Juvenile Bay Scallops Research Articles

Comparison of Visual Surveys Versus Dredging for Monitoring Bay Scallops (Argopecten irradians) in Seagrass Beds

Bay scallops (Argopecten irradians), once a profitable fishery species in North Carolina, have declined in population size following harmful algal blooms in the late 1980s. To prepare for future scallop restoration efforts, appropriate survey methods should be identified to evaluate the status of the population with respect to managing harvest. The North Carolina Division of Marine Fisheries monitors bay scallop populations in seagrass beds using a scallop dredge, though it is still unclear what the current natural densities of bay scallops are. In this study, visual surveys in three time treatments (5, 10, and 20 min) were compared with dredging in terms of quantifying scallop density and the catch per unit effort (CPUE) in seagrass beds in Core Sound, North Carolina. There was no significant difference in scallop density and the CPUE among visual survey time treatments. Scallop densities observed during visual surveys were three times higher than densities using a scallop dredge; however, the CPUE was five times higher using a scallop dredge compared with visual surveys. If visual surveys indicate the true natural density of bay scallops in this study system, then the dredge sampling efficiency for evaluating bay scallop density was 33%. Dredging for bay scallops can uproot seagrass and displace juvenile bay scallops. Visual surveys provide a low-impact method for identifying the distribution and abundance of bay scallops in seagrass bed habitats.

Diurnal Fluctuations in Acidification and Hypoxia Reduce Growth and Survival of Larval and Juvenile Bay Scallops (Argopecten irradians) and Hard Clams (Mercenaria mercenaria)

Diurnal variations in pH and dissolved oxygen (DO) concentrations are common seasonal phenomena in many eutrophic estuaries, yet few studies have investigated the concurrent effects of low pH and low DO on marine organisms inhabiting these coastal systems. Here, we assess the effects of diurnal variations in pH and DO on the early-life history of two bivalve species native to Northeast US estuaries, the bay scallop (Argopecten irradians) and hard clam (Mercenaria mercenaria). In one set of experiments, larval- and juvenile-life stage bivalves were exposed to ambient conditions (pHT ~ 7.9), two continuously-low pH levels (pHT ~ 7.3 and 7.6), and diurnal fluctuations between the ambient and low conditions yielding mean pH levels equal to the intermediate pH levels. In a second set of experiments, larval bivalves were exposed to ambient conditions (pHT ~7.9, DO ~ 7 mg L-1), two levels of low pH and DO (pHT ~ 7.2, DO ~1 mg L-1; pHT ~7.4, DO ~ 4 mg L-1) and diurnal fluctuations of both pH and DO between the ambient and low pH/DO levels that resulted in mean pH and DO levels equal to the intermediate pH and DO levels. Diurnal acidification treatments with ambient DO levels yielded survival rates for both species at both life stages that were consistent with the survival of individuals exposed to the same mean level of chronic pH with juveniles being more resistant to acidification than larvae. In contrast, when both pH and DO varied diurnally, the survival rates of larval bivalves were significantly lower than the survival of individuals chronically exposed to the same mean levels of pH and DO, an indication that bivalves were physiologically more vulnerable to concurrent fluctuations of both parameters compared to acidification alone. While both species displayed sensitivities to diurnal fluctuations in pH and DO, scallops were relatively more susceptible than hard clams. Since many shallow eutrophic estuaries presently experience diurnal cycles of both pH and DO when early-life stages of bivalves are present in estuaries, the populations of the bivalves studied are likely impacted by these conditions which are likely to intensify with climate change.

Structured habitat provides a refuge from blue crab, Callinectes sapidus, predation for the bay scallop, Argopecten irradians concentricus (Say 1822)

Predation in estuarine systems is an important source of mortality for benthic organisms. In laboratory mesocosm experiments, we assessed the survival of bay scallops (Argopecten irradians concentricus) of various sizes (10–19mm, 20–29mm, 30–39mm, and 40–50mm shell height; SH) as a function of female blue crab, Callinectes sapidus, predation as it varied with habitat (oyster shell, sand, Gracilaria spp.) and predator size (>140mm, ≤140mm carapace width; CW) in a balanced two-by-three factorial design. Scallops of all sizes were afforded higher proportional survival with small female crabs (0.61, SE=0.05) compared to that with large female crabs (0.36, SE=0.05), and the proportion of scallops surviving was highest in oyster shell (0.61, SE=0.08), as compared to the Gracilaria spp. and sand treatments, at 0.41 (SE=0.06) and 0.42 (SE=0.07), respectively. Subsequent field-tethering experiments conducted in the Lynnhaven River sub-estuary of the lower Chesapeake Bay further illustrated the effect of habitat on the survival of juvenile bay scallops (<30mm SH); survival after 48h differed significantly by habitat and location, but not size, and there were no interactions. Proportional survival was significantly higher in Gracilaria spp. treatment (0.60, SE=0.07) as compared to other habitats, and it was higher at Alanton's Cove (0.60, SE=0.10) compared to other locations. Overall, scallop survival was low; however, transplanting scallops in structured substrates with protection against predation, such as oyster shell and Gracilaria spp., will likely increase the success of restoration efforts.

Epifaunal invertebrates as predators of juvenile bay scallops (Argopecten irradians)

Predation strongly influences populations of numerous benthic invertebrates, although many predation studies to date have focused on macroscopic individuals, ignoring critical early life stages. Juveniles of the bay scallop, Argopecten irradians, settle and grow on the blades of eelgrass, Zostera marina, then migrate to the sediment surface when their mobility and size provide a refuge from benthic predators. During their time in the eelgrass canopy, scallops co-occur with a diverse array of small invertebrates, including peracarid and small decapod crustaceans, whose role as predators is largely unexplored. We measured consumption by amphipods, isopods, and a shrimp on recently settled bay scallops ranging in size from 0.5 to >1.5mm in a series of 24-hour experimental laboratory assays. These invertebrate predators, which were common concurrent epifaunal surveys of restored eelgrass beds in the mid-Atlantic, consumed up to 63% day−1 of juvenile scallops when the scallops were <1mm, but predation impacts decreased as scallops exceeded this size. Our data have implications for current restoration of both bay scallops and their eelgrass habitat, suggesting that previously unrecognized consumers may significantly affect scallop population dynamics at early life stages.

Bioactive effects of Prorocentrum minimum on juvenile bay scallops (Argopecten irradians irradians) are dependent upon algal physiological status

The harmful dinofl agellate Prorocentrum minimum reportedly has variable toxicity to grazing animals. We used the bay scallop Argopecten irradians irradians as a bioassay organism to compare expression of harmful effects of P. minimum cultures in growth or senescent phases. The non-toxic alga Rhodomonas sp. was used as a control. Exposure to both types of P. minimum cultures decreased the degree of shell opening, amount of biodeposits produced, motility and byssal-thread attachment. As P. minimum cultures approached senescence, effects became more severe, and mortality increased to 15 % in scallops exposed to senescent P. minimum . Pathological effects of P. minimum on scallops included derangements of scallop digestive tubules and the adductor muscles, and abnormal hemocyte distributions, which were more severe in scallops exposed to senescent cultures. These fi ndings help to explain the variable toxicity of P. minimum to scallops and other bivalves reported in the literature. Further, these fi ndings demonstrate that the defi nition of a “ harmful algal bloom ” should probably be expanded to include physiological status along with identity and abundance of a phytoplankton species.

Acquisition of thermotolerance in bay scallops, Argopecten irradians irradians, via differential induction of heat shock proteins

Many cells and organisms are rendered transiently resistant to lethal heat shock by short exposure to sublethal temperatures. This induced thermotolerance is thought to be related to increased amounts of heat shock proteins (HSPs) which, as molecular chaperones, protect cells from stress-induced damage. As part of a study on bivalve stress and thermotolerance, work was undertaken to examine the effects of sublethal heat shock on stress tolerance of juveniles of the northern bay scallop, Argopecten irradians irradians, in association with changes in the levels of cytoplasmic HSP70 and 40. Juvenile bay scallops heat-shocked at a sublethal temperature of 32 °C survived an otherwise lethal heat treatment at 35 °C for at least 7 days. As determined by ELISA, acquisition of induced thermotolerance closely paralleled HSP70 accumulation, whereas HSP40 accrual appeared less closely associated with thermotolerance. Quantification of scallop HSPs following lethal heat treatment, with or without conditioning, suggested a causal role for HSP70 in stress tolerance, with HSP40 contributing to a lesser, but significant extent. Overall, this study demonstrated that sublethal heat shock promotes survival of A. irradians irradians juveniles upon thermal stress and the results support the hypothesis that HSPs have a role in this induced thermotolerance. Exploitation of the induced thermotolerance response shows promise as a means to improve survival of bay scallops in commercial culture.

Characterization of the toxicity of Cochlodinium polykrikoides isolates from Northeast US estuaries to finfish and shellfish

Harmful algal blooms caused by Cochlodinium polykrikoides are annual occurrences in coastal systems around the world. In New York (NY), USA, estuaries, bloom densities range from 103 to 105mL−1 with higher densities (≥104cellsmL−1) being acutely toxic to multiple fish and shellfish species. Here, we report on the toxicity of C. polykrikoides strains recently isolated from New York and Massachusetts (USA) estuaries to juvenile fish (Cyprinodon variegates) and bay scallops (Argopecten irradians), as well as on potential mechanisms of toxicity. Cultures of C. polykrikoides exhibited dramatically more potent ichthyotoxicity than raw bloom water with 100% fish mortality occurring within ∼1h at densities as low as 3.3×102cellsmL−1. More potent toxicity in culture was also observed in bioassays using juvenile bay scallops, which experienced 100% mortality during 3 days exposure to cultures at cell densities an order of magnitude lower than raw bloom water (∼3×103cellsmL−1). The toxic activity per C. polykrikoides cell was dependent on the growth stages of cultures with early exponential growth cultures being more potent than cultures in late-exponential or stationary phases. The ichthyotoxicity of cultures was also dependent on both cell density and fish size, as a hyperbolic relationship between the death time of fish and the ratio of algal cell density to length of fish was found (∼103cellsmL−1cm−1 yielded 100% fish mortality in 24h). Simultaneous exposure of fish to C. polykrikoides and a second algal species (Rhodomonas salina or Prorocentrum minimum) increased survival time of fish, and decreased the fish mortality suggesting additional cellular biomass mitigated the ichthyotoxicity. Frozen and thawed-, sonicated-, or heat-killed-, C. polykrikoides cultures did not cause fish mortality. In contrast, cell-free culture medium connected to an active culture through a 5μm nylon membrane caused complete mortality in fish, although the time required to kill fish was significantly longer than direct exposure to the whole culture. These results indicate that ichthyotoxicity of C. polykrikoides isolates is dependent on viability of cells and that direct physical contact between fish and cells is not required to cause mortality. The ability of the enzymes peroxidase and catalase to significantly reduce the toxicity of live cultures and the inability of hydrogen peroxide to mimic the ichthyotoxicity of C. polykrikoides isolates suggests that the toxicity could be caused by non-hydrogen peroxide, highly reactive, labile toxins such as ROS-like chemicals.

Characterization, dynamics, and ecological impacts of harmful Cochlodinium polykrikoides blooms on eastern Long Island, NY, USA

We report on the emergence of Cochlodinium polykrikoides blooms in the Peconic Estuary and Shinnecock Bay, NY, USA, during 2002–2006. Blooms occurred during late summer when temperatures and salinities ranged from 20 to 25 °C and 22 to 30 ppt, respectively. Bloom patches achieved cell densities exceeding 10 5 ml −1 and chlorophyll a levels exceeding 100 μg l −1, while background bloom densities were typically 10 3–10 4 cells ml −1. Light, scanning electron and ultrathin-section transmission electron microscopy suggested that cells isolated from blooms displayed characteristics of C. polykrikoides and provide the first clear documentation of the fine structure for this species. Sequencing of a hypervariable region of the large subunit rDNA confirmed this finding, displaying 100% similarity to other North American C. polykrikoides strains, but a lower similarity to strains from Southeast Asia (88–90%). Bioassay experiments demonstrated that 24 h exposure to bloom waters (>5 × 10 4 cells ml −1) killed 100% of multiple fish species (1-week-old Cyprinodon variegates, adult Fundulus majalis, adult Menidia menidia) and 80% of adult Fundulus heteroclitus. Microscopic evaluation of the gills of moribund fish revealed epithelial proliferation with focal areas of fusion of gill lamellae, suggesting impairment of gill function (e.g. respiration, nitrogen excretion, ion balance). Lower fish mortality was observed at intermediate C. polykrikoides densities (10 3–10 4 cells ml −1), while fish survived for 48 h at cell densities below 1 × 10 3 cells ml −1. The inability of frozen and thawed-, or filtered (0.2 μm)-bloom water to cause fish mortality suggested that the thick polysaccharide layer associated with cell membranes and/or a toxin principle within this layer may be responsible for fish mortality. Juvenile bay scallops ( Argopecten irradians) and American oysters ( Crassostrea virginica) experienced elevated mortality compared to control treatments during a 9-day exposure to bloom water (∼5 × 10 4 cells ml −1). Surviving scallops exposed to bloom water also experienced significantly reduced growth rates. Moribund shellfish displayed hyperplasia, hemorrhaging, squamation, and apoptosis in gill and digestive tissues with gill inflammation specifically associated with areas containing C. polykrikoides cells. In summary, our results indicate C. polykrikoides blooms have become annual events on eastern Long Island and that bloom waters are capable of causing rapid mortality in multiple species of finfish and shellfish.

IMPACTS OF A CYANOBACTERIUM CONTAMINATING LARGE-SCALE AQUACULTURE FEED CULTURES OF TETRASELMIS CHUI ON SURVIVAL AND GROWTH OF BAY SCALLOPS, ARGOPECTEN IRRADIANS IRRADIANS

Abstract Large-scale, open, microalgal feed cultures for hatchery and nursery production of marine invertebrates inevitably becomes contaminated with various microbes that can affect productivity and usability of the harvested biomass. In the Greenhouse for Research on Algal Mass Production Systems (GRAMPS) at the NMFS Laboratory in Milford, CT, cultures of Tetraselmis chui (PLY429) often become contaminated with a cyanobacterium; preliminary observations suggested that juvenile bay scallops, Argopecten irradians irradians showed reduced performance when the feed culture became contaminated with this cyanobacterium. We isolated a cyanobacterium from a contaminated culture of PLY429 and conducted a feeding study to determine if this isolate affects survival and growth of juvenile bay scallops, either alone or in combination with PLY429, thereby simulating feeding of a contaminated culture. Bay scallops were given a diet of either 100% PLY429, 50% PLY429 with 50% cyanobacteria, 100% cyanobacteria, or starve...

Toxicity of Un-ionized Ammonia, Nitrite, and Nitrate to Juvenile Bay Scallops, Argopecten irradians irradians

Juvenile bay scallops (7.2-26.4 mm) were exposed for 72 h to different concentrations of un-ionized ammonia, nitrite, or nitrate. Using the Trimmed Spearman Karber method, 50% lethal concentrations (LC(50)) and 95% confidence limits were calculated individually for each. Un-ionized ammonia concentrations above 1.0 mg N-NH(3)/L resulted in 100% scallop mortality within 72 h. The 72-h LC(50) for un-ionized ammonia was calculated at 0.43 mg N/L. At nitrite concentrations of 800 mg N/L or higher 100% mortality was observed. The 72-h LC(50) for nitrite was calculated at 345 mg N/L. Nitrate was the least toxic, with 100% mortality observed at a concentration of 5000 mg N/L. The calculated nitrate 72-h LC(50) was 4453 mg N/L. Our results indicate that un-ionized ammonia is the most lethal nitrogenous waste component to bay scallops.

Comparison of early life history stages of the bay scallop, Argopecten irradians: Effects of microalgal diets on growth and biochemical composition

The culture of bay scallops, Argopecten irradians, is limited by a reliable and affordable supply of spat and the ability to ensure that animals attain market size within a single growing season. The main goals of our study were thus: (1) to develop growth-optimizing algal diets for implementation in hatcheries, and (2) to identify and compare bay scallop postlarval and juvenile dietary requirements, especially of lipids and fatty acids, which if met may enhance production. Nutritional needs of postlarval bay scallops (present study) are compared with those of sea scallops, Placopecten magellanicus, offered the same diets in a previous companion study. To this end, postlarval (initial shell height, SH = 240 μm) and juvenile (initial SH = 10 mm) bay scallops were offered 6–7 microalgal diet combinations at 20 °C, for 3 weeks. A similar growth ranking among diets was observed between the two developmental stages. A combination diet of Pavlova sp. (CCMP 459) and Chaetoceros muelleri was far superior to any other diet tested, yielding growth rates of 58 and 357 μm day − 1 which were 65% and 25% higher than the next highest performing diet of Tetraselmis striata/ C. muelleri in postlarvae and juveniles, respectively. The T. striata/ C. muelleri diet, which is limited in the n-3 fatty acid docosahexaenoic acid (DHA), yielded very poor growth of sea scallop postlarvae in a prior study, indicating that bay scallops may have less stringent requirements for DHA than sea scallops. The Pav 459/ C. muelleri diet, which also supported the highest growth of sea scallop postlarvae, is characterized by elevated levels of the n-6 fatty acids, arachidonic (AA) in C. muelleri and 4,7,10,13,16-docosapentaenoic (DPA) in Pav 459. The two diets deficient in AA and n-6 DPA, Pavlova lutheri/ Thalassiosira weissflogii and P. lutheri/ Fragilaria familica, yielded the lowest growth rates in both bay scallop postlarvae and juveniles. Tissue enrichment of these two fatty acids relative to the diet, as well as overall enrichment in ∑n-6 fatty acids was observed across developmental stages and dietary treatments. A similar pattern has previously been observed in sea scallop postlarvae, suggesting a dietary requirement for n-6 fatty acids in pectinids that has often been overlooked in the past.

Growth, condition, reproductive potential, and mortality of bay scallops, Argopecten irradians, in response to eutrophic-driven changes in food resources

Anthropogenic nutrient enrichment of coastal waters is changing the habitat and food resources of bay scallops, Argopecten irradians. As land-derived nitrogen (N) enters estuaries, phytoplankton abundance, particulate organic matter, and nitrogen content of seston may increase, providing a higher quantity and quality of food. Understanding these changes is important for monitoring declining populations or developing field aquaculture systems. To examine if changes in food resources due to nutrient enrichment will affect growth, condition, reproductive potential, and mortality, we conducted a field experiment in seven estuaries each having a different land-derived N load. We placed juvenile bay scallops within these estuaries for approximately 12 weeks while monitoring food quantity and quality. Stable isotopic signatures suggested that scallops assimilated food from the specific estuaries in which we placed them. Growth rates were relatively high and did not increase with higher phytoplankton concentrations, suggesting that at the densities we deployed, ambient phytoplankton concentrations were in excess of consumption ability. Growth rates decreased at sites with lower salinities, and where high densities of competitors (barnacles and slipper shells) fortuitously settled on the scallops. Condition index significantly increased with higher growth rates. Gonad index and mortality were not related to food resources, but mortality increased with lower salinities. Land-derived N load seems unlikely to directly alter condition, reproductive potential, or mortality. These results suggest that estuaries undergoing anthropogenic nutrient additions may provide food concentrations above the maximum ration assimilable, resulting in high bay scallop growth rates.

Influence of seagrass habitat patch size on growth and survival of juvenile bay scallops, Argopecten irradians concentricus (Say)

Patchy seagrass meadows are common features on high energy sand shoals in temperate coastal waters. We tested the effect of seagrass patch size within patchy seagrass meadows on growth and survival of juvenile bay scallops, Argopecten irradians concentricus (Say), during spring (two sites) and fall (one site) settlement periods in Back Sound, North Carolina, USA. Because plant characteristics such as density and species composition (proportion of Zostera marina L. and Halodule wrightii Aschers) may influence growth and predation rates, we also quantified these variables during each time period. Weekly growth rates of scallops in the spring were more than twice that of the fall (∼1.5 vs. 0.6 mm per week). Spring growth rates were statistically greater at Cape Lookout than at Oscar Shoal. The actual difference in growth, however, was <1 mm (7.34±0.13 mm and 6.45±0.19 mm over 5 weeks, respectively), and most likely would not have produced significant biological effects such as differential predation rates. Average growth of juvenile scallops placed in the center (0.75×0.75 m area) of replicate small (∼1 m across) patches was faster than in large (>8 m across) patches over 5–7 week periods during both seasons. The difference in growth, however, was small (∼1 mm) and would probably have little biological significance. In the spring, scallop survivorship after 2 weeks did not differ between sites or among seagrass patches of varying size. After an additional 3 weeks however, scallop survivorship was approximately 30% greater at the site dominated by Zostera marina with lower shoot density, but there was no difference in survivorship among seagrass patch sizes. The same pattern was observed over the cumulative 5-week period. In the fall, predation rates on juvenile scallops were significantly higher in small patches than in large over very short time intervals (e.g., 24 h). Over 3-, 4- and 7-week time intervals, however, there was no effect of patch size on scallop survival, even though shoot densities were significantly greater in medium- and large-sized patches than in small. Lack of a long-term patch size and/or seagrass density effect on scallop survivorship may be due to differential responses to patch size by the many potential predators that prey on juvenile scallops.

An automated rearing chamber system for studies of shellfish feeding

Producing large volumes of high quality microalgae to feed shellfish and other organisms is a limiting factor in the development of the aquaculture industry. Feeding regimes yielding the highest conversion efficiencies of algal feed to molluscan growth are required to maximize the return on algal-culture investments. In the past we have used 12 specialized, manually-controlled molluscan rearing chambers to study nutritional requirements and growth of oysters, clams, and scallops. A computer-controlled, solenoid-valve system was added to automate seawater flow, volume of microalgal food delivered, and feeding duration independently for each chamber. Labor was reduced from 7 h per week to 3 h, while adding flexibility. Each chamber represents a model for a programmed nursery system. Evidence that superior growth of bivalves can be achieved by feeding regimes made possible by this apparatus are provided by an experiment with juvenile bay scallops ( Argopecten irradians). Published by Elsevier Science B.V.

Ontogenic Changes in Microhabitat Distribution of Juvenile Bay Scallops, Argopecten irradians irradians (L.), in Eelgrass Beds, and Their Potential Significance to Early Recruitment.

Ontogenetic changes in the vertical distribution of a cohort of juvenile bay scallops, Argopecten irradians, on eelgrass, Zostera marina, were followed throughout the summer and early fall in two Long Island embayments (New York, USA). Despite site-specific differences in eelgrass height and density, more than 95% of post-settlement scallops remained attached above the bottom until they reached a shell height of about 11 mm. Over a 5-week period, scallops gradually relocated until, at a mean size of 31 mm, all occurred on the bottom. The decline in percent attachment coincided with a 5-fold increase (from 16 to 84 μmoles min-1 g muscle dry wt-1) in the activity of octopine dehydrogenase (proposed here as an index of the scallops' capacity for burst swimming activity), and in maximum rate of increase in the shell aspect ratio. While attached to eelgrass, scallops were nonuniformly distributed, with greatest concentration at mid-canopy. Following disturbance, they rapidly regained above-ground position, attaining asymptotic heights within 3-10 h. This and prior studies suggest that the climbing behavior of the bay scallop is an adaptive response to high predation pressure at small sizes. Enhanced scope for activity (predator avoidance) may enhance survival of scallops at intermediate sizes, when they become too heavy to maintain elevation but have not yet attained effective refuge in size.

Nursery culture of the bay scallop, Argopecten irradians irradians, in suspended mesh nets

Scallops were suspended subtidally in pyramid-shaped (pearl) nets in Long Island Sound, Milford, Connecticut, USA to assess effects of density and temperature at deployment on growth and survival. Juveniles as small as 4 mm were grown at densities ranging from 250 to 15 000/m 2. There was an inverse relationship between the size of the scallops and the density that affected growth. Scallops with a mean shell height of 16–17 mm exhibited no reduction in growth when held at densities as high as 7500/m 2. Similarly, scallops with mean shell heights of 24 to 25 mm exhibited no reduction in growth when held at densities as high as 2500/m 2. Growth of scallops held at biovolumes as high as 1.1 l/net was not affected substantially. Survival typically exceeded 80%, and was not affected by density. Different size groups of juvenile bay scallops were deployed at ambient temperatures of 5, 11.5 and 14 °C. Survival for all but one group was greater than 80%. Deploying scallops early in the season at temperatures lower than 14 °C appears to have no advantage relative to growth.

The eelgrass canopy an above-bottom refuge from benthic predators for juvenile bay scallops Argopecten irradians

Juvenile bay scallops Argopecten irradians commonly attach to shoots of eelgrass Zostera marina using byssal threads. Although this behavior has long been recognized, its adaptive value is poorly understood. This study examined (1) the size-specif~c nature of scallop attachment on eelgrass, and (2) the possible role of vertical attachment in providing refuge from benthic predators. Laboratory experiments using artificial eelgrass showed strong, inverse relationships between scallop size (over the range 6 to 20 mm) and several measures of attachment performance (percent attachment, rate of attachment, and height-above-bottom attained). Field experiments in which 10 to 15 mm scallops were tethered to natural eelgrass in Lake Montauk, Long Island, New York (USA), demonstrated a dramatic, highly significant enhancement of scallop survival at greater heights of attachment. Scallops tethered at 20 to 35 cm above bottom experienced > 59 O/ O survival over 4 d , compared to < l 1 O/O sunrival near the sediment surface. A similar pattern was observed in laboratory tethering experiments using transplanted natural eelgrass and 3 crab predators common in mid-Atlantic embayments: Carcinus maenas, Libinia dubia, and Dyspanopeus sayi. The refuge value of vertical attachment was found, however, to be less with D. sayi than with the other predators tested, since individuals of t h s species climbed eelgrass to feed on scallops in the upper canopy. These data demonstrate that vertical attachment on eelgrass functions as an effective predator-avoidance mechanism for juvenile bay scallops by placing them out of reach of many benthic predators. It has also been previously shown that scallop predatory risk decreases with scallop size, with a partial size refuge attained at about 40 mm. It is therefore suggested that scallops undergo an ontogenetic shift from a spatial to a size refuge from predation a s they move from the eelgrass canopy to the sediment surface over the course of their post-settlement life history. We hypothesize that this shift may result in a critical window of high predatory risk between ca 15 and 40 mm, when scallops are expected to be most susceptible to benthic predators.

The eelgrass canopy an above-bottom refuge from benthic predators for juvenile bay scallops Argopecten irradians

Juvenile bay scallops Argopecten irradians commonly attach to shoots of eelgrass Zostera marina using byssal threads. Although this behavior has long been recognized, its adaptive value is poorly understood. This study examined (1) the size-specif~c nature of scallop attachment on eelgrass, and (2) the possible role of vertical attachment in providing refuge from benthic predators. Laboratory experiments using artificial eelgrass showed strong, inverse relationships between scallop size (over the range 6 to 20 mm) and several measures of attachment performance (percent attachment, rate of attachment, and height-above-bottom attained). Field experiments in which 10 to 15 mm scallops were tethered to natural eelgrass in Lake Montauk, Long Island, New York (USA), demonstrated a dramatic, highly significant enhancement of scallop survival at greater heights of attachment. Scallops tethered at 20 to 35 cm above bottom experienced > 59 O/ O survival over 4 d , compared to < l 1 O/O sunrival near the sediment surface. A similar pattern was observed in laboratory tethering experiments using transplanted natural eelgrass and 3 crab predators common in mid-Atlantic embayments: Carcinus maenas, Libinia dubia, and Dyspanopeus sayi. The refuge value of vertical attachment was found, however, to be less with D. sayi than with the other predators tested, since individuals of t h s species climbed eelgrass to feed on scallops in the upper canopy. These data demonstrate that vertical attachment on eelgrass functions as an effective predator-avoidance mechanism for juvenile bay scallops by placing them out of reach of many benthic predators. It has also been previously shown that scallop predatory risk decreases with scallop size, with a partial size refuge attained at about 40 mm. It is therefore suggested that scallops undergo an ontogenetic shift from a spatial to a size refuge from predation a s they move from the eelgrass canopy to the sediment surface over the course of their post-settlement life history. We hypothesize that this shift may result in a critical window of high predatory risk between ca 15 and 40 mm, when scallops are expected to be most susceptible to benthic predators.

Effects of flow speed, turbulence, and orientation on growth of juvenile bay scallops Argopecten irradiansconcentricus (Say)

Experiments were conducted using controlled pipe flows to test the extent to which growth of juvenile bay scallops Argopecten irradians concentricus (Say) is affected by flow speed, turbulence, and orientation of individuals relative to flow direction. When flow speed was held within a narrow range (1.7–3.9 cm · s −1), scallop growth was unaffected by the presence and intensity of turbulence in flows. Scallops with exhalent pseudosiphonal openings (posteriors) oriented upstream grew significantly less than those with anteriors oriented upstream at identical flow speeds. Shell growth declined gradually with increasing velocity from 1.4–17.2 cm · s −1, but the extent of flow effects on growth varied significantly with orientation to the flow. Negative effects of higher flows on growth were apparent for scallops with posteriors oriented upstream, whereas in the reverse position effects of flow speed on growth were less obvious. Flume tests suggested that the unfavorable position, with posterior pointed upstream, was not often maintained at flow speeds > 9 cm · s −1, whereas the more favorable reverse orientation was maintained at all flow speeds examined. Velocity profiles measured in natural scallop beds, and in situ observations of the elevation of juvenile bay scallops above the bottom, revealed that juveniles routinely experienced current speeds ⩾ 17 cm · s −1. Thus, changes in orientation may provide a significant means of maintaining high growth rates under field conditions. The effect of orientation suggests that feeding by juvenile Argopecten is inhibited by back pressure established at the exhalent opening by external flow. Hydrodynamic scaling arguments indicate that adult scallops should be less sensitive than juveniles to back pressure, yet, paradoxically, previous evidence, coupled with our results, indicate that growth of adult Argopecten is more sensitive to current speed than is the growth of juveniles.

Effects of flow velocity, food concentration and particle flux on growth rates of juvenile bay scallops Argopecten irradians

Feeding rates, and hence growth, of suspension-feeding organisms are determined by many interacting factors including efficiency of particle capture, food-particle concentration, and ambient flow regimes. Studies in a recirculating seawater flume were conducted to examine the growth rates of juvenile bay scallops Argopecten irradians irradians (L.) (3–10 mm in shell height) in four flow velocities (0, 1, 6, and 15 cm · s −1) over a range of food concentrations (0–75 000 cells · ml −1) in a four-by-four factorial design. Particle flux (algal cells · cm −2· s −1) encountered by the scallops is the product of food concentration (cells · ml −1) and flow velocity (cm · s −1). The experimental levels of food concentration and flow velocity were chosen to produce a wide range of fluxes. Several treatments with equal flux derived from different flows and food concentrations were used to decouple the effects of each factor. Within a given flux of particles, growth rates were not significantly different between flows or particle concentrations. Growth was only weakly correlated to flux of particles over the range of fluxes tested. The effects of algal concentration were more pronounced than the effects of flow velocity suggesting that differences in growth were a result of differences in algal concentrations. Additional experiments were run to determine the clearance rates of scallops at different food concentrations. These experiments support the findings of the flume experiments; clearance rates were seen to decrease with increasing food concentrations. Flux of food particles is not a predictor of growth, rather it is the combination of the effects due to food concentration and flow velocity which determine the response of the individual juvenile bay scallop.