Comparative survival of bay scallops in eelgrass and the introduced alga, Codium fragile, in a New York estuary

Eelgrass, Zostera marina, is generally regarded as the preferred habitat of bay scallops, but in some cases scallop populations have persisted or increased in areas lacking eelgrass. This suggests that some other substrate(s) may serve important ecological functions for bay scallops. One candidate is Codium fragile, a macroalgal species with which bay scallops are known to associate and in which we commonly find juvenile and adult bay scallops in eastern Long Island, New York. In this study, we examined whether survival of planted bay scallops differed in Codium, eelgrass, and Codium + eelgrass substrates at two sites during August and October of 2 years. Survival of tethered scallops and recoveries of live free-planted individuals varied with scallop size, planting season and year, but no differences were observed between the three substrates for a given scallop size and planting date. Crab (particularly Dyspanopeus sayi) and whelk predation were implicated as important causes of tethered scallop mortalities while emigration and removal by predators likely contributed to scallop losses. Densities of naturally recruited 0+ years scallops recovered by visual and suction dredge sampling were similar in the eelgrass and Codium substrates. While our results suggest that Codium may offer some degree of predation refuge for bay scallops, further work needs to weigh the potential disadvantages of this substrate (such as low DO levels, potential attachment and transport of scallops, and differences in current flow, food availability and sedimentation relative to eelgrass) to determine if Codium may serve as a valuable habitat for bay scallops throughout their lifespan.     

Genetic structure and gene flow of eelgrass <Emphasis Type="Italic">Zostera marina</Emphasis> populations in Tokyo Bay,Japan: implications for their restoration

Massive losses of eelgrass Zostera marina beds in Japan have occurred over the past 100 years. Toward their restoration, transplantation of eelgrass has been attempted in some areas, including Tokyo Bay. This study examined population genetic structures and gene flow in eelgrass in Tokyo Bay to establish guidelines for conducting restoration. Genotypes of a total of 360 individuals from 12 beds were determined using five microsatellite markers. The eelgrass beds in inner bay had above-average genetic diversity. A neighbor-joining tree based on F _ST values among beds revealed that a strong gene flow had occurred among six beds in the inner bay. Genetic assignment testing of drifting shoots indicated that those with seeds migrate in both directions between the inner and outer bay. We suggested that the restoration of eelgrass in the innermost part of Tokyo Bay, where natural habitats have been lost, should be conducted using the inner bay beds.     

A model for measuring leaf growth rate of vegetative shoot in the seagrass, Zostera marina

A new model for determining leaf growth in vegetative shoots of the seagrass Zostera marina (eelgrass) is described. This model requires the weights of individual mature and immature whole leaves and leaf plastochrone interval (P_L) as parameters, differing from the conventional leaf marking technique (CLM) that requires cutting and separation between new and old tissue of leaves. The techniques required for the model are the same as for the plastochrone method, but the parameters differ between both methods in use of the weight of individual immature leaves. In a mesocosm study, eelgrass growth was examined, and parameters for the new model and plastochrone method (the weights of individual mature and immature leaves and P_L) were measured. Leaf growth rate was measured using the CLM and determined by the new method and the plastochrone method. The results were then compared between the CLM, the new model, and the plastochrone method. The results obtained with the new model were similar to those obtained with the CLM. However, the results of the plastochrone method differed from those of the CLM, while the weight of immature leaves varied seasonally. The new model was also used to determine leaf growth in a natural eelgrass bed in Mikawa Bay, Japan, and revealed the growth rates in all shoots and those of different ages. This method would be advantageous as an accurate means of direct measurement in fieldwork, and should therefore be a useful tool for monitoring seagrass growth.     

Production dynamics of the eelgrass, <Emphasis Type="Italic">Zostera marina</Emphasis> in two bay systems on the south coast of the Korean peninsula

Production dynamics of eelgrass, Zostera marina was examined in two bay systems (Koje Bay and Kosung Bay) on the south coast of the Korean peninsula, where few seagrass studies have been conducted. Dramatically reduced eelgrass biomass and growth have been observed during summer period on the coast of Korea, and we hypothesized that the summer growth reduction is due to increased water temperature and/or reduced light and nutrient availabilities. Shoot density, biomass, morphological characteristics, leaf productivities, and tissue nutrient content of eelgrass were measured monthly from June 2001 to April 2003. Water column and sediment nutrient concentrations were also measured monthly, and water temperature and underwater irradiance were monitored continuously at seagrass canopy level. Eelgrass shoot density, biomass, and leaf productivities exhibited clear seasonal variations, which were strongly correlated with water temperature. Optimal water temperature for eelgrass growth in the present study sites was about 15–20°C during spring period, and eelgrass growths were inhibited at the water temperature above 20°C during summer. Daily maximum underwater photon flux density in the study sites was usually much higher than the light saturation point of Z. marina previously reported. Densities of each terminal, lateral, and reproductive shoot showed their unique seasonal peak. Seasonal trends of shoot densities suggest that new eelgrass shoots were created through formation of lateral shoots during spring and a part of the vegetative shoots was transformed into flowering shoots from March. Senescent reproductive shoots were detached around June, and contributed to reductions of shoot density and biomass during summer period. Ambient nutrient level appeared to provide an adequate reserve of nutrient for eelgrass growth throughout the experimental period. The relationships between eelgrass growth and water temperature suggested that rapid reductions of eelgrass biomass and growth during summer period on the south coast of the Korean peninsula were caused by high temperature inhibition effects on eelgrass growth during this season.     

Occurrence and detrimental effects of the bivalve-inhabiting hydroid <Emphasis Type="Italic">Eutima japonica</Emphasis> on juveniles of the Japanese scallop <Emphasis Type="Italic">Mizuhopecten yessoensis </Emphasis>in Funka Bay,Japan: relationship to juvenile massive mortality in 2003

In November 2003, we first observed prevalent occurrence of a hydroid, Eutima japonica, on soft body tissues of age zero Japanese scallop (Mizuhopecten yessoensis) juveniles cultured in large areas of Funka Bay, Hokkaido. The occurrence coincided with massive death of juvenile scallops. A major objective was to clarify ecological relationships between the symbionts, and to infer the relationship between symbiosis and the massive mortality. To do this, we investigated distributions of association rates of hydroids with juvenile scallops at 15–34 sites over 3 years (2003–2005), with age one adult scallops at 24 sites in 2003, and with mussels at 13 sites in 2004. We studied seasonal changes in association rates with juvenile scallops, and numbers of polyps per juvenile scallop at three sites from November 2003 to June 2004. We also quantified the hydroid impacts on juvenile scallop shell length growth and triglyceride accumulation in the digestive gland. The association rate of E. japonica polyps with juvenile scallops was high in large areas of Funka Bay in 2003, and overlapped the distribution of mussels bearing polyps. Association rates with age one adult scallops were very low in November 2003, even at the sites where polyps were very common on juvenile scallops. Levels of hydroid occurrence in juvenile scallops varies by year. We found that hydroids presence in juvenile scallops declined drastically in 2004 and 2005. The association rates with juvenile scallops, and numbers of polyps per juvenile scallop declined during winter, until they disappeared completely in the following June. Since polyps were rare in adult scallops, we believe that infection of juvenile scallops was probably initiated from the planulae produced by medusae released from polyps growing on Mytilus spp., especially M. galloprovincialis. Subsequently, the inhabitation spread intraspecifically and interspecifically. In juvenile scallops, inhabitation of polyps reduced shell length growth by 43%, and triglyceride accumulation in digestive glands by 24–47%. Inhabitation of E. japonica on juvenile scallop is best regarded as parasitism, rather than inquilinism or commensalism. Occurrence of polyps was probably not a direct lethal factor for juvenile scallops, because there were some sites where association rates were high, but mortalities were low. Massive mortalities in 2003 may have resulted from simultaneous impacts of heavy polyp load and stresses caused by the way in which the animals were handled (transferred from cages for pre-intermediate culture to cages for intermediate culture), because the massive mortality occurred within a month of the transfer. The presence of polyps in juvenile scallops does not affect the quality of the product in Funka Bay, because market size scallops are hydroid-free.     

Sea level rise and eelgrass (Zostera marina) production: A spatially explicit relative elevation model for Padilla Bay, WA

The dynamics that govern the elevation of a coastal wetland relative to sea level are complex, involving non-linear feedbacks among opposing processes. Changes in the balance between these processes can result in significant alterations to vegetation communities that are adapted to a specific range of water levels. Given that current sedimentation rates in Padilla Bay, Washington are likely less than historical levels and that eustatic sea level rise is accelerating, the extensive Zostera marina (eelgrass) meadows in the bay may be at risk of eventual submergence. We developed a spatially explicit relative elevation model and used it to project changes in the productivity and distribution of eelgrass in Padilla Bay over the next century. The model is mechanistic and incorporates many of the processes and feedbacks that govern coastal wetland elevation change. Accretion estimates made using ²¹⁰Pb dating of sediment cores, sediment characteristics measured within cores, and eelgrass productivity and decomposition data were used to initialize and calibrate the model. Validation was performed using an elevation change rate measured with a network of surface elevation tables. Both the field data and model simulations revealed a net accretion deficit for the bay. Simulations using current rates of sea level rise indicated an overall expansion of eelgrass within Padilla Bay over the next century as it migrates from the center of the bay shoreward.     

Experimental examination of movement of the giant scallop,Placopecten magellanicus

We quantified movements of tagged giant scallops, Placopecten magellanicus (Gmelin, 1791), (40 to 115 mm in shell height) released in 1991 and 1992 at nine stations in Port Daniel Bay, Baie des Chaleurs, Gulf of St. Lawrence, Canada, for periods of 10 to 52 d. Both the mean distances moved per day and dispersion distances from release points were usually greater at the two stations with sand substratum, low scallop densities and high rock crab, Cancer irroratus, densities. Movement rate and dispersion of scallops from the release points at the other seven stations were similar, even though there were marked differences in substratum type (gravel, cobble or bedrock), predator density, and scallop density. Most mean dispersal directions were random, and scallops did not appear to migrate from unsuitable to suitable habitats. Although movement did reduce predation rate, scallop movement was weakly correlated with the abundance of only one predator, C. irroratus.     

Observations on shell deformities,ultrastructure, and increment formation in the bay scallop Argopecten irradians

Shell morphology and ultrastructure were examined in the bay scallop Argopecten irradians, cultured in recirculating seawater systems under various conditions of feeding, lighting, and handling. On a unialgal diet of Thalassiosira pseudonana, scallop growth ranged from 120 to 183 m d^-1 at 20°C in the laboratory, about two-thirds of the growth rate found in the field. However, shell deposited in the laboratory differed from that in the field in several ways. In the field, scallops formed costae as an unpigmented, corrugated marginal shell layer; shell deposited in the laboratory lacked this layer and was therefore darker. Also, microstructure of the exterior shell surface of field scallops was coarsely granular, while that of cultured scallops was relatively smooth. Excessive handling of scallops in the laboratory resulted in marginal thickening of valves, a deformity which was completely arrested by a change from daily to weekly handling. Scallops cultured in the same tank with oysters developed shell-thickening on the interior of the valves. It is postulated that shell abnormalities in bivalves result from disruption of complex behavioral processes associated with shell deposition and may be elicited by a variety of natural and experimental irritants. Under natural lighting regimes and optimal conditions for growth, scallops deposited exactly one shell increment per day, but under continuous lighting, deposition of growth increments often became aphasic. In one 28-d experiment, there was a strong correlation between number of growth increments formed and increase in shell height, suggesting that shell ridge formation occurred intermittently, rather than daily, when shell growth rates fell below approximately 150 m d^-1.     

A genetic basis for geographic variation in shell morphology in the bay scallop, Argopecten irradians

The bay scallop, Argopecten irradians, exhibits extensive variation in morphology among geographically separated populations, resulting in the recognition of three major subspecies (A. i. irradians, A. i. concentricus, A. i. amplicostatus). The extent to which morphological variation results from differing environmental conditions is unknown. In the present study, bay scallops from Massachusetts, North Carolina, Florida, and Texas were collected, spawned and the offspring reared in a common garden experiment to determine if scallops cultured under similar environmental conditions exhibited the morphology expected given the geographic origin of their parents. Significant differences among populations were indicated by ANOVA in both the wild-caught (13/14 morphological characters) and cultured (11/14 characters) scallops. Principal components analysis clustered wild-caught scallops according to geographic origin and cultured scallops according to geographic ancestry. The morphological characters most influential in resolving groups were plical width, plical spacing, number of plicae and valve convexity. Geographic variation in morphology apparently has a strong genetic basis, and reflects significant differentiation among disjunct populations of bay scallops. Received: 12 September 1996 / Accepted: 15 October 1996     

Balancing the edge effects budget: bay scallop settlement and loss along a seagrass edge

Edge effects are a dominant subject in landscape ecology literature, yet they are highly variable and poorly understood. Often, the literature suggests simple models for edge effects-positive (enhancement at the edge), negative (enhancement at the interior), or no effect (neutral)--on a variety of metrics, including abundance, diversity, and mortality. In the marine realm, much of this work has focused on fragmented seagrass habitats due to their importance for a variety of commercially important species. In this study, the settlement, recruitment, and survival of bay scallops was investigated across a variety of seagrass patch treatments. By simultaneously collecting settlers (those viable larvae available to settle and metamorphose) and recruits (those settlers that survive some period of time, in this case, 6 weeks) on the same collectors, we were able to demonstrate a "balance" between positive and negative edge effects, resulting in a net neutral effect. Scallop settlement was significantly enhanced along seagrass edges, regardless of patch type while survival was elevated within patch interiors. However, recruitment (the net result of settlement and post-settlement loss) did not vary significantly from edge to center, representing a neutral effect. Further, results suggest that post-settlement loss, most likely due to predation, appears to be the dominant mechanism structuring scallop abundance, not patterns in settlement. These data illustrate the complexity of edge effects, and suggest that the metric used to investigate the effect (be it abundance, survival, or other metrics) can often influence the magnitude and direction of the perceived effect. Traditionally, high predation along a habitat edge would have indicated an "ecological trap" for the species in question; however, this study demonstrates that, at the population level, an ecological trap may not exist.     

Hawksbill sea turtles in seagrass pastures: success in a peripheral habitat

Hawksbill sea turtles, Eretmochelys imbricata, are closely associated with coral reef and other hard-bottom habitats. Seagrass pastures are peripheral habitats for Caribbean hawksbills. With the decline in quality and quantity of coral reefs, seagrass habitats may become more important for hawksbills. We use data from a 30-year mark-recapture study of hawksbills and green turtles, Chelonia mydas, in the southern Bahamas to assess the quality of a seagrass habitat for hawksbills. Size distribution, residence times, and body condition index for the seagrass hawksbill aggregation are similar to those of hawksbill aggregations over Caribbean reefs. Somatic growth rates of seagrass hawksbills are in the upper range of those reported for reef hawksbills. Based on these parameters, peripheral seagrass habitats can support healthy, productive hawksbill aggregations. During the 30-year study, a sixfold variation in green turtle density in the study area did not affect the productivity or body condition of hawksbills.     

Influences of abundance,behavior, species composition,and ontogenetic stage on active emergence of meiobenthic copepods in subtropical habitats

The emergence of meiobenthic copepods in subtidal sand and seagrass sites in Tampa Bay, Florida (USA), was investigated on nine dates from March 1983 to August 1984. Numbers of emerging copepods were dependent on habitat, date and diel sampling period. Greater numbers typically emerged in seagrass sites, and differences between habitats may be related to lower copepod abundances in sand sites. Both copepod abundance and behavior significantly affected numbers emerging among dates. An increase in emergent behavior alone resulted in consistently greater numbers emerging during postsunset periods. Over 30 species in 15 families were found to emerge. Total numbers of emerging copepods were affected by the sporadic presence and seasonal behavior of certain species. Numbers of the predominant speciesParadactylopodia brevicornis, Tisbe furcata, Parategastes sp.,Mesochra pygmea, Laophontid sp., andHarpacticus sp. emerging depended on differences in both species abundance and behavior. The emergent behavior ofParategastes sp.,Harpacticus sp., andT. furcata also was influenced by total copepod densities in the sediment. Juvenile copepods constituted only a small proportion of the total numbers emerging. The consistent postsunset entry of between 10³ to 10⁵ copepods m^?2 into the water column in subtidal sand and seagrass habitats will contribute to the increased probability of copepod dispersal and reassortment of the benthic community, while providing a pathway for benthic-pelagic exchange.     

Gpi genotypic effect on quantitative traits in the northern bay scallop,Argopecten irradians irradians

The relationships between glucose-6-phosphate isomerase (Gpi) genotype and quantitative variation related to reproduction and growth were explored over the lifespan of a single cohort of northern bay scallops, Argopecten irradians irradians (Lamarck), from September 1986 to April 1988 in the Niantic River estuary, Connecticut, USA. Analyses revealed that Gpi genotype explained an increasingly significant proportion of variation in scallop size (up to 15%) as the cohort aged. The genotype-specific effects were consistent across sampling dates and among measured traits. There was no evidence for heterosis with respect to size at this locus; however, rare genotypes contributed substantially to the relationship and showed some tendency to fall on the extremes of the phenotypic distribution. The strength and consistency of the genotypic effect on scallop size suggest that genetic variation for Gpi, or some locus in linkage disequilibrium with Gpi, may translate into biochemical and/or physiological variation and affect fitness in this species.     

Substrate catabolism related to reproduction in the bay scallop Argopecten irradians concentricus,as determined by O/N and RQ physiological indexes

Weight specific rates of oxygen consumption carbon dioxide production, and ammonia-N excretion, measured for a Florida population of the bay scallop Argopecten irradians concentricus between May and September, 1982 and October and November, 1983 were significantly correlated (P<0.0005) to environmental factors that co-varied seasonally with metabolic shifts related to reproduction. Mean O/N and CO₂/O₂ (RQ) molar ratios indicates that scallop energy metabolism varied over the course of the reproductive cycle. Resting stage individuals (May-early June) had RQ values close to 0.7, indicative of a predominantly lipid-based metabolism. During the initial stages of gametogenesis (late June-early July) scallops catabolized primarily carbohydrate, as evidenced by maximum O/N (>22) values and RQ values close to 1.0 RQ values >1.0 indicated a possible carbohydrate to lipid conversion during the period of cytoplasmic growth (late July-early September). As gametes matured and spawning commenced (late September-November), metabolism became primarily protein based, as indicated by O/N and RQ values around 9.0 and 0.8, respectively. This pattern of substrate catabolism supports existing data on the storage and utilization of specific energy reserves with respect to reproduction in this species.     

Trophic diversity in amphipods within a temperate eelgrass ecosystem as determined by gut contents and C and N isotope analysis

A conjoint analysis of gut contents and stable C and N isotopes was applied to determine the main food sources and feeding habits of dominant amphipods in an eelgrass bed (Zostera marina) in Gwangyang Bay, Korea. Gut content observations demonstrated that, while Gammaropsis japonicus and Jassa slatteryi are herbivorous, feeding on epiphytes and detritus, Pontogeneia rostrata and Monocorophium acherusicum are omnivorous, feeding on mesozooplankton fragments and detritus. Stable isotope data confirmed that epiphytes, detritus, and mesozooplankton fragments were major food sources for amphipods in the eelgrass bed. Isotopic mixing model calculations clearly showed an interspecific difference in diet composition. A high isotopic dissimilarity between amphipod taxa demonstrated interspecific trophic diversity, reflecting their herbivorous (G. japonicus and J. slatteryi) and omnivorous (P. rostrata and M. acherusicum) feeding habits and confirmed the detrivorous feeding habits of caprellids. Such trophic diversity at interspecific level of the amphipod species indicates that they use different food resources within their microhabitats and play species-specific functional roles as mediators in trophic pathways from producers to higher-level consumers of the eelgrass ecosystem. Finally, our findings suggest that information on the species-specific trophic ecology of amphipods is needed to better understand their potential role in the trophic dynamics and carbon flow of seagrass bed ecosystems.     

Behavioral response of fish larvae to low dissolved oxygen concentrations in a stratified water column

Density stratification and respiration lead to vertical gradients in dissolved oxygen in many aquatic habitats. The behavioral responses of fish larvae to low dissolved oxygen in a stratified water column were examined during 1990–1991 with the goal of understanding how vertical gradients in dissolved oxygen may directly affect the distribution and survival of fish larvae in Chesapeake Bay, USA. In addition, the effects of low oxygen on 24-h survival rates were tested so that results of behavior experiments could be interpreted in the context of risk to the larve. Naked goby [Gobiosoma bosc (Lacépède)] and bay anchovy [Anchoa mitchilli (Valenciennes)] larvae strongly avoided dissolved oxygen concentrations <1 mg 1^-1, which were lethal within 24 h at 25 to 27°C. In addition, naked goby larvae, whose behavior was tested at a wider range of dissolved oxygen concentrations, also showed a reduced preference for an oxygen concentration of 2 mg 1^-1, which leads to reduced survival during long-term exposures and to reduced feeding rates. There were no major differences in behavior or survival between the two species, or between the two age classes of naked gobies tested. Results suggest that behavioral responses to oxygen gradients will play a large role in producing marked vertical changes in abundance of feeding-stage larvae in Chesapeake Bay; mortality from direct exposure to low oxygen will likely be much less important in producing vertical patterns of larval abundance.     

Small-scale spatial and temporal genetic structure of the Atlantic sea scallop (Placopecten magellanicus) in the inshore Gulf of Maine revealed using AFLPs

Despite long planktonic durations, many species of broadcast spawning invertebrates exhibit genetic structure at small spatial and temporal scales. Amplified fragment length polymorphisms were used to assess genetic variation in the sea scallop, Placopecten magellanicus, among four inshore and one offshore location in the Gulf of Maine and temporal genetic variation among age classes of sea scallops at one site. Our results indicated that genetic structure for P. magellanicus exists on smaller spatial scales (tens to hundreds of kilometers) than expected given the 40-day planktonic larval period. In addition, genetic differences among age classes may be influenced by inter-annual differences in larval supply or reproductive success. Future genetic studies should sample multiple age classes prior to comparison among locations.     

Growth and content of energy reserves in juvenile sea scallops,Placopecten magellanicus,as a function of swimming frequency and water temperature in the laboratory

The effects of swimming frequency and water temperature on shell growth, tissue mass, and stored energy reserves of juvenile sea scallops, Placopecten magellanicus Gmelin, were examined in a factorial laboratory experiment spanning six weeks in July and August 1992. Individually tagged scallops of similar initial size (22.5±0.1 mm shell height, n=240) were induced to swim to exhaustion at three different swimming frequencies (every day, twice a week, or not at all) in two different water temperature regimes (4 to 7 or 7 to 13°C). The scallops were fed an ad libitum mixture of cultured microalgae. At the end of the experiment, cumulative increase in shell height, dry weight of soft tissues, condition index of dry adductor muscle (adductor muscle dry weight/soft tissue dry weight x 100) and total carbohydrate content of dry adductor muscle were measured for each scallop. Scallops at the higher temperature had significantly greater shell heights, and were in better metabolic condition as evidenced by significantly higher condition indices and muscle carbohydrate contents. The dry soft tissue weights did not differ significantly from their low temperature counterparts. Swimming frequency had no significant effect on shell height, dry tissue weight, or carbohydrate content, but condition index of the adductor muscle increased significantly with swimming frequency. These results show that not only was there no cumulative cost of swimming in terms of shell growth, total soft tissue weight, or carbohydrate content in young scallops, but that condition of adductor muscle tissue was higher in scallops that swam.     

Testing the robustness of primary production models in shallow coastal areas: a case study

In this paper we investigate the robustness of a dynamic model, which describes the dynamic of the seagrass Zostera marina, with respect to the inter-annual variability of the two main forcing functions of primary production models in eutrophicated environments. The model was previously applied to simulate the seasonal evolution of this species in the Lagoon of Venice during a specific year and calibrated against time series of field data. In the this paper, we present and discuss the results which were obtained by forcing the model using time series of site-specific daily values concerning the solar radiation intensity and water temperature. The latter was estimated by means of a regression model, whose input variable was a site-specific time series of the air temperature. The regression model was calibrated using a year-long time series of hourly observations. The Z. marina model was first partially recalibrated against the same data set that was used in the original paper. Subsequently, the model was forced using a 7-year-long time series of the driving functions, in order to check the reliability of its long-term predictions. Even though the calibration gave satisfactory results, the multi-annual trends of the output variables were found to be in contrast with the observed evolution of the seagrass biomasses. Since detailed information about the air temperature and solar radiation are often available, these findings suggest that the testing of the ecological consistency of the evolution of primary production models in the long term would provide additional confidence in their results, particularly in those cases in which the scarcity of field data does not allow one to perform a formal corroboration/validation of these models.     

Cadmium bioaccumulation in organs of the scallopMizuhopecten yessoensis

Cadmium and Zinc content was determined in organs of the Japanese scallopMizuhopecten yessoensis, collected in 1984 from the Sea of Japan and aged between 1 and 8 yr. Under conditions of background Cd, Cd concentration in hepatopancreas and kidney increased linearly with age from 39 to 400µg g^?1 dry wt in hepatopancreas, and from 100 to 640µg g^?1 dry wt in kidney. Such a pattern was not determined for Zn. Cd concentration in muscle, mantle and gill did not exceed 6µg g^?1 dry wt in the oldest scallops. In subcellular fractions of the hepatopancreas, cytosolic Cd accounted for 71.7% in 1-yr-old scallops and 98.8% in 8-yr-olds. A similar ratio was established for gills, although gill Cd content was an order of magnitude lower. Analysis of Cd distribution in cytoplasmic proteins (of different molecular weight) in the hepatopancreas, showed that the amount of Cd bound to metallothionein-like proteins increases with scallop growth. A considerable amount of Cd also was detected in high molecular weight proteins. Thus, Cd accumulation, with age, in organs of the Japanese scallop is due to metal binding by cytoplasmic proteins.