Adult Cockles Research Articles

Parasites and Pathologic Conditions of the Cockle Cerastoderma edule Populations of the Coast of Galicia (NW Spain)

A survey of pathological conditions affecting cockle populations of the most economically important natural beds of Galician estuaries in NW Spain was performed. Samples of 30 adult cockles were collected from each of 34 natural beds in the spring of 1999 and processed by histological techniques. Disseminated neoplasia were seen in samples from most of the natural beds, in some cases with a high prevalence. The gregarine Nematopsis sp., larval trematode stages, and branchial extracellular large cysts enclosing bacteria-like microorganisms were the most prevalent parasites. Paravortex cardii, intracellular colonies of rickettsiae-like organisms in digestive and gill epithelium, Pseudoklossia sp. coccidians, Trichodina sp., and other ciliates were frequently seen in the samples. Copepods in gills and intestine and unidentified gregarines in intestine epithelia and surrounding connective tissue were less prevalent and were observed in samples of some natural beds. Large foci of heavy hemocytic infiltration were detected in a few sites only. Cysts of Steinhausia sp. and plasmodia and spores of a haplosporidian were seen in cockles from two localities. Inflammation was frequently observed in the samples. Some of the parasites and pathological conditions could be associated with mortality.

Stable isotope evidence of benthic microalgae-based growth and secondary production in the suspension feeder Cerastoderma edule (Mollusca, Bivalvia) in the Marennes-Oléron Bay

The contribution of natural food sources to the growth and secondary production of the suspension feeding bivalve Cerastoderma edule (L.) was estimated under field conditions in the Marennes-Oluron Bay (Atlantic coast, France). Monthly estimates of abundance, biomass and cockle growth were combined with seasonal analyses of δ 13 C and δ 15N ratios of juvenile and adult cockles, together with their potential food sources [i.e. suspended particulate organic matter (POM), microphytobenthos, macroalgae and seagrass] sampled at mid-tide level in a muddy sandflat. Adult cockles grew mainly in spring, whereas juveniles grew in summer and autumn, following spat recruitment in early summer. Total annual production and elimination of cockles were estimated to be 32.5 and 34.7 g AFDW m−2 yr−1. Relative contributions of each year class to production were ca 40, 41, 11 and 6% for 0-group, 1-, 3- and 4-yrold cockles in 1995, respectively. Quantitative assessment of proportions of food sources to the annual secondary production of cockles was obtained by using a simple carbon isotope-mixing model with microphytobenthos (δ 13 C = —16.0+0.6%o) and POM (δ 13C = —22.2+1.1%o) as end-members. On average, more than 70% of the total annual cockle production originated from microphytobenthos, with a much higher contribution for the 0-group (88%) than for adult cockles (60%). The between-age difference was induced mainly by changes in the availability of food resources (benthic versus planktonic) during the non-synchronous growing seasons of juvenile and adult cockles.

Food sources of an infaunal suspension-feeding bivalve Cerastoderma edule in a muddy sandflat of Marennes-Oléron Bay, as determined by analyses of carbon and nitrogen stable isotopes

Stable carbon and nitrogen isotope ratios were used to define the trophic base of the infaunal suspension-feeding bivalve Cerastoderma edule (L.) living on an intertidal muddy sandflat in Marennes-Oleron Bay, France. Suspended particulate organic matter (POM) collected from adjacent marine channels had a mean δ 13 C value of -22.2 ± 1.1‰ (n = 19). Benthic primary producers on Ronce-les-Bains tidal flats had δ 13 C values distinct from that of the POM: microphytobenthos (-16.0 ± 0.6‰), the macroalgae Enteromorpha compressa (-17.6 ± 2.8‰), Fucus serratus (-17.4 ± 2.6‰), Fucus vesiculosus (-18.5 ± 1.6‰), Phorphyra umbilicalis (-19.7 ± 0.5‰), Ulva rigida (-14.7 ± 2.0‰) and the seagrass Zostera noltii (-11.1 ± 1.0‰). Mean δ 15 N values of all macroalgae species ranged from 7.6 ± 1.3 to 8.9 ± 1.0‰, while those for microphytobenthos, POM and seagrass were 5.3 ± 0.8, 5.0 ± 0.9 and 6.5 ± 1.3‰, respectively. The mean δ 13 C value of spat (-15.3 ± 0.8‰) and juvenile cockles (-15.7 ± 0.7‰) varied within a smaller range than those of 1 to 4 yr old adults (-18.2 ± 1.2‰). The δ 15 N values of 0-group cockles (spat and juveniles) and adult cockles are similar (8.0 ± 0.9‰ for 0-group and 8.4 ± 1.1‰ for adults). The results suggest, based on the average trophic enrichment found in the literature for C and N, and the relative abundance of each food source, that there are 2 major sources of organic matter assimilated by cockles of the studied flats: microphytobenthos and POM. Seasonal variations in δ 13 C values reflect a higher dependence of adult cockles on POM variability while spat and juveniles are more closely linked to microphytobenthos. The isotopic shifts indicate that the relative importance of the 2 major food sources depends on the age of the cockles and the season.

Predation and the survival of juvenile Cerastoderma glaucum Bruguière (Mollusca: Bivalvia) in a coastal lagoon

The effects of predators on the survival of juvenile Cerastoderma glaucum Bruguière, were examined in a lagoon in Holkham, North Norfolk during 2 years (1993 and 1995) using cages to exclude predators from areas of sediment. A range of cage designs were employed to either control for caging artifacts, exclude all predators or a subset of the predators. Cages were seeded with adult cockles at field densities, and juvenile cockles were counted and measured at regular intervals from the time of spat settlement at 350 μm until reaching an overwintering size of 30 mm, some 4–5 months later. Numbers and sizes of cockles surviving in caged areas were compared to control caged areas and unmanipulated areas of sediment. The evidence from controls in the field experiments suggests that the cages themselves, had no detrimental effect on the survival or size of juvenile cockles. Cage treatments which excluded all predators produced the greatest survival of the cockle spat during the 2 years of experiments. The treatment excluding only epibenthic predators provided a short-term improvement in cockle survival suggesting that this group contributes to the lagoonal cockles mortality. However, there is no firm evidence for one predator playing a more important role than the others over the whole experiment. Additional factors influencing juvenile C. glaucum mortality are discussed and it is concluded that dystrophic crises caused by factors such as decaying algae, anoxia and the presence of hydrogen sulphide could contribute to the mortality found in these juvenile lagoonal bivalves.

Trial of a tractor dredger for cockles in Burry Inlet, South Wales

The effects on cockle (Cerastoderma edule L.) populations in Burry Inlet, South Wales of mechanical harvesting using a tractor dredger were investigated with an experimental trial conducted on 29 October 1992. Previously, only hand gathering methods were used, and the trial was intended to assist a licensing decision for mechanical dredging. Six blocks of dredged and undredged (control) plots were set out in each of two areas, one having a low density of cockles, the other high. Approximately 82% of the dredged areas was lifted by the blade of the dredger. The catch consisted almost exclusively of adult cockles (≥2 years old) over 25 mm in length. Appreciable losses of spat and one-year-olds from the dredged plots were also observed even though they were not taken in the catch. Possible reasons are discussed. Counts of damaged individuals rem-classes showed further mortalities attributable to dredging, and changes to shell growth were either minor or absent. Spatfall success in 1993 was depressed by 11% on dredged plots compared to that on control plots in the low density area, but was increased slightly (not significant p≥0.05) in the high density area. It is concluded that delayed effects of the dredging on cockle stocks were negligible.

The Influence of the Cockle, Cerastoderma edule, on the Macrozoobenthic Community of Tidal Flats in the Wadden Sea

Abstract. The cockle Cerastoderma edule is one of the large and important species of the tidal flats of the Wadden Sea. C. edule disturbs the upper sediment layer due to its crawling and “shaking” behaviour. About 7% of a cockle population move within one week. The mean distance of movement is ‐4 cm per week, which, in the case of a 2‐year‐old cockle with a length of ˜3 cm, means that ˜20 cm2 of sediment is disturbed to a depth of ˜3 cm. Besides these measurable movements, cockles also regularly “shake” themselves, thereby disturbing the sediment around them to at least ˜0.5 cm. At a density of 500 cockles per m2 of a mean length of 3 cm, ˜16% of the sediment surface is occupied by cockles and another ˜29% is disturbed by cockles within one week.To study the effects of cockles on recruitment of other benthic species, densities of 0, 125, 250, 375 and 500 cockles (mean length 3 or 3.5 cm) were added to 1‐m2 plots within large depopulated squares in April. Subsequently, sediment samples were sieved once a month (from June to September) and the densities of all macrozoobenthic species were estimated. The presence of cockles significantly reduced the densities of juveniles of the bivalve species C. edule, Macoma balthica, Mya arenaria, Tellina (Angulus) tenuis, and Ensis directus and of the worm species Pygospio elegans, Lanice conchilega, Eteone longa, Anaitides spec., Nephtys hombergii, Heteromastus filiformis, Scoloplos armiger, and Tharyx marioni and of the amphipods Corophium volutator and C. arenarium. Non‐significant effects were found only in Capitella capitata and Nereis diversicolor. For all species mentioned above a negative effect was already observed at the lower cockle densities of 125 ‐ 250 per m2. Reductions of about 50% or more were found at densities of 500 per m2, which was more than could be expected based on the area occupied by the cockles (˜16%), but close to the area occupied plus disturbed (˜45%). It is concluded that (dense) assemblages of adult cockles can have a strong influence on the macrozoobenthic community.

9.—On the Ecology and Sedimentation of the Cardium Shellsands and Transgressive Shellbanks of Traigh Mhor, Island of Barra, Outer Hebrides

SynopsisHolocene carbonates rest unconformably on Lewisian gneiss in an almost land-locked bay sheltered from Atlantic waves by sand dunes: frosts are rare and cockles, the dominant species, may reach 16 years of age. Five intertidal zones are recognised: algal mat, Arenicola sand, Cardium shell sand, Lanice sand, Ensis sand. Transgressive shellbanks composed solely of cockles and linguoid in form, are advancing over the Arenicola sand towards the shore, earlier seaward positions being indicated by hard-packed areas of shell pavement. Comparison of 1973 positions with those on aerial photographs taken in 1965 demonstrates a maximum transgression of 150 m over the past eight years, but in 1948 banks were smaller and most material was onshore as a chenier.Different age-groups are dominant in different parts of the beach: young cockles high on the shore; older, partially epifaunal individuals towards low water. In any one age-group shell height is inversely related to distance from LWM. By analysing the age and growth of dead shells from the banks it is possible to estimate where they originated, and thus how much post-mortem transport they have suffered. In the centre of the bay banks receive 90 per cent of their shells from seaward, the maximum distance travelled being 400 m.During westerly gales adult cockles living high on the shore are blown seawards, though young forms escape by speedier burrowing. This, together with the concentration of oystercatcher predation on small individuals from the more emergent parts of the bay, probably explains why older animals are more abundant on the lower reaches of the shore.Feeding oystercatchers leave many conjoined valves of Cardium on the sediment surface which roll in easily with the tide and replenish the shellbanks. The valves are rarely damaged, though gulls shatter shells by dropping them onto hard areas of pavement. In calm conditions shoreward flotation of both fragments and single valves is important: banks are high and narrow, and accrete by avalanche co-sets of haphazardly arranged whole valves. In stormy conditions much comminuted shell sand is blown onto the beach from the dunes: banks are more spread out, and the shell fabric locally becomes vertically imbricate in response to directly opposed wave systems generated by refraction around the banks.

Cercaria bucephalopsis haimeana(Lacaze-Duthiers, 1854) (Digenea: Bucephalidae) in the cockle,Cardium eduleL. in South Wales

Summary Some variations in the percentage infection of the cockle, Cardium edule L., on Llanrhidian sands, South Wales with Cercaria bucephalopsis haimeana (Lacaze-Duthiers, 1854) are described. Vertical variations in percentage infection are related to the population density of the final hosts and the seasonal variation with the breeding cycle of the first intermediate host. Only spent adult cockles, over 18·0 mm long and one year old, are infected, the highest percentage infection occurring in three-year old cockles. A decline in the percentage infection in older cockles suggests an increase in the mortality of infected specimens together with an increase in resistance to infection with age. Medium-sized cockles have a higher percentage infection than smaller and larger cockles of the same age. The parasite causes a decrease in the host's shell growth rate but an improvement in the flesh yield. The possible reasons for the variations in percentage infection and the effect of the parasite on the host are...