Growth reduction in the soft-shell clam Mya arenaria from a heavily oiled lagoon in Chedabucto Bay,Nova Scotia

Growth rates were determined in Mya arenaria collected from a heavily oiled lagoon lagoon shortly after the Arrow disaster (Nova Scotia, Canada). The clams, ranging in length from 30 to 50 mm, were compared with a sample collected from the same location 9 years later. Both samples from Janvrin Lagoon were compared with Mya arenaria collected in 1979 from a non-oiled neighbouring lagoon on Potato Island. Annual internal growth bands seen in shell cross-sections rather than shell external annuli were used for age determination. The 1979 Janvrin Lagoon sample, compared with the 1970 sample, suffered growth retardation in tissue and shell growth by 2–5 years.     

Differential reproductive strategies of two bivalves in the Dutch Wadden Sea

Cerastoderma edule and Mya arenaria are two common bivalve species in European waters. Longevity and maximum size are much greater in the latter species. Because comparison of species life-history strategies states that a long life span (i.e. high annual survival) generally goes with lower fecundity, we hypothesise that reproductive output would be lower in M. arenaria than in C. edule. In the present paper, we studied the reproductive strategies of these two species in an intertidal and a subtidal area of the western Dutch Wadden Sea, by following seasonal changes in absolute and relative weights of somatic and gonadal tissues in these bivalves. Starting of spawning was similar in the two species, around May, except for intertidal M. arenaria, which initiated spawning in August. Individual energy investment in reproduction was similar for the two species but, unlike M. arenaria, C. edule spawned completely, releasing all energy of gonadal mass in the form of gametes. Mya arenaria used the gonad not only for reproduction but also for storage. In the intertidal area, we found a trade-off between longevity and reproduction, i.e. maximum reproductive output (expressed as a proportion of body mass) was higher in C. edule than in M. arenaria. However, since body size is larger and life span longer in M. arenaria than in C. edule, mean lifetime reproductive output per individual must be higher in the first than in the latter. Based on the differences in reproductive strategies of these two species, we hypothesise that the negative effects of warming climate on bivalve population dynamics in the Wadden Sea will be stronger in C. edule than in M. arenaria.     

Eiders Somateria mollissima scavenging behind a lugworm boat

The eider is one of the most important molluscivorous birds in the Wadden Sea, where it feeds mainly on blue mussels Mytilus edulis and edible cockles Cerastoderma edule. These prey species are within reach of the birds at all times. Other potential prey of suitable size that are abundantly present, such as several polychaete worms, or the clam Mya arenaria, are taken to a much lesser extent, possibly because they live buried in the sediment and digging them out would take too much effort. Mya may pose another problem because they grow to sizes that prevent eiders from swallowing them. Large Mya also live too deep down in the sediment, but young (small) specimens should be available to eiders. Yet, even these have only rarely been found as prey in eiders in the Wadden Sea. However, diet studies in relation to food abundance have been few, and may have missed prey that do not leave large shell fragments (i.e. in faeces studies). This paper describes observations on eiders taking both Mya and polychaete worms. The eiders fed on these prey in a fashion reminiscent of gulls that scavenge behind fishing vessels: some eiders have learnt to follow professional worm-digging boats that supply a bycatch of molluscs (mainly Mya arenaria) and polychaete worms (mainly Arenicola marina and Nephtys hombergii). Mya and worms were also the main targets of the eiders that fed in a dense flock close to the boat's stern. Faeces found on the flats at low tide comprised mainly cockle shell fragments, a prey rarely taken by the eiders behind the boat. Faeces studies may thus give a highly biased impression of local eider diet.     

A comparison of the incidence of five pathological conditions in soft-shell clams, Mya arenaria, from environments with various pollution histories

The distribution of the occurrences and co-occurrences of five pathological conditions in samples of the soft-shell clam, Mya arenaria, from 17 locations on the east coast of the United States were compared by two different analytical techniques. 1. The relative fit of Poisson and Lagrange-Poisson distributions to the data were related to the mortality rate and spread of the pathological conditions. 2. A severity index based on a matrix of occurrences and co-occurrences, and the analysis of its eigenvectors were used to measure and to characterise the severity of the spread. These two techniques were compared with the help of a cluster analysis and discussed in the context of the pollution history of the seventeen locations studied. Although causal inferences cannot be drawn, the analyses suggested that crude oils and heavy metals had less effect on the incidence of hyperplasia and neoplasia than more refined petrochemicals. However, neoplasia was found in both clean and polluted environments.     

Histopathological alterations in Mya arenaria following a # 2 fuel oil spill in the arthur kill, Elizabeth, New Jersey

Following a 2270000-liter #2 fuel oil spill on 1 January 1990 studies were carried out to evaluate histopathological alterations in a population of Mya arenaria inhabiting an impacted tidal mud flat in the Arthur Kill, Elizabeth, New Jersey. Approximately 30 clams were collected 1 week after the spill and on a monthly basis thereafter. The lesion incidence found in M. arenaria following the spill was compared to a base-line incidence established at the same location during 1988 and 1989. There was an increased incidence of moderate and severe lesions of the digestive gland, gills, and mantle 1 week after the spill, returning to pre-spill levels by 1 month. During the first 3 months post-spill an increase in lesion incidence of the intestines was observed which returned to previous levels by 6 months post-spill. The heart and kidney showed a latent increase in lesion incidence 2 and 3 months following the spill, which by 4 months returned to previous levels. Examination of the gonadal development indicated that ova and sperm development were not adversely affected and spawning occurred both in late spring and late summer. The siphon, gill, stomach, foot muscle, and body epithelium were not found to be adversely affected.     

基于数量性状的砂海螂(Mya arenaria)选择指标评估

采用多元回归和通径分析的方法,研究了砂海螂(Mya arenaria)形态性状对重量性状的影响,建立了利用形态性状估计重量性状的回归方程,以评估砂海螂育种的选择指标。结果表明,各形态性状与体重、软体部重之间的相关系数均达到极显著水平(P<0.01);壳长对体重和软体部重的直接效应均为最大,其通径系数分别为0.4338和0.3265,是影响体重和软体部重的主要因素。壳宽对体重和软体部重的直接影响效应次之,但壳高对重量性状的影响主要是间接效应。决定系数分析的结果与通径分析结果具有一致的变化趋势。上述结果为砂海螂选择育种提供了依据,以壳长作为首选指标,能有效地增加体重和软体部重。     

A severity index to assess and monitor the incidence of pollution-related pathological conditions in marine organisms

A normalised matrix of the occurrences and co-occurrences of n-pathological conditions in a hypothetical population is termed a severity matrix and was used to define a volume in an n-dimensional vector space representing the severity or extent of the spread of the conditions in the population. The changes in this volume resulting from varying the numbers of occurrences and co-occurrences in the severity matrix are described by the varying determinant of the matrix. The natural logarithm of this determinant was shown to be a consistent measure or index of severity for comparing the spread of the same pathological conditions among different populations or among chronological samples from the same population. While the eigenvalues of the severity matrix are used to determine the relative contribution of each pathological condition to the index of severity, the eigenvectors serve to investigate possible associations among the conditions. Actual data from a population of the soft-shell clam Mya arenaria were used to demonstrate the application of this methodology to characterise the incidence of five pathological conditions assumed to be indicators of pollution related stress on the clam population.     

Copper accumulation by two bivalve molluscs: Salinity effect is independent of cupric ion activity

Oysters (Crassostrea virginica) and soft shell clams (Mya arenaria) were exposed to two different total copper concentrations (10 μg liter⁻¹ and 30 μg liter⁻¹) and two different cupric ion activities (10⁻¹⁰ m and 10⁻⁹ m) under high salinity and low salinity conditions. Cupric ion activities were adjusted by adding nitrilotriacetic acid (NTA) to the experimental media.Copper accumulation by both species was inversely related to salinity and positively associated with total copper and cupric ion activity. This salinity effect was proportionately greater for lower copper concentrations and activities and it was concluded that salinity would, therefore, be an important influence on metal accumulation in the natural environment. The salinity effect was still apparent when the cupric ion activity was held constant indicating that the effect is, at least in part, independent of the copper speciation in the medium.     

Increased benthic grazing: An alternative explanation for low phytoplankton biomass in northern San Francisco Bay during the 1976–1977 drought

Among the consequences of extremely low river flow into northern San Francisco Bay during a two-year drought were (1) a gradual increase in salinity, (2) an unusual decline in chlorophyll a concentration, and (3) the upstream migration of estuarine benthic invertebrates to the normally brackish area of the bay. Total abundance in the benthos at a shallow monitoring site increased from a normal 2000 to greater than 20 000 individuals m^?2 during the summer of 1977, presumably in response to the increased salinity. Estimated filtration rates derived from equations in the literature for one of the species, the suspended-feeding bivalve Mya arenaria ranged from 1 to 4 m³ m^?2 day^?1 during 1977 depending on abundance and mean size on sampling dates. Because water depth at this site is less than 2 m, Mya could have filtered all of the particles (including diatoms) from the water column on the order of once per day. Several other immigrant species undoubtedly contributed to the removal of particles from the near-bottom water as well. Increased benthic grazing, therefore, could have accounted for the anomalously low phytoplankton biomass observed during the drought. These results suggest that during periods of prolonged low river flow and increased salinity benthic food webs could become more important than planktonic food webs in the upper part of the estuary.     

Age compositions of flatfish stocks as determined by a new otolithometric method,its application in the estimation of growth,spawning potential and fisheries management

We developed an ageing methodology and examined age composition of three flatfish stocks inhabiting the Seto Inland Sea, Japan. Ages were difficult to determine for three-lined tongue sole (Cynoglossus abbreviates) and ridged-eye flounder (Pleuronichthys cornutus) because the first year annulus ring was often indistinct; therefore, we used directional change in otolith growth to distinguish it. Sectioning and etching methods were powerful tools for identifying annual checks for red tongue sole (Cynoglossus joyneri). Using these ageing methods, we determined age–length relationships and growth curves. The age composition of the populations studied and of the landings showed that a large proportion of the latter consisted of individuals under the mean age of sexual maturity, thereby reducing the percent spawning potential ratio (%SPR) to ≈ 20% for all species. These findings suggest that fishing pressure on immature fish is leading to overfishing of these flatfish stocks.     

Spatial and temporal changes in abundance of the infaunal bivalve Soletellina alba (Lamarck, 1818) during a time of drought in the seasonally-closed Hopkins River Estuary,Victoria, Australia

The infaunal bivalve Soletellina alba is susceptible to mass mortalities during annual winter flooding in the Hopkins River Estuary, southern Australia. Periods of low salinity (≤1) are the likely cause of these mass mortality events, which can occur in seasonally-closed estuaries when high winter flows are sufficient to flush all salt water from the estuary. Core samples of S. alba were collected from two water depths across four times and at three sites near the mouth of the estuary. Minimal to zero abundances of large S. alba (>1 mm) were expected to be sampled, particularly at the shallower water depth, during a typical winter flood event. However, the present study occurred during a period of drought, which led to the absence of winter flooding. This absence of winter flooding prevented the occurrence of lethal salinities (i.e. ≤1) in the estuary during this period and a greater number of living S. alba adults were sampled. Abundances of juvenile and adult S. alba were still variable, even in the absence of winter flooding, and reflected an interaction between date, site and water depth. However, no mass mortalities of adults were observed during the drought conditions in contrast to what occurs during typical winter flood events and provides support for the hypothesis that winter flooding is responsible for past mass mortalities.     

Spatial and temporal variations in the population structure of the soft-shell clam Mya arenaria in the Pomeranian Bay (southern Baltic Sea)

The population structure of Mya arenaria has been investigated in the sublittoral zone of the Pomeranian Bay (southern Baltic Sea). Box-corer samples were collected during a 1.5-year period in 1993/94 to follow changes in size and age structure of the clam populations in different parts of the study area. Large spatial differences in population structure were found between the sheltered southwest of the Bay and the shallow and exposed Oder Bank in the centre. The stock of the Oder Bank consisted of two different clam types. A slow-growing cohort was assumed to be autochtonous on the Oder Bank, whereas a fast-growing one was assumed to have immigrated from the surrounding area. The contribution of the two cohorts to the total density varied seasonally. Because of bedload transport of clams, the contribution to the reduction of the clam stock by wintering sea ducks could not be quantified. In the southwest of the Pomeranian Bay erosion was of minor importance. High mortality rates during the first two years of life were assumed to be caused by predation. Mortality rates of older cohorts remained stable until old age. Variations in cohort strength were related to interannual differences in the reproductive success. A mild winter presumably lowers the reproductive success in the subsequent summer. Severe oxygen depletion in summer 1994 caused a strong reduction in the clam stock at stations deeper than 10 m.     

Reconstruction of food conditions for Northeast Atlantic bivalve species based on Dynamic Energy Budgets

Required assimilated energy to support observed growth was reconstructed for four common bivalve species (Mya arenaria, Cerastoderma edule, Mytilus edulis and Macoma balthica) from various Northeast Atlantic coastal areas, along the species distributional range. The approach applied was based on the Dynamic Energy Budget (DEB) theory whereby observed growth patterns in the field, in combination with prevailing temperatures, were used to reconstruct the average food intake experienced in the field scaled to the maximum possible. For all species, results suggest food limitation over the range of locations. In general, reconstructed food intake indicated better conditions for C. edule compared to the other species, while M. edulis presented the lowest food conditions in all the areas. Despite the indications for a latitudinal trend in primary production, no clear pattern or relationship between reconstructed food conditions and latitude was observed suggesting that any trend may be overruled by local conditions.     

Distinctly variable mudscapes: Distribution gradients of intertidal macrofauna across the Dutch Wadden Sea

The Wadden Sea is a shallow coastal region, with a large area of sedimentary tidal flats that extends from The Netherlands to Denmark and has been declared a site of international importance in the Dutch and German parts (Ramsar status and UNESCO World Heritage Site). Benthic macrofauna are central to the ecosystem functioning of this area, as they recycle nutrients, decompose organic matter and are an important food source for many secondary consumers, like fish and waterbirds. Due to the environmental gradients characteristic of estuarine systems, it is expected that changes in assemblage composition will be observed across the physical and environmental gradients of the Wadden Sea. First, we explored the spatial variation in assemblage composition of benthic macrofauna across the intertidal part of the Dutch Wadden Sea using 3 years of biomass data. Then, we identified the relative importance of six environmental variables for explaining and predicting changes in assemblage composition across the intertidal areas of the Wadden Sea using generalised dissimilarity modelling (GDM). In accordance with the environmental gradients across this system, the biomass contributed by a few common species differed from west to east and were distinct in the Dollard. In the west, bivalves Mya arenaria, Cerastoderma edule and Ensis directus contributed a relatively large and equal share of the total biomass, whereas C. edule contributed the sole largest share of the total biomass towards the east. The polychaete Alitta succinea became a large share of the total biomass in the upper Ems and in the Dollard estuary, but contributed little elsewhere. Similar to the observed differences in species composition, the spatial patterns in assemblage composition, as predicted by the GDM models, identified the Dollard as distinct and that the prevalence of assemblage types in the west differed to the east. Median grain size, followed by microphytobenthic biomass, and exposure time were the most important variables describing differences in assemblage composition. That the Wadden Sea forms a heterogeneous landscape where assemblage composition varies across multiple gradients has repercussions for management and monitoring.     

Phytoplankton community structure in the Arabian Sea during and after the SW monsoon, 1994

Diatoms, dinoflagellates, coccolithophores, nanoflagellates, picophytoplankton and procaryote algae (Synechococcus spp. and prochlorophytes) were quantified by microscopy and flow cytometry, and their biomass determined, at 12 stations along a 1600 km transect across the Arabian Sea at the end of the SW monsoon in September, and during the inter-monsoon period of November/December 1994. The transect spanned contrasting oceanic conditions that varied from seasonally eutrophic, upwelling waters through mesotrophic, downwelling waters to permanently oligotrophic, stratified waters. The overall diversity of diatoms, dinoflagellates and coccolithophores along the transect was not significantly different between the SW monsoon and inter-monsoon. However, diatoms showed greatest diversity during the SW monsoon and coccolithophores were most diverse during the inter-monsoon. Integrated phytoplankton standing stocks during the SW monsoon ranged from 3 to 9 g C m^-2 in the upwelling eutrophic waters, from 3 to 5 g C m^-2 in downwelling waters, and from 1 to 2 g C m^-2 in oligotrophic waters. Similar phytoplankton standing stocks were found in oligotrophic waters during the inter-monsoon, but were ca. 40% lower compared to the SW monsoon in the more physically dynamic waters. Phytoplankton abundance and biomass was dominated by procaryote taxa. Synechococcus spp. were abundant (often >10⁸ cells l^-1) during both the SW monsoon and inter-monsoon, where the nitrate concentration was ⩾0.1 μ mol l^-1, and often dominated the phytoplankton standing stocks. Prochlorophytes were restricted to oligotrophic stratified waters during the SW monsoon period but were found at all stations along the transect during the inter-monsoon, dominating the phytoplankton standing stocks (>40%) in the oligotrophic region during this period. Of the nano- and micro-phytoplankton, only diatoms contributed significantly to phytoplankton standing stocks, and then only in near-shore upwelling waters during the SW monsoon. There were significant changes in the temporal composition of the phytoplankton community. In nearshore waters a mixed community of diatoms and Synechococcus spp. dominated during the SW monsoon. This gave way to a community dominated by Synechococcus spp. in the intermonsoon. In the downwelling zone, a Synechococcus spp. dominated community was replaced by a mixed procaryote community of Synechococcus spp. and prochlorophytes. In the oligotrophic stratified waters, the mix of procaryote algae was replaced by one dominated by prochlorophytes alone.     

The long-term impacts of fisheries on epifaunal assemblage function and structure, in a Special Area of Conservation

Fisheries can have profound effects on epifaunal community function and structure. We analysed the results from five dive surveys (1975-1976, 1980, 1983, 2003 and 2007), taken in a Special Area of Conservation, Strangford Lough, Northern Ireland before and after a ten year period of increased trawling activity between 1985 and 1995. There were no detectable differences in the species richness or taxonomic distinctiveness before (1975-1983) and after (2003-2007) this period. However, there was a shift in the epifaunal assemblage between the surveys in 1975-1983 and 2003-2007. In general, the slow-moving, or sessile, erect, filter-feeders were replaced by highly mobile, swimming, scavengers and predators. There were declines in the frequency of the fished bivalve Aequipecten opercularis and the non-fished bivalves Modiolus modiolus and Chlamys varia and some erect sessile invertebrates between the surveys in 1975-1983 and 2003-2007. In contrast, there were increases in the frequency of the fished and reseeded bivalves Pecten maximus and Ostrea edulis, the fished crabs Cancer pagurus and Necora puber and the non-fished sea stars Asterias rubens, Crossaster papposus and Henricia oculata between the surveys in 1975-1983 and 2003-2007. We suggest that these shifts could be directly and indirectly attributed to the long-term impacts of trawl fishing gear, although increases in the supply of discarded bait and influxes of sediment may also have contributed to changes in the frequency of some taxa. These results suggest that despite their limitations, historical surveys and repeat sampling over long periods can help to elucidate the inferred patterns in the epifaunal community. The use of commercial fishing gear was banned from two areas in Strangford Lough in 2011, making it a model ecosystem for assessing the long-term recovery of the epifaunal community from the impacts of mobile and pot fishing gear.     

Changes in survivorship,behavior, and morphology in native soft‐shell clams induced by invasive green crab predators

Many studies on invasive species show reduced native densities, but few studies measure trait‐mediated effects as mechanisms for changes in native growth rates and population dynamics. Where native prey face invasive predators, mechanisms for phenotypic change include selective predation, or induced behavioral or morphological plasticity. Invasive green crabs, Carcinus maenas, have contributed to declines in native soft‐shell clams, Mya arenaria, in coastal New England, USA. We tested the hypothesis that clam ability to detect chemical cues from predators or damaged conspecifics would induce greater burrowing depth as a refuge from invasive crabs, and greater burrowing would require increased siphon growth. To determine how crab predation affected clam survivorship and phenotypic traits in the field, clams in exclosure, open, and crab enclosure plots were compared. Crab predation reduced clam density, and surviving clams were deeper and larger, with longer siphons. To determine whether the mechanism for these results was selective predation or induced plasticity, phenotypes were compared between clams exposed to chemical cues from crab predation and clams exposed to seawater in laboratory and field experiments. In response to crab predation cues, clams burrowed deeper, with longer siphons and greater siphon mass. Overall, crab predation removed clams with shorter siphons at shallow depths, and crab predation cues induced greater burrowing depths and longer siphons. Longer siphons and greater siphon mass of deeper clams suggests clams may allocate energy to siphon growth in response to crabs. By determining native behavior and morphological changes in response to an invasive predator, this study adds to our understanding of mechanisms for invasive impacts and illustrates the utility of measuring trait‐mediated effects to investigate predator–prey dynamics.     

黄海小黄鱼生物学特征对多重压力的响应

Temporal changes in biological characteristics of small yellow croaker Larimichthys polyactis in the Yellow Sea were examined for the period of 1960–2008. The body size and age of small yellow croaker decreased substantially, in particular, average length of fish in 2008 was reduced by ~85% than those occurring in 1985, and at that time ~93% of the total catch was dominated by one-year-old individuals. Correspondingly, growth parameters also varied significantly over the years, i.e., k(growth coefficient) and t_0(zero-length age) gradually increased from 0.26 and –0.58 year in 1960 to 0.56 and –0.25 year in 2008, respectively. Although, L∞(body length)sharply decreased from 34.21 cm in 1960 to 24.06 cm in 2008, and t_r(inflexion age) decreased from 3.78 year in1960 to 1.61 year in 2008. There was a great increase both in natural mortality coefficient and fishing mortality coefficient. However, according to the gray correlation analysis, changes in the biological characteristics of small yellow croaker were induced by different stressors ranked as: fishing vessel powerfeeding gradesea surface temperature. This study suggests that the active fishery management measures for biological characters of fish populations should be considered.     

Iron fertilization of Trichodesmium off the west coast of Barbados: A one-dimensional numerical model

A one-dimensional quasi-Eulerian model, describing the biological and chemical interactions of autotrophic and heterotrophic plankton populations within the upper 100 m of the water column, was used to explain the relative impact of iron fertilization from Saharan dust on phosphorus cycling by the cyanobacterium, Trichodesmium spp. We examined the Caribbean surface waters off the west coast of Barbados using seasonal cases of dust delivery (summer peak) in relation to periods of elevated phosphorus stocks found in episodic pulses of Amazon River water advected past Barbados by the Guiana Current. The additional iron and phosphorus supplied by fluvial sources, available after biological depletion of nitrogen, could alleviate the growth limitation of Trichodesmium far downstream of the river plumes. The model results were compared with measurements made of Trichodesmium stocks 8 km off the west coast of Barbados. Modeled concentrations within the low-salinity Amazon plumes reached ∼7.4 mg chl m⁻², as seen at Barbados. In the absence of the low-salinity signal, the Trichodesmium biomass shifted to phosphorus limitation with little response to iron fertilization. This indicates that Trichodesmium stocks may indeed be mainly a function of phosphorus availability within the upper water column. Therefore, the supply of phosphorus from subtropical/tropical rivers, such as the Amazon and Orinoco, limits the potential cyanophyte response, despite adequate atmospheric iron delivery. This alters our view not only of the western North Atlantic nitrogen budget, but potentially several regions downstream of low N:P river systems.     

Differences in abundance and distribution of Alexandrium cysts in Sendai Bay,northern Japan,before and after the tsunami caused by the Great East Japan Earthquake

The tsunami caused by the Great East Japan Earthquake on 11 March 2011 greatly influenced the coastal benthic environment on the northern Pacific coast of Japan. We used the direct count method to investigate changes in the abundance and distribution of Alexandrium (Alexandrium tamarense and Alexandrium catenella) cysts before and after the tsunami in Sendai Bay. Densities of Alexandrium cysts in sediments collected in summer 2011 ranged from 0 to 8,190 cysts cm^?3. In the western part of the bay, the density increased greatly after the tsunami, the highest density being approximately 10 times the density recorded in 2005. Molecular identification of single cysts with multiplex polymerase chain reaction (PCR) showed that Alexandrium tamarense dominated the cyst population in the southwestern part of the bay in 2011. Furthermore, accumulation of cysts on the surface sediment after disturbance of the sediment was confirmed by a laboratory experiment. The main factor causing the drastic changes in abundance and distribution of Alexandrium cysts after the earthquake was considered to be vertical and horizontal redistribution of the cysts in sediments after the tsunami.