Articles

Pleistocene and Holocene procurement of the clam Donax serra (Bivalvia: Donacidae) along the southern Cape coast of south Africa

Humans began to exploit aquatic resources routinely since the late Middle Pleistocene (c. 162 ka), as evidenced by PP13B Cave in the southern Cape of South Africa. Hard bottom or rocky shore species were among the first to be collected, and although they dominate Pleistocene and Holocene archaeomalacological assemblages locally, the soft-bottom Donax serra clam began to be collected systematically near PP13B by 110 ka. Since then, D. serra appears intermittently in early and Holocene middens. Earlier studies on PP13B D. serra assemblages dated to 110–91 ka concluded that initial collection of D. serra ~ 110 ka was unselective in terms of shell size and was undertaken along the entire tidal gradient. In later visits ~ 91 ka, foragers narrowed their collection to the mid-intertidal where most larger clams live, thus increasing the efficiency of D. serra procurement. In this paper, further metrical data from other Pleistocene and late Holocene assemblages is presented in order to test these conclusions. Pleistocene data from mainly the Klasies River main site also reflect small sizes as in contemporary levels from PP13B; however, it is not possible to generalise that the increase in foraging efficiency observed in PP13B also applies to post 90 ka contexts overall due to a lack of Pleistocene samples dating to this latter period. However, it is clear that foraging efficiency was comparatively higher in the Plettenberg Bay and Mossel Bay areas since the early Holocene. Variability among Holocene D. serra assemblages appear to be explained also in terms of foraging ranges. Settlement patterns, group size and/or its composition may be factors behind the generally intermittent presence of D. serra in the southern Cape middens.

CASA in invertebrates.

Sperm movement has been described in several phyla of invertebrates. Yet, sperm motility has only been quantified using computer-aided sperm analysis (CASA-Mot) in externally fertilising species (broadcast spawners) of two phyla, molluscs and echinoderms. In the present study we quantified in detail the nature of the sperm tracks, percentage motility groupings and detailed kinematics of rapid-, medium- and slow-swimming spermatozoa in the oyster Crassostrea gigas and four species never previously studied by CASA-Mot, namely the molluscs Choromytilus meridionalis, Donax serra and Haliotis midae and the echinoderm Parechinus angulosus. A feature common to all these species are the helical tracks, the diameter of which seems to be species specific. Using CASA-Mot, the behaviour of spermatozoa was also studied over time and in the presence of egg water and Ca2+ modulators such as caffeine and procaine hydrochloride. For the first time, we show that hyperactivation can be induced in all species in the presence of egg water (sea water that was mixed with mature eggs and then centrifuged) and/or caffeine, and these hyperactivated sperm tracks were characterised using CASA-Mot. We relate the different patterns of sperm motility and behaviour to reproductive strategies such as broadcast spawning and spermcasting, and briefly review studies using CASA-Mot on other invertebrates.

Demographic history, marker variability and genetic differentiation in sandy beach fauna: What is the meaning of low FST's?

This note demonstrates the effect of locus variability and demographic history on the estimation of genetic differentiation and its interpretation in terms of ecological connectivity. Cytochrome c oxidase subunit I (COI, mtDNA) sequences of the beach clam Donax serra from four sites along the South African coast were analysed. D. serra showed low COI haplotype diversity (h = 0.30 ± 0.069, 7 haplotypes), suggesting expansion from a small source population into extant habitats. As a consequence, statistical power to measure genetic connectivity was low. The lack of genetic population structure therefore does not necessarily demonstrate high connectivity. Although COI has been used successfully to identify species and populations isolated for prolonged periods of time, lack of differentiation has to be interpreted with caution, especially in terms of extant patterns of connectivity.

Changing collecting strategies of the clam Donax serra Röding (Bivalvia: Donacidae) during the Pleistocene at Pinnacle Point, South Africa

South Africa's Middle Stone Age (MSA) coastal sites have played a prominent role in documenting early evidence of systematic shellfish collection and adaptation to aquatic environments in the context of anatomically modern humans. Pinnacle Point 13B cave is important among these MSA sites not only because it holds the earliest yet known evidence for human use of marine resources (∼162 ka [thousands of years ago]), but because shellfish observations have been integrated more fully into discussions of MSA adaptations. This is particularly the case of Donax serra procurement on sandy beaches, where skills that are usually indicative of an aspect of behavioural modernity (in the context of hunting) were apparently used. In this paper, D. serra from 110 to 91 ka old assemblages are studied in detail by way of metrical analyses and relevant biological and ecological literature of this species. Existing seasonality studies derived from oxygen isotope analyses on the same molluscs are incorporated into this reconstruction. Shellfish appear to have been collected in winter over many millennia when D. serra are most nutritious due to high gonad content. A dramatic change in collection strategies took place during the same millennia-long period. Earliest systematic collection of D. serra consisted of mostly unselective procurement of animals in terms of shell size along the tidal gradient and beach depth. In later visits, people collected mostly larger individuals by narrowing their collection to the mid-intertidal. This change increased the efficiency of D. serra collection, which reflects a positive adaptive behaviour that endured into Later Stone Age (LSA) times.

Holocene geoarchaeology of the Sixteen Mile Beach barrier dunes in the Western Cape, South Africa

Holocene evolution and human occupation of the Sixteen Mile Beach barrier dunes on the southwest coast of South Africa between Yzerfontein and Saldanha Bay are inferred from the radiocarbon ages of calcareous dune sand, limpet shell ( Patella spp.) manuports and gull-dropped white mussel shells ( Donax serra). A series of coast-parallel dunes have prograded seaward in response to an overall marine regression since the mid-Holocene with dated shell from relict foredunes indicating periods of shoreline progradation that correspond to drops in sea level at around 5900, 4500 and 2400 calibrated years before the present (cal yr B.P.). However, the active foredune, extensively covered by a layer of gull-dropped shell, has migrated 500 m inland by the recycling of eroded dune sand in response to an approximate 1 m sea level rise over the last 700 yr. Manuported limpet shells from relict blowouts on landward vegetated dunes indicate human occupation of coastal dune sites at 6200 and 6000 cal yr B.P. and help to fill the mid-Holocene gap in the regional archaeological record. Coastal midden shells associated with small hearth sites exposed in blowouts on the active foredune are contemporaneous (1600–500 cal yr B.P.) with large midden sites on the western margin of Langebaan Lagoon and suggest an increase in marine resource utilisation associated with the arrival of pastoralism in the Western Cape.

Population structure, growth and production of the surf clam Donax serra (Bivalvia, Donacidae) on two Namibian sandy beaches

Population structure, growth and production of the surf clam Donax serra (Bivalvia, Donacidae), inhabiting highly exposed sandy beaches of Namibia, were investigated between November 1997 and December 1999. From length–frequency distribution and tagging–recapture data, a von Bertalanffy growth function with an asymptotic length ( L ∞ ) of 82 mm and a growth constant ( K) of 0.274 yr −1 was established. Regarding growth performance of Donacidae, D. serra fits in a group of species inhabiting cold temperate and upwelling regions. The intertidal biomass of the studied population ranged between 141 and 546 g ash-free dry mass (AFDM) m −2 yr −1. Individual production was maximal at 56.5 mm shell length (0.83 g AFDM ind. −1 yr −1), and annual production ranged between 167 and 637 g AFDM m −2 yr −1, resulting in productivity values (P/ B ̄ ) between 1.167 and 1.589 yr −1. These data underline the importance of D. serra for the beach/surf ecosystem. Further, the findings of this study are crucial to support future aquaculture or exploitation activities and management.

A Social Cost-benefit Analysis of a Small-scale Clam Fishery in the Eastern Cape, South Africa

When a proposal was advanced in 1991 to harvest the wedge clam Donax serra in the Eastern Cape, South Africa, for commercial gain, it elicited a huge public outcry. In order to shed light on the issue, a social cost-benefit analysis (CBA), based on a biologically sustainable extraction rate of 100 ton of clams per year, was carried out on its commercial exploitation. Because fishery activities might exact a price on society in the form of negatively altering the quality of people's recreation experience in the area, as well as damage the aesthetic and ecological attributes of the beach system, such external effects were identified and valuated. Results from the CBA yielded positive net present values (NPV s) for project options involving live clams being sold on the export market (to Hong Kong) for R24.70 kg−1 and negative NPV s for domestic market options (Johannesburg and Cape Town), where the clam would be sold for R6.50 kg−1. Assuming a discount rate of 8%, the highest NPV was obtained for the scenario in which total production was sold on the foreign market and the firm rented facilities of an existing organization. The external cost was the single largest component of the cost of the fishery. The conclusion drawn is that the social benefit of the project exceeds the social cost – a conclusion which supports commercial exploitation of clam stocks along the St Francis Bay beach, but one which does not include the costs associated with policing.

Genetic and morphological variation in four populations of the surf clam Donax serra (Röding) from southern African sandy beaches

The surf clam Donax serra (Bivalvia, Donacidae) dominates sandy-beach communities of two southern African biogeographical regions, a cold (Benguela current) and warm province (Agulhas current). Morphometric and behavioural differences led to a controversial discussion of whether or not populations from the two provinces belong to the same species. Shell size measurements confirmed morphological differences: clams from the cold province were significantly rounder, flatter and less wedge-shaped than clams from the warm province. In this study a genetic approach was used to relate phenotypic differences to genetic variability of four populations of D. serra separated by up to 2500 km of shoreline. Genetic analysis of 22 protein-coding loci was carried out by starch-gel electrophoresis. Populations studied are conspecific (genetic distances range from 0.003 to 0.044) and possess genetic variation (alleles per locus: 1.73–1.91; mean heterozygosity: 18–22%; percentage polymorphism: 45.5–59.1%) in the range of most other marine bivalves, which allows for potential adaptation to environmental changes. Wright’s fixation indices show little to moderate genetic divergence among the subpopulations relative to the limiting amount under complete fixation ( F ST=0.016–0.089), moderate divergence of individuals relative to the total population ( F IS=0.265–0.452), and comparably high divergence of individuals relative to the compound population ( F IT=0.300–0.473). The effective number of individuals exchanged between populations in each generation is high enough (1.44–8.65) to counteract genetic drift. We propose that the observed differences represent phenotypic plasticity enabling this species to inhabit different biogeographic regions. Gene flow, balanced selective pressure and evolutionary inertia are discussed as explanations for similarities of the two outlying populations. The substantial subdivision of the two Namibian populations indicates a potential biotic barrier and requires separate studies of the population dynamics.

Investigating biotic and abiotic factors affecting the recruitment of an intertidal clam on an exposed sandy beach using a generalized additive model

The existence of biotic interactions among macroinfauna resident on exposed sandy beaches and the potential roles that such interactions might play with respect to structuring these assemblages is currently a controversial topic. To elucidate the relative importance of biotic versus abiotic factors in determining the spatial distribution of beach fauna, we use a generalized additive model, which can account for nonlinear relationships between beach clam recruitment and four explanatory variables: two abiotic (beach gradient and distance from a major river mouth); and two biotic (the abundance of adult clams and the abundance of their juveniles). The study organism, Donax serra Röding, is a fairly typical intertidal filter feeder that dominates the macrofaunal communities on many southern African sandy beaches. Results indicate that a mixture of biotic and abiotic factors mediates recruit abundance, with beach gradient being the most influential, followed by adult and juvenile abundances. Of particular importance was strong evidence of intraspecific interactions between D. serra adults and recruits. Furthermore, despite the existence of a published hypothesis stating that the highest recruitment of this species is found beside river mouths and that the subsequent alongshore distribution of older age classes is determined primarily by distance from a major river mouth, we found the opposite. Lowest recruit abundance was in the vicinity of a river mouth and maximum recruit abundance was near the center of the beach. Distance from the river mouth was also less important in structuring the distribution of recruits than the biotic factors. This emphasizes the complex ecology of organisms inhabiting apparently simple intertidal landscapes, and illustrates how the generalized additive modelling approach can be fruitfully applied to problems in beach ecology.

Survivorship of juvenile surf clams Donax serra (Bivalvia, Donacidae) exposed to severe hypoxia and hydrogen sulphide

Toxic “sulphide eruptions” sporadically occur in the highly productive inshore regions of the central Namibian Benguela upwelling system. The surf clam Donax serra (Röding, 1798) dominates the intertidal and upper subtidal of large exposed sandy beaches of southern Africa and its recruitment seems to be affected by sulphide events. The reaction of juvenile surf clams to low oxygen concentrations and sulphide occurrence (0.1 mmol l −1) was examined by in vitro exposure experiments in a gas-tight continuous flow system. After 2 h of hypoxic- and hypoxic/sulphidic conditions, clams moved to the sediment surface, aiding their passive transport to areas with more favourable conditions. The clams showed a high sulphide detoxification capacity by oxidising the penetrating hydrogen sulphide to non-toxic thiosulphate. Moreover, juvenile D. serra switched to anaerobic energy production, indicated by the significant accumulation of succinate and, to some extent, alanine. Test animals were not able to reduce their energy requirements enough to withstand long periods of exposure, leading to a median survival time (LT 50) of 80 h under hypoxic sulphide incubation. In conclusion, natural “sulphide eruptions”, especially those with a large spatial and temporal extension, have to be considered as an important factor for D. serra recruitment failures. Hydrogen sulphide is assumed to be a potential community structuring factor.

The effect of grain size on the burrowing of two Donax species

Donax serra and Donax sordidus are bivalves inhabiting the intertidal area of sandy beaches in the Eastern Cape, South Africa. This study investigated the effect of grain size on the burrowing performance of these two species in the laboratory to elucidate the influence of grain size on the longshore distribution of Donax populations and, consequently, the importance of particle diameter in swash exclusion. Burial time, burrowing cycles and a burrowing rate index were obtained for both species in nine well-sorted sediments with grain sizes ranging 90–2000 μm. The burial times of both D. serra and D. sordidus were positively correlated with shell length. Burrowing performance of both species was influenced by grain size. Fastest burrowing times were measured in the fine and medium sediments (125–500 μm) but increased towards the very fine (90–125 μm) and coarse extremes (500–2000 μm). Burial time experiments were extended to test how D. serra would behave in sediments with more natural properties and were, therefore, repeated in mixed sands of different sortings. Sediment sorting had a positive influence on the burrowing times of D. serra. Burrowing time was fastest in well-sorted sediments and slowest in moderately sorted sediments, especially those containing fractions of sands >500 μm. Burial times approximating the swash periods on dissipative beaches (∼32 s) were measured for grain sizes <500 μm. It is, therefore, predicted that most D. serra individuals are able to burrow between swashes on dissipative beaches. However, only the smallest individuals (∼15 mm) would be able to burrow between swashes on reflective beaches with swash periods approximating 15 s. The smaller (maximum) size of D. sordidus reduces burial time and should, therefore, enable this slower burrowing species to bury successfully under most dissipative/intermediate conditions.

Reproduction and recruitment patterns of the surf clam Donax serra (Bivalvia, Donacidae) on two Namibian sandy beaches

Reproduction and recruitment of the surf clam Donax serra on two Namibian beaches were studied over a period of two years. Histological examination of the gonads indicated a discontinous annual reproductive cycle, related to monthly mean sea surface temperature. The spawning season lasted from August/September to February, but juveniles (2–6 mm anterior-posterior shell length) were only present for three months in the intertidal zone. The condition index indicates that the species spawns during autumn and summer, but histological validation is needed. The period when juveniles are abundant is decoupled from the spawning period and therefore cannot be predicted clearly, even if the spawning period is known. Starvation, hydrodynamic processes, chemical parameters and different release times during the spawning period are thought to cause the differences in settlement time and recruitment strength between locations. Keywords: condition index, Donax serra , histology, Namibia, recruitment, reproduction, sandy beach ecology African Journal of Marine Science 2001, 23: 53–60

Lessons from a Disturbance Experiment in the Intertidal Zone of an Exposed Sandy Beach

Exposed sandy beaches are important, sensitive and widespread coastal habitats. Although they have been studied for more than 50 years, investigators have been reluctant to attempt manipulative experiments due to the dynamic nature of these environments. Consequently, the ecology of exposed sandy beaches remains relatively poorly understood. We conducted a community-level, manipulative experiment involving a simulated anthropogenic disturbance on an exposed microtidal sandy beach in the Eastern Cape, South Africa; the first of its kind and scale. This study comprised pre- and post-impact sampling at an experimental site and two control sites. The impact involved excavating and removing a 200m2quadrat of sand from the mid-intertidal of the experimental site to a depth of 0·3m. The intention was to address the prediction that anthropogenic disturbances would be detectable if appropriate spatial and temporal scales were investigated. The following variables were monitored: transect gradient; species richness; macrofaunal abundance; and both the abundance and biomass of the dominant infaunal species, the beach clam Donax serra Röding. Analyses revealed significant differences in temporal patterns of all response variables amongst sites. Some evidence linked these changes to the experimental disturbance, although impacts appear temporary, being ameliorated within, at most, one semi-lunar cycle. This confirms that it is possible to successfully conduct manipulative experiments on exposed sandy beaches. However, the uncontrollable, natural dynamics of the beach face, as expressed by intertidal gradient, contributed significantly to the description of spatio-temporal variation in biotic response variables. It is concluded that to isolate treatment effects from those of natural variation, two advances are necessary on the current research approach. First, experimental designs must take cognizance of the fact that exposed, microtidal sandy beaches have little in common with other intertidal habitats; and second, large-scale treatments must be replicated in space.

An assessment of longshore movement in Donax serra Röding (Bivalvia: Donacidae) on an exposed sandy beach

Peak abundances of intertidal populations of a donacid bivalve, Donax serra, occur 10–23 km from major river mouths along exposed sandy beaches of log-spiral bays on the coast of southeast Africa. Postsettlement eastward movement of D. serra from areas of spat settlement by prevailing longshore currents has been suggested as the mechanism which results in the observed population distributions. To estimate the net rates of longshore movement in intertidal D. serra on one of these exposed beaches, we developed and used a novel technique. A metal detector was used to track net change in the positions of intertidal juvenile and adult clams (37–67 mm shell length, 1–3+ years) with aluminum tags glued to the shells over a 3-month period during the austral fall and winter. Estimated net longshore movement of tagged clams ranged from 0.19 to 0.80 m day −1. The net movement of tagged clams was primarily to the east along the beach but the direction of longshore movement varied during the study. Net rates of longshore movement were not correlated with clam size. The burrowing rates and the condition of recaptured tagged individuals were not significantly different from untagged clams after 2 months. The results of this study suggest that average longshore movement rates of intertidal clams, >1 year of age, along the beach are low and the animals are relatively sedentary. The technique developed and used here to estimate net longshore movement rates in D. serra allowed us to successfully recapture and track tagged individuals with very minimal disturbance to the intertidal habitat and associated fauna. Use of this novel technique could potentially enhance investigations of postsettlement movement, growth, zonation, competitive interactions and other aspects of the ecology of mobile soft sediment macrofauna.

Effect of Feed Type and Age-at-weaning on Growth and Survival of Clownfish Amphiprion percula (Pomacentridae)

To simplify the weaning of clownfish Amphiprion percula from live food to artificial food, and to reduce the costs of feeding live food, three trials were undertaken to determine the best age for weaning. Twenty-nine-day-old post-hatch A. percula were weaned onto a fishmeal/caesin-based dry food which was an acceptable substitute for either Artemia or a moist food comprising Penaeus indicus and Donax serra. Larvae weaned 4 days after hatch (DAH) had lower survival than fish weaned 7 or 10 DAH. In juveniles weaned from 10 to 30 DAH good survival was obtained between 15 and 20 DAH suggesting that this is a suitable window for the weaning of this species.

MICROEVOLUTION AND PHENOTYPIC PLASTICITY IN DONAX SERRA RÖDING (BIVALVIA: DONACIDAE) ON HIGH ENERGY SANDY BEACHES

Morphological differences between populations of the wedge clam Donax serra inhabiting two different coasts and biogeographic regions of South Africa were investigated. Both adults and juveniles occupied different positions along the beach gradient depending on the coast: on the southeast coast adults occurred in the mid intertidal and juveniles and recruits were low intertidal to subtidal; on the west coast the zonation pattern was reversed. Not only adults but also juvenile clams had shapes differing significantly between the two coasts; west coast clams were thinner, rounder and had a higher body density than the southeast ones-recruits were less dense in the former coast. Differences in shell shape between coasts are probably the result of directional selection on the adults with the microevolutionary changes being maintained by geographical isolation. Shell density, on the other hand, seems to be environmentally determined through physiological control of shell calcification, i.e. more mobile intertidal clams having lower shell density than less mobile subtidal clams. Ontogenetic changes in shape and density are presumably adaptive and appear to be related to mobility, i.e. the larger, heavier and denser adults being more stable in the substrate, and the smaller, thinner and less dense juveniles being more mobile and dispersive. Phenotypic plasticity in present D. serra populations is an important factor that enabled this species to occupy different habitats and biogeographic regions and to survive 5 million years of environmental changes.

Carbon and nitrogen exchange between sandy beach clams ( Donax serra ) and kelp beds in the Benguela coastal upwelling region

Carbon consumption and nitrogen requirements were estimated for populations of the sandy beach bivalve Donax serra on nine beaches of the west coast of South Africa. Subtidal populations composed mainly of adult clams were responsible for the bulk of standing stock (3538 g C m−1), annual carbon consumption (13 444 g C m−1 yr−1), faeces production (6478 g C m−1 yr−1 ) and nitrogen regeneration (2525 g N m−1 yr−1). Kelp detritus, bacteria and kelp consumers' faeces available in the water column surpass several times the carbon and nitrogen requirements of intertidal and subtidal clam populations. Individual Donax serra pop ulations, in turn, may regenerate up to 3.2% of the total nitrogen requirements of all primary producers from kelp beds and 14% of the requirements of phytoplankton. These high standing stocks of clams are presumably supported mainly by organic matter originating from kelp which, in contrast to phytoplankton, is in constant supply and comprises the largest proportion of the annual production of particulate organic matter on this coast. Wide and shallow continental shelves with gentle slopes probably limit the penetration of upwelled waters to the nearshore waters, decreasing the influence of external inputs and increasing the importance of internal flows of nutrients and carbon within the nearshore zone. In this context, sandy beaches, rocky shores and kelp beds may be more closely interlinked compartments of a larger ecosystem encompassing the whole nearshore than traditionally thought.

Disturbance effects of stranded kelp on populations of the sandy beach bivalve Donax serra (Röding)

Twelve beaches on the coast west of Cape Agulhas, South Africa, were surveyed to examine the influence of several physical factors, including stranded kelp, on the zonation, density and biomass of populations of the wedge clam Donax serra. Biomass and density of adult clams were significantly higher on beaches with lower cover of stranded kelp; this pattern did not hold for juveniles. Discriminant analysis of beaches with and without Donax classified these beaches 100% correctly — the most important factor being kelp cover followed by average winter temperature, wave height, morphodynamic state, beach length, wave period and beach slope. Beaches with Donax had a significantly lower percentage of the drift line covered by stranded kelp (mean=30%) than beaches without Donax (mean=77%). On two beaches of the southwest coast with similar morphodynamics and temperature regimes but different kelp cover, adult populations were centered in different zones: in the low intertidal to subtidal where kelp cover was higher (mean=42%) and in the mid-intertidal where no kelp was found. A disturbance hypothesis is developed to explain the observed patterns: kelp stranding in varying quantities interferes with Donax feeding and burrowing activities, dislodging animals of increasing size gradually downshore to the saturation and inner surf zones, where adult populations are eventually established. In the presence of extremely high quantities of stranded kelp, both juveniles and adults would be dislodged from the intertidal to the subtidal. The disturbance process may interact with several other physical factors, being magnified when coupled with low temperatures and small waves on reflective pocket beaches with steep slopes and coarse sands. Additionally, kelp gulls predate dislodged animals, explaining the absence of clams in a stretch of 20 m above the swash zone. It is thus concluded that the zonation patterns here observed are the result of a disturbance effect triggered by stranded kelp and mediated by multiple interacting factors. The adaptations of Donax serra that enable it successfully to colonize intertidal and subtidal habitats of the beach ecosystem are discussed.

Physical factors in benthic ecology:effects of changing sand particle size on beach fauna

This paper reports on the disposal of diamond mine tailings on a Namibian sandy beach. Coarse sand in the tailings greatly increases the grain slze of the affected parts of the beach and thereby provides the opportunity to assess the effects of changing sand grain size on a beach when other physical vanables are kept constant. Elizabeth Bay (Namibia) is 4 km long and was originally composed of fine sand which, exposed to moderate to heavy wave acbon, produced a log spiral bay with a disslpative beach. Tailings disposal in the centre of the bay has increased mean sand particle size from original values of 110 to 160 pm to present values of 500 to 800 pnl w t h a concomitant conversion of beach morphodynanlic state from dissipative to intermediate. Surveys of the 2 ends of the bay, which are relatively undffected by disposal, and of an undisturbed similar bay nearby revealed intertidal benthlc macrofauna communities with 15 to 20 species occurring in h ~ g h abundance (24 120 to 129276 m ' ] . In 3 transects in the affected area , species nchness was 8 to 12 per transect and abundance was 640 to 4710 m ' Beds of the large sand mussel Donax serra have d~sappea red from the affected sector of the bay and peracarids typical of finer sands have been replaced by a more robust species. Regression analysis revealed signlf~cant correlations between community parameters (species richness and abundance) and both beach slope and par t~cle size; ANOVA confirmed the significantly lower abundances of fauna in the affected areas Smothering effects appeared to be localised and 11mited. This study has supported the hypothesis that a n increase In sand particle size (on a beach where tide range and wave energy have remained constant) results in a change in beach state and a decrease In species richness and abundance

Organisms associated with the sandy-beach bivalveDonax serraRöding, with a description ofCercaria serraesp. nov. (Trematoda)

Examination of two collections of Donax serra from a South African west coast beach revealed the presence of Ciliophora, Trematoda, Nematoda and a parasitic pycnogonid. This is the first record of a pycnogonid from the genus Donax and the first published report of such a parasite from any southern African bivalve mollusc. One of the two trematode species found is described as new and its epidemiology is discussed briefly. No epibiota were found on the bivalve shell.