Adult Cockles Research Articles

Decadal population dynamics of an intertidal bivalve (Austrovenus stutchburyi) bed: pre- and post- a mass mortality event

ABSTRACT Three independent annual surveys with some spatial–temporal overlap were amalgamated to describe the population dynamics of the New Zealand intertidal cockle (Austrovenus stutchburyi) at Lews Bay, Whangateau Harbour from 1997–2018. After more than a decade of relative stability there was a dramatic decline in the biomass (from 7.4 to 1.8 kg wet weight m−2), density (1049 to 409 individuals m−2) and mean length (22.5 to 17.7 mm) of cockles following a mass mortality event in 2009. Large (≥30 mm) cockles were more severely impacted by the die-off, suffering mortality rates of more than 80%, compared with approximately 44–55% for cockles <30 mm. Since 2009 the density of A. stutchburyi at two survey sites has steadily increased, and at one of these sites has recovered close to pre-2009 densities. However, the density of large cockles has not increased since 2009, and the mean shell length has remained stable at 17.9 mm despite protection from harvesting. This hysteresis in the recovery of population size structure suggests exceptionally slow growth rates, high mortality of young adult cockles or a change in environmental conditions that no longer supports large individuals.

Overview and challenges of blood cockle culture in Malaysia

The blood cockle (Tegillarca granosa) is an important aquaculture species in Southeast Asia and plays a major role on economic importance in Malaysia since 1948. In comparing food value among other bivalves, cockles showed a cheap source of protein among Malaysians. Malaysia used to have a coastline which was suitable for breeding cockles, particularly, Selangor was enriched with a large number of cockles spat naturally developed from mudflats. Unfortunately, production of cockles spat in farming and marketing started to decline in 2000 and stopped production in year 2005 where there are only two main peaks for spawning season which are in March and November. Based on statistics from Department of Fisheries, unstable production of adult cockles occurred since 2011 and drastically declined in 2015, where the total yield for year 2015 was only 16,866.22 tonnes compared to 57,544.40 tonnes in 2011. With the decreased of cockle production, cockles are no longer considered as a cheap seafood resources but a delicacy marketed to the locals and tourists. There are several factors leading to the mortality of seed and adult cockles such as environmental issue, smuggling and overharvesting and low food availability in culture areas. The overview of the cockle status in Malaysia over 28 years, the challenges and the potential of aquaculture of cockles will be presented in this paper.

The interactive role of predation, competition and habitat conditions in structuring an intertidal bivalve population

Habitat characteristics, predation and competition are known to interactively drive population dynamics. Highly complex habitats, for example, may reduce predation and competition, allowing more individuals living together in a certain area. However, the strength and direction of such interactions can differ strongly and are context dependent. Furthermore, as habitat characteristics are rapidly changing due to anthropogenic impacts, it becomes increasingly important to understand such interactions. Here, we studied the interactive effects of predation and competition on common cockle (Cerastoderma edule) recruitment, growth and survival under different habitat characteristics in the Wadden Sea, one of the world's largest intertidal ecosystems. In a predator-exclosure experiment, we manipulated cockle densities (100 vs. 1000 individuals m-2) and shorebird predation at two sites differing in habitat characteristics, namely at the wake of a blue mussel bed (Mytilus edulis) and at an adjacent sandy site. We found that recruitment was higher in the mussel-modified habitat, most likely due to reduction of hydrodynamic stress. Although bird predation strongly reduced recruit density, the combined effects still yielded more recruitment at the vicinity of the mussel bed compared to the sandy area. Furthermore, we found that high cockle densities combined with high densities of other potential prey (i.e. mussels) at the mussel-modified site, mitigated predation effects for adult cockles. Apart from these positive effects on adults, mussel-modified habitat reduced cockle growth, most likely by reducing hydrodynamics in the wake of the mussel bed and by increasing inter-specific competition for food. Our study experimentally underpins the importance of habitat characteristics, competition and predation in interactively structuring intertidal communities.

Effects of cockle abundance and cockle fishery on bivalve recruitment

Understanding possible effects of cockle dredging on bivalve dynamics should include studies of the influence of cockle abundance on bivalve recruitment and of effects of fishery on survival of recruits present at the time of fishing and on future recruitments. Numerical densities of adult cockles and of recruits of Cerastoderma edule, Limecola balthica and Mya arenaria were monitored by twice annual sampling for 21 years (1973–1993) along 7 transects in the fished part of Balgzand, a 50-km2 tidal flat area in the westernmost part of the Wadden Sea. Between-year variability in recruitment success in all 3 species was negatively correlated with adult-cockle abundance. Recruit densities as estimated in August were relatively low in years of cockle fishing in all of these species. This was so already well before fishing started in September. So these low densities were not an effect of fishery, but of the high cockle abundance that made cockle dredging profitable. The proportions of recruit numbers estimated in August (just before cockle dredging) that were still present half a year after fishing (in March) were not different between fishing and non-fishing years nor between fished and unfished areas in fishing years. Densities of new recruits in years following cockle-dredging somewhere in the area were not different from densities in years not preceded by any fishery nor were they lower at the fished sites in the year after fishing than in other years. The lack of significant influences of fishery in the studied area and period was observed in all 3 bivalve species. The above results were obtained in areas with relatively muddy sediments. Earlier reported negative effects of cockle-dredging on bivalve recruitment may be limited to the minority of tidal flats characterized by relatively coarse sediments with <1% of silt.

The effect of macrofaunal disturbance on Cerastoderma edule post-larvae

Populations of the Common European cockle (Cerastoderma edule) often have highly patchy distributions and variable recruitment success. One of the proposed reasons is that high densities of filter feeders and/or bioturbators are thought to reduce the success of larval settlement and post-settlement survival, but the direct causal processes driving these observations are not clearly identified and validated. Through combined field and laboratory experiments, we test the hypothesis that macrofauna cause decreases in post-larval density through feeding and movement activities. The effect of excluding the bioturbating lugworm Arenicola marina and filter-feeding adult cockles on post-larval cockle densities was estimated in separate field experiments at two locations from the time of initial larval settlement in May 2012 to late summer August 2012. Lugworm exclusion led to a significant increase in cockle post-larval densities whereas the opposite was true for adult cockles, where exclusion led to a reduction in C. edule post-larval density. Although clear effects were observed in the field, experiments conducted in the laboratory failed to detect changes in mortality or byssus drifting of post-larvae as a consequence of macrofaunal activity. This study demonstrates that the presence of macrofauna can have both positive and negative effects on post-settlement density of C. edule post-larvae. Thus the density, distribution and identity of macrofauna have significant effects on the density and spatial distribution of C. edule post-larvae during the post-settlement period. These observations have implications for conservation and fishery management of this species.

Long-term epidemiological study of disseminated neoplasia of cockles in Galicia (NW Spain): temporal patterns at individual and population levels, influence of environmental and cockle-based factors and lethality.

The dynamics of disseminated neoplasia (DN) affecting cockles Cerastoderma edule (L.) in Galicia was addressed at individual and population levels. Early stage of DN was characterized by isolated neoplastic cells occurring in branchial vessels or in the connective tissue of gills, mantle, gonad or digestive gland. As disease progressed, the neoplastic cells appeared loose in foci and became widely distributed throughout the organs. In advanced stages, the connective tissue of most organs was infiltrated by neoplastic cells, which displaced normal cells, leading to the loss of the normal tissue/organ architecture. Host defence reaction was occasionally observed. A field survey performed for 7years, in two cockle beds located in different Galician Rías, showed that DN is a hyperendemic disease usually present all year-round at high prevalence in adult cockles but with annual prevalence minima in spring likely due to the death of heavily affected cockles, concurrently with gonad ripeness-spawning. DN was detected in the cockles ranging from 10 to 39mm in size; the highest DN prevalence and severity corresponded to the cockles of intermediate size/age (22-29mm/0.7-1year old). Sex did not appear to influence DN occurrence. An inhibitory effect of DN on cockle gametogenesis was detected.

Relative sensitivity of two marine bivalves for detection of genotoxic and cytotoxic effects: a field assessment in the Tamar Estuary, South West England

The input of anthropogenic contaminants to the aquatic environment is a major concern for scientists, regulators and the public. This is especially relevant in areas such as the Tamar valley in SW England, which has a legacy of contamination from industrial activity in the nineteenth and twentieth centuries. Following on from previous laboratory validation studies, this study aimed to assess the relationship between genotoxic and cytotoxic responses and heavy metal concentrations in two bivalve species sampled from locations along the Tamar estuary. Adult cockles, Cerastoderma edule, and blue mussels, Mytilus edulis, were sampled from five locations in the Tamar and one reference location on the south Devon coast. Bivalve haemocytes were processed for comet and neutral red retention (NRR) assays to determine potential genotoxic and cytotoxic effects, respectively. Sediment and soft tissue samples were analysed for metal content by inductively coupled plasma mass spectrometry. Sediment concentrations were consistent with the physico-chemical nature of the Tamar estuary. A significant correlation (P = 0.05) was found between total metal concentration in sediment and C. edule soft tissues, but no such correlation was found for M. edulis samples. DNA damage was elevated at the site with highest Cr concentrations for M. edulis and at the site with highest Ni and Pb concentrations for C. edule. Analysis of NRR revealed a slight increase in retention time at one site, in contrast to comet data. We conclude that the comet assay is a reliable indicator of genotoxic damage in the field for both M. edulis and C. edule and discuss reasons for the apparent discrepancy with NRR.

A review of the biology of European cockles (Cerastodermaspp.)

This review examines the biology of the two main cockle speciesCerastoderma eduleandC. glaucumfound in coastal areas around the north-east Atlantic from Norway to Morocco and through the Baltic, Mediterranean and Black Sea. It considers those factors in particular that impact on the overall health and survival of individuals as well as populations. Methods for the discrimination of the species are reviewed as well as the approaches being taken to delineate different populations, which is crucial to appropriately manage individual fisheries. Cockle populations generally undergo sexual maturation during their second summer and sexes are separate. Eggs are pelagic, with larvae being both benthic and pelagic before settling on the sediment and becoming benthic adults. However, data are lacking on basic larval biology and dispersal mechanisms. Data are provided on predator–prey relationships including information on types of food of importance to cockles. Main predators of cockles include brown shrimp, shore crabs, gastropods, polychaetes, fish and a variety of birds and these can be important in structuring cockle populations. Predation of larval cockles by adult cockles through larviphagy can lead to reductions of up to 40% of the population. Cockles are sensitive to a wide range of chemical contaminants but few data are published on impacts on cockles, in particular larval stages. The review concludes with an assessment of future climate change scenarios on cockles and considers some areas of future research required to preserve this ecologically and economically important species.

Is the mangrove cockle Anadara tuberculosa a candidate for effluent bioremediation? Energy budgets under combined conditions of temperature and salinity

To determine extreme and optimum temperature and salinity for growth in the mangrove cockle Anadara tuberculosa, we measured the scope for growth in adult cockles exposed to 16 treatments composed of combinations of four salinities (20, 30, 40, and 50 psu) and four water temperatures (23, 26, 29, and 32 °C). Higher clearance rates were recorded in cockles maintained at 30 and 40 psu and 32 °C. Consistently high clearance rates (0.9 to 1.14 l g –1 h –1) were recorded in cockles held at 40 psu, regardless of temperature. Absorption efficiency was higher at 20 psu and lowest at 40 and 50 psu. Absorbed energy increased with decreasing salinity and increasing temperature. Respiration energy was higher at low to moderate salinities (20 to 40 psu) and at intermediate temperatures (26 and 29 °C). The lowest respiration energy and excretion energy were recorded in cockles held at 50 psu, regardless of temperature. Excretion energy was inversely related to temperature and salinity, with high values in cockles held at low salinity (20 and 30 psu) and low temperature (23 and 26 °C). Scope for growth was directly related to temperature and inversely related to salinity. The highest scope for growth occurred in cockles held at 32 °C (120.4 to 327.3 J g –1 h –1) and the lowest in cockles held at 23 °C (4.3 to 84.1 J g –1 h –1). The mangrove cockle can tolerate a wide range of temperature and salinity and maintain positive scope for growth. This species could be grown under a wide variety of conditions, especially in tropical lagoons receiving fresh water runoff. However it can also be grown in challenging environments such as shrimp ponds effluents and take advance of the cockle tolerance to extreme conditions.

Effects of experimental small‐scale cockle (Cerastoderma eduleL.) fishing on ecosystem function

AbstractThe cockleCerastoderma eduleL. occurs at commercially exploited densities within both species‐poor and species‐rich assemblages, dominating the faunal biomass and potentially acting as a key contributor to a number of ecological functions. The questions considered were: Is the delivery of ecological functions affected by the removal ofC. edule? Does the biodiversity of the affected assemblage affect the extent to which it is altered? To explore these questions, adult cockles were removed from experimental plots at two shores in the North‐West of England: a species‐poor shore at Warton Sands, Morecambe Bay, and a more diverse shore at Thurstaston, Dee estuary. Impacts on the functioning of the systems were determined during a 4‐month study in summer 2007 by direct measurements of benthic primary production, organic matter content and sediment granulometry, and by indirect analyses using Biological Traits Analysis. Removal of adult cockles led to significant changes in faunal assemblage composition and in the distribution of biological traits, with increased biodiversity and an increased prevalence of traits relating to opportunistic taxa observed following the removal ofC. edule. These changes occurred at both study sites and hence were independent of the initial biodiversity of the assemblages. Sediment granulometry was significantly affected; however, surface chlorophyll and organic matter were not affected. This implies a minimal impact on these functions and a high degree of functional redundancy within the macrofauna. The implications of these findings and future areas of research are discussed.

Intrapopulational distribution of Meiogymnophallus minutus (Digenea, Gymnophallidae) infections in its first and second intermediate host

Host size and age are generally assumed to play a pivotal role in digenean trematode infection patterns, accounting for much of the variation found within intermediate host populations. However, knowledge is based on a limited number of studied host-parasite systems. We investigated the shell length class distribution of Meiogymnophallus minutus infections within populations of the first intermediate host Scrobicularia plana and second intermediate host Cerastoderma edule. Infections occurred very early in the life of the two intermediate hosts. Both prevalence and intensity of infections increased with host shell length and displayed extremely high values amongst large individuals. Whilst metacercarial infection patterns in juvenile C. edule could be best explained by differences in host shell length, in adult cockles, the effect of host age on infection levels prevailed. The microsporidian hyperparasite Unikaryon legeri, occurring in the metacercarial stage of M. minutus, was particularly abundant in aged cockles, strongly influencing infection patterns of the gymnophallid. Our results are consistent with the intrapopulational distribution reported from other digenean trematode parasites. The relative influence of both host size and age and the underlying mechanisms as well as the impact of hyperparasitism on M. minutus infection patterns are discussed.

Context-dependent effects of suspension feeding on intertidal ecosystem functioning

Species loss or gain can change ecosystem functions according to the ecological context (e.g. assemblage composition, habitat heterogeneity and disturbance regime). We document how the suspension-feeder cockle Cerastoderma edule (L.) affects some properties of intertidal sediments during macrofauna recovery after deliberately induced anoxia. Forty adult cockles were transplanted to each of 12 plots (25 × 25 cm) in 3 physically homogeneous patches (4 × 4 m). Two patches had recolonised for 4 or 7 mo (DJ and DM, respectively) and had a very similar number of species (DJ: 4.5 ± 0.3 and DM: 5 ± 0.0). The other patch (C) was natural sediment with a relatively high spe- cies diversity (6.5 ± 0.6). Species composition was different among the 3 patches (DJ < DM < C) and indicated an increasing number of individuals and functional groups in accordance with the duration of the recovery period (DJ < DM < C). Changes in abundance of the resident macrofauna, in total benthic mineralization rates (oxygen uptake and carbon dioxide release) and in the exchange of in- organic nitrogen at the sediment-water interface were compared between transplanted and undis- turbed plots. The functional response of the sediment ecosystem to the gain of C. edule varied among the patches. Following C. edule addition, ammonium release increased 3 to 4 times in the sediment which had been recolonising for 7 mo (DM), while uptake of nitrate decreased at similar levels in the other 2 patches (DJ and C). Effects were related to variations in the densities of 2 species, namely new recruits of the surface deposit-feeder Macoma balthica (≤5 mm) and the surface-detritivore and bio- irrigator Hediste diversicolor. Cockles caused a decrease in the densities of M. balthica (≤5 mm) and an increase in those of H. diversicolor, mainly in DM. Cockles also modified how densities of these species and of Heteromastus filiformis correlated to sediment-water exchanges of ammonium and oxygen. C. edule also enhanced oxygen uptake at all sites, mainly due to its specific respiration rate. Variability in the functional response among patches of habitat (e.g. context-dependence) resulted as a consequence of biotic interactions between the cockles and the resident infauna.

OPTIMIZING THE SUCCESS OF AUSTROVENUS STUTCHBURYI RESTORATION: PRELIMINARY INVESTIGATIONS IN A NEW ZEALAND ESTUARY

Degradation of coastal habitats can result in loss or depletion of valued vegetation, fish, and shellfish stocks. Consequently, there are many initiatives to protect and restore these areas or populations to their previous state. Here we describe a study designed to provide necessary information on dispersal, mortality and growth rates of transplanted adult (25–32-mm shell length) cockles, Austrovenus stutchburyi (Gray) with which to evaluate the density of transplants and need for predation protection that should be used in larger-scale projects. Marked Austrovenus were transplanted at high (75 ind) and low (20 ind) densities, at two sites on an intertidal sandflat. To distinguish between epibenthic predator-associated mortality, other mortality and natural dispersal, caged and uncaged plots (30 × 30 cm) were used. Around 30% of the adult Austrovenus transplanted remained in the plots 12 mo later, and abundances were enhanced relative to pretransplant ambient densities. Death rates increased in later months and were significantly higher for Austrovenus kept at high densities in cages. There was no effect of caging on plot sediment characteristics, no obvious targeting of the plots by predators and no size-dependency of the deaths. However, being caged at high densities appeared to act as a stressor that rendered Austrovenus less able to cope with the extreme environmental conditions that occurred in later months. Growth rates were low over the 12-mo trial (<0.2 cm mo−1), did not differ significantly with site, transplant density, or the presence/absence of cages. Results indicate adult Austrovenus should be transplanted at densities intermediate between those used here and that caging is unnecessary. Our observations of dispersal of cockles out of the uncaged plots suggest that future trials should investigate transplanting cockles in patches within larger areas than the plots used here.

Human trampling as short-term disturbance on intertidal mudflats: effects on macrofauna biodiversity and population dynamics of bivalves.

The effect of physical disturbance in the form of trampling on the benthic environment of an intertidal mudflat was investigated. Intense trampling was created as unintended side-effect by benthic ecologists during field experiments in spring and summer 2005, when a mid-shore area of 25 × 25 m was visited twice per month by on average five researchers for a period of 8 months. At the putatively-impacted location (I) (25 × 25 m) and two nearby control locations (Cs) (25 × 25 m each), three sites (4 × 4 m) were randomly selected and at each site, three plots (50 × 50 cm) were sampled after 18 and 40 days from the end of the disturbance. Multivariate and univariate asymmetrical analyses tested for changes in the macrofaunal assemblage, biomass of microphytobenthos and various sediment properties (grain-size, water content, NH4 and NO3 concentrations in the pore water) between the two control locations (Cs) and the putatively-impacted location (I). There were no detectable changes in the sediment properties and microphytobenthos biomass, but variability at small scale was observed. Microphytobenthos and NH4 were correlated at I to the number of footprints, as estimated by the percentage cover of physical depressions. This indicated that trampling could have an impact at small scales, but more investigation is needed. Trampling, instead, clearly modified the abundance and population dynamics of the clam Macoma balthica (L.) and the cockle Cerastoderma edule (L.). There was a negative impact on adults of both species, probably because footsteps directly killed or buried the animals, provoking asphyxia. Conversely, trampling indirectly enhanced recruitment rate of M. balthica, while small-sized C. edule did not react to the trampling. It was likely that small animals could recover more quickly because trampling occurred during the growing season and there was a continuous supply of larvae and juveniles. In addition, trampling might have weakened negative adult-juvenile interactions between adult cockles and juvenile M. balthica, thus facilitating the recruitment. Our findings indicated that human trampling is a relevant source of disturbance for the conservation and management of mudflats. During the growing season recovery can be fast, but in the long-term it might lead towards the dominance of M. balthica to the cost of C. edule, thereby affecting ecosystem functioning.

Ecophysiology in environmental impact assessment: implications of spatial differences in seasonal variability of bivalve condition

Nucleic acid ratios (RNA:DNA, RNA:protein or total RNA content) have been success- fully used as indirect measures of short-term growth in a range of marine organisms. However, using biochemical and physiological indicators in environmental impact assessment requires changes to be evaluated against a background of natural variability (e.g. spatial differences in the seasonal variabil- ity and size-dependence in bivalve condition). The natural temporal and spatial variability in nucleic acid ratios and condition indices in adult cockles Austrovenus stutchburyi was investigated along a gradient of increasing sediment grain size and wave exposure in an estuary on the east coast of New Zealand. The results were examined in relation to bivalve reproductive state, water temperature, and sediment and hydrodynamic characteristics of the estuary. The magnitude of seasonal variability in RNA was site-specific and increased towards more exposed sites, likely due to enhanced food avail- ability at these sites. In addition, investigation of the effect of bivalve size class on nucleic acid ratios and condition indices revealed site-specific patterns. Thus, by integrating information over a range of spatial and temporal scales, we can increase our understanding of the interaction between the phys- iology of estuarine soft-sediment bivalves and their spatially and temporally complex environment to provide a sound basis for assessing the effects of added environmental stressors.

Metazoan parasites in intertidal cockles Cerastoderma edule from the northern Wadden Sea

At four intertidal sites near the island of Sylt (eastern North Sea), 13 metazoan parasite taxa were found in 1400 cockles investigated, with digenean trematodes being dominant. Almost all cockles were infected by parasites and most individuals harboured more than one parasite species. We observed four conspicuous patterns: (1) Adult cockles harboured a two times higher species richness (2003: 6.1 ± 0.7 species/host; 2004: 7.1 ± 0.7) than juveniles (2003: 2.9 ± 0.8; 2004: 3.4 ± 0.8) and total parasite community composition significantly differed between age groups. (2) Infection levels were 2–52 times higher in adult cockles than in juveniles both in trematode species and in non-trematode species. In the dominant trematodes, species utilising cockles as first intermediate host ( Gymnophallus choledochus, Labratrema minimus, Monorchis parvus) only occurred in adult cockles, and prevalences were low (2–12%). Prevalences of up to 100% were reached by trematodes using cockles as second intermediate host ( Himasthla elongata, H. continua, H. interrupta, Renicola roscovita, Psilostomum brevicolle, Meiogymnophallus minutus, Gymnophallus gibberosus). Metacercariae of these species were segregated between body parts within cockles. (3) High spatial heterogeneity in parasite community composition and infection levels occurred between sampling sites. However, communities in juveniles were more similar than communities in adults. (4) Temporal variation in parasite community composition was low between two consecutive years, especially for adult cockles. The omnipresence of parasites in this dominant bivalve species has important implications for sampling designs and as a potentially confounding variable in e.g. physiological studies. It suggests strong and cumulative negative effects on the cockle hosts.

Annual cockle Cerastoderma edule production in the Wadden Sea usually fails to sustain both wintering birds and a commercial fishery

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 309:189-204 (2006) - doi:10.3354/meps309189 Annual cockle Cerastoderma edule production in the Wadden Sea usually fails to sustain both wintering birds and a commercial fishery J. J. Beukema*, R. Dekker Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands *Email: janb@nioz.nl ABSTRACT: Low cockle Cerastoderma edule stocks in the Dutch Wadden Sea over the last ~15 yr have caused ecological problems, including declines in numbers of some specialized bird species and a sharpening conflict between nature conservationists and fishermen. To study why cockle production and resulting biomass of adult cockles failed, we analyzed long-term (1973 to 2003) data on annual and cohort production of cockles in the Balgzand tidal flat area in the westernmost part of the Wadden Sea. Somatic production was estimated from summed growth increments per half-year period and expressed in ash-free dry weight (AFDW). In adults, positive values in April to August regularly alternated with negative values in September to March, when up to 60% of individual weight gains in the preceding season were lost. Life-time production averaged 0.06 g AFDW for ~3 mo old recruits and 0.21 g for ~10 mo old recruits. The numerical recruit density of a cohort as observed after their first winter predicted subsequent life-time cohort production within narrow limits. The 31 yr mean of net somatic production amounted to 7.3 g AFDW m–2 yr–1. The period of largest production during the lifetime of a cohort occurred during their second growing season (when cockles grow from spat to adult size). Years with high production values occurred 1 yr after a highly successful reproductive season, and high biomasses were achieved 1 to 3 yr after a highly successful season. In contrast, low values were observed when a successful recruitment had been >3 yr in the past, and also after severe winters (causing high cockle mortality). Local variation in mean annual production was primarily related to local variation in recruitment success; however, production per recruit was significantly higher at low than at high intertidal levels. The main cause of low biomass values observed in recent years was failing recruitment in off-shore areas. As a consequence, in nearly all of the last 15 years, cockles have been too scarce in the western Wadden Sea to sustain both wintering birds and a commercial fishery. KEY WORDS: Long-term data · Seasonal growth · Annual recruitment · Secondary production · Tidal flats Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 309. Online publication date: March 15, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Geochemical survey and metal bioaccumulation of three bivalve species ( Crassostrea gigas, Cerastoderma edule and Ruditapes philippinarum) in the Nord Médoc salt marshes (Gironde estuary, France)

A 15-month experiment combining a geochemical survey of Cd, Cu, Zn and Hg with a bioaccumulation study for three filter-feeding bivalve species (oysters, Crassostrea gigas; cockles, Cerastoderma edule; and clams, Ruditapes philippinarum) was conducted in a breeding basin of the Nord Médoc salt marshes connected to the Gironde estuary, which is affected by historic polymetallic pollution. Regular manual surface measurements of temperature, salinity, pH and dissolved O 2 concentration and hourly multiprobe in situ measurements throughout several periods for 6–8 weeks were performed. The geochemical behavior of metals in water, suspended particulate matter and sediment and their ecotoxicological impact on the three bivalve species were evaluated by in situ exposure of juvenile oysters (water column) and adult cockles and clams (sediment surface). The physico-chemical parameters reflected seasonal variations and basin management. A distinct daily periodicity (except salinity) indicated intense photosynthesis and respiration. In summer, low dissolved O 2 saturations (∼40–50%) occurred in the early morning at 30 cm above the sediment, whereas in depressions, the water column near the sediment surface was suboxic. Cadmium, Zn and Cu concentrations in suspended particulate matter exceeded typical estuarine values and were much higher than the homogeneously distributed concentrations in different depth ranges of the basin sediment. Particles collected in sediment traps showed intermediate metal concentrations close to sediment values. These results suggest trace metal recycling due to reductive dissolution under suboxic conditions at the sediment surface resulting in trace metal release to the water column and adsorption onto suspended particles. Dissolved Cd, Zn and Hg concentrations (e.g. 13–136 ng l −1; 0.3–25.1 μg l −1 and 0.5–2.0 ng l −1, respectively) in the basin corresponded to the concentration range typically observed in the Gironde estuary, except for some maximum values attributed to metal recycling. In contrast, dissolved Cu concentrations (1.08–6.08 μg l −1) were mostly higher than typical estuarine values, probably due to recycled Cu complexation by dissolved organic matter. Growth, bioaccumulation rates and kinetics in the whole soft body of the bivalves were analyzed every 40 days. Although Cd bioaccumulation of oysters was lower in the basin than in the estuary during the same period (27,000 ng g −1, dry weight and 40,000 ng g −1, respectively) these values are largely above the new human consumption safety level (5000 ng g −1, dw; European Community, 2002). For cockles and clams, Cd bioaccumulation was lower, reaching 1400 ng g −1 and 950 ng g −1, respectively. Similar results were obtained for Zn and Cu suggesting physiological differences between the species and/or differences in the exposure of the organisms due to physico-chemical conditions and metal distribution between dissolved and particulate phases. In contrast, Hg bioaccumulation was highest for cockles reaching bioconcentration factors of ∼200,000, which even exceeded that of Cd in oysters (50,000) for the same exposition period. Nevertheless, Hg concentrations remained relatively low in the three bivalve species.

Physical and numerical modeling of the role of hydrodynamic processes on adult-larval interactions of a suspension-feeding bivalve

The importance of hydrodynamic processes for adult-larval interactions in the cockle, Cerastoderma edule, was examined through physical and numerical modeling. A set of physical experiments in a flow-tank using adult cockles and larval mimics showed that the settlement of particles was affected by adult cockles. Settlement was reduced by 20% in an area of 2.5 cm(2) Surrounding the siphons, and the most marked decrease occurred near the inhalant siphon. On a larger spatial scale downstream of the siphons, settlement was more heterogeneous compared to surfaces without cockles. The experimental results near individual cockles were compared with numerical models of settlement dynamics in conditions with no horizontal flow. The models suggest that the vertical position of the siphon orifice determines whether any small-scale reduction in larval settlement should be expected near suspension-feeding benthic invertebrates. The results are compared qualitatively and quantitatively with previous observations of small-scale patterns (approximate to1 cm) around individual C edule and with observations of larger-scale (1-10 m) differences among patches with varying densities of cockles. These comparisons indicate that passive hydrodynamic processes can explain patterns around individual cockles, whereas a combination of active and passive processes are necessary to explain differences among patches. Such hydrodynamic modification of larval behavior has previously been reported to greatly increase rates of mortality for settling bivalve larvae.

Cockle emergence at the sediment surface: 'favourization' mechanism by digenean parasites?

The aim of the present work was to assess the effect of digenean trematodes on indirect mortality of the cockle Cerastoderma edule, an infaunal bivalve. The tested hypothesis was that parasites altered the burrowing capacity of cockles and thus exposed them at the sediment surface, where they are more vulnerable to predators. If the predator is the final host, this mechanism, which drives the cockle out of the sediment, is considered as a 'favourization'. Cockle populations from 2 stations in Arcachon Bay (France)-Banc d'Arguin (oceanic situation) and La Canelette (lagoonal situation)--were sampled for 1 yr. At La Canelette, monitoring every 2 d showed that 50% of adult cockles regularly migrated to the sediment surface at a rate of 5 cockles m(-2) yr(-1) and disappeared in a few days. In the laboratory, 67% of these 'surface cockles' did not burrow again, suggesting that they would die in the field. Moreover, mortality measured after 7 d in the laboratory was 2 to 5 times higher than mortality of 'buried cockles', at both stations and particularly during summer. Species richness and abundance of digeneans from both stations were compared in 'buried' and 'surface' individuals to determine whether parasites played a role in cockle migration and mortality. Ten and 9 digeneans were found at Banc d'Arguin and La Canelette, respectively, with Himasthla quissetensis and Labratrema minimus being the most prevalent and abundant species at both stations. The abundance of H. quissetensis was slightly higher in surface cockles at Banc d'Arguin, but the difference fluctuated with station and cockle age (or size). L. minimus prevalence was only higher in surface cockles at La Canelette. In the latter station, we estimated that L. minimus and H. quissetensis were responsible for the emergence of 9 and 2%, respectively, of the buried cockles. Although this favourization mechanism may induce some mortality in cockles, it does not alone explain the magnitude of the observed mortalities (41 and 57% at La Canelette and Banc d'Arguin, respectively). A correspondence analysis did not show the presence of a particular parasite community in buried or surface cockles, which could explain these high surface cockle mortalities in association with the 2 dominant digeneans.