Optimizing the design of giant scallop (Placopecten magellanicus) spat collectors: flume experiments

Flume experiments were carried out in 1993 with specially designed collectors and inert particles to optimize the effect of various structural components of scallop spat collectors. The structural components considered were monofilament diameter and density, and mesh size of external bags. Results showed that abundance of particles trapped in silicone-treated collectors did not vary significantly with monofilament diameter. The heterogeneous arrangement of monofilament also proved to be an important factor. Combinations of monofilament of three different diameters (0.17, 0.55 and 0.90 mm), the thinnest being the upstream one, collected at least twice as many particles than all other single one-sized monofilament collectors. Particle abundance decreased with decreasing density of monofilament, and the densest arrangement of monofilament captured over four times more particles than the least dense arrangement. Collectors with an external bag of intermediate mesh size (0.4×0.5 cm) were the most efficient at collecting particles. The effect of flow speed was also investigated. Particles were trapped in significantly greater numbers at flow velocities of 5 cm s^-1 than at lower or higher velocities. These results are consistent with results of field experiments carried out to optimize the design of scallop collectors. Enhancement of spat collection is possible by modifying collectors. Results from both field and flume experiments confirm the hypothesis that larvae of the species collected in the field are passive at settlement at the scale of the collectors.Contribution to the program of GIROQ (Groupe Interuniversitaire de Recherches Océanographiques du Québec), and OPEN (Ocean Production Enhancement Network)     

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.     

Influence of temperature and food availability on the ecological energetics of the giant scallop Placopecten magellanicus

Scallops (Placopecten magellanicus) were collected in 1981–1983 from two water depths at a location in Newfoundland, Canada, where temperature and food conditions associated with shallower water have been shown to be more favourable for somatic growth and gamete production. To gain insight into the seasonal energy balance for this species, metabolic and clearance rates were measured monthly under ambient temperature conditions and natural seston levels. Stereological techniques were used to determine the gamete volume fraction in the gonad in order to establish the annual reporductive cycle. The less favourable conditions associated with deeper water were relfected in reduced rates of gamete development, but the diameter of spawned eggs and the timing of spawning appeared unaffected by poorer conditions in the natural environment. Estimates of scope for growth were low or negative during the winter, but consistently high during the spring bloom, corresponding to a period of rapid gamete maturation. Somatic weight declined in both populations as gamete development proceeded but increased again during periods of low gametogenic activity, suggesting a close relationship between energy available for growth and the reproductive cycle. Oxygen uptake and clearance rate varied seasonally in relation to ambient temperature and food conditions, all of which appeared to be interrelated in a complex fashion with the energy demands of gametogenesis.     

Relation of spawning of the giant scallop,Placopecten magellanicus,to temperature fluctuations during downwelling events

We examined the relation of spawning to biological and physical factors for the scallop Placopecten magellanicus (Gmelin, 1791) over 8 yr (1984 to 1991) in the Baie des Chaleurs, southwestern Gulf of St. Lawrence, Canada. Spawning was always abrupt and occurred between July and mid-September. It did not appear to be related to the abundance of phytoplankton or particulate organic carbon and nitrogen in the water. It further showed no relationship to lunar or tidal phascs or to current velocity. Spawning consistently occurred during the summer temperature maximum, but did not coincide with any critical temperature or cumulative temperature threshold. All but one of the 33 spawning events, for which temperature data were recorded, were associated with temperature changes; 25 of these were sharp temperature increases and 7 were during strong temperature fluctuations when the mean temperature was 9 to 14°C. Both types of temperature changes were caused by downwelling of warm surface water. The delay by about 1 d in time of spawning between sites coincided with the rate at which downwelling events propagated into the bay. Virtually all of the spawning events resulted in gametes being ejected into warm water masses where conditions are likely to favour larval development.     

Incorporation of dietary sterols by the sea scallop Placopecten magellanicus (Gmelin) fed on microalgae

The anatomical distributions of sterols and the incorporation of dietary phytosterols into different organs were studied in two populations of sea scallops, Placopecten magellanicus Gmelin, collected in 1989 from Georges Bank (Nova Scotia) and St. Pierre Bank (Newfoundland), respectively. In contrast to the well-established organ-specific lipid classes and fatty-acid compositions usually found in marine animals, the major organs of wild sea scallops (adductor muscle, digestive gland, gonads, gills and mantle) had the same sterol compositions. In order to know if anisomyarian bivalves require a uniform anatomical distribution of sterols, wild scallops were subjected to a microalgal diet containing high concentrations of brassicasterol, -sitosterol and cholesterol. The sterol composition of the scallop adductor muscle was not changed by 6 wk of feeding on the experimental diet. In contrast, the proportions of brassicasterol, -sitosterol and cholesterol in the digestive gland, and of brassicasterol and cholesterol in the male gonad, increased significantly (p<0.05). These results showed that the typical even anatomical distribution of sterols of bivalves can be disrupted by a drastic change in diet and is therefore not subject to strict internal regulation. Furthermore, the P. magellanicus results indicate that, although sea scallops may be capable of sterol biosynthesis, the incorporation of unmodified dietary phytosterols plays an influential role in establishing their sterol composition.ORNL is managed by Martin Marietta Energy Systems for the US Department of Energy, under contract DE-ACOS-8-10R21400     

Postingestive selection in the sea scallop (Placopecten magellanicus) on the basis of chemical properties of particles

Suspension-feeding bivalves are adapted to use a highly variable mixture of particles by sorting them before and after ingestion. Postingestive sorting in bivalves has been confirmed for several bivalve species, but few studies have attempted to isolate the factors influencing postingestive selection among different particles presented simultaneously. The ability of the sea scallop, Placopecten magellanicus (Gmelin), to sort a mixture of organic (¹⁴C-labeled dinoflagellates, Prorocentrum minimum) and inorganic (⁵¹Cr-labeled beads, diameter 16-18 µm) particles of similar size and shape within the stomach was examined. The study was carried out in August 1997 using scallops collected near Deer Island, New Brunswick, Canada. Sorting of the two particle types was measured by dissecting the scallops at intervals after feeding and comparing the ¹⁴C:⁵¹Cr ratios within the stomach, digestive gland, and feces. The ¹⁴C:⁵¹Cr ratio in the stomach decreased over time, indicating that sea scallops were sorting organic from inorganic particles. The ability of P. magellanicus to sort particles solely on the basis of chemical properties was tested in a second study by presenting them with a mixture of protein-coated and uncoated beads of two different colors. This study was carried out in February 2000 using scallops collected near Maces Bay, New Brunswick, Canada. Gut retention times of the two types of beads were measured using flow cytometry. Scallops retained protein-coated beads in the gut longer than uncoated beads, indicating postingestive selection by chemical properties. This study provides the first evidence for simultaneous postingestive sorting by bivalves of particles based solely on chemical properties. This would potentially enable P. magellanicus to preferentially retain particles of higher food quality longer than those of poor quality, thereby enhancing digestive efficiency.     

The response of sea scallop (Placopecten magellanicus) veligers to a weak thermocline in 9-m deep mesocosms

We examined the vertical distributions of scallop (Placopecten magellanicus) veligers in deep (0.6 m diameter, 9.5 m deep) polyethylene mesocosms from December 1991 to January 1992. In the mesocosms temperature stratification varied from 0 to 1.5 °C. Profiles of vertical distribution revealed several repeated patterns. Peaks in veliger numbers often appeared at the water surface and just above the thermocline. Higher density patches were seen below the surface peaks, and revealed the presence of bio-convective cells. Distribution away from these discontinuities was usually even. Distribution of veligers was affected by thermoclines above 1.0 °C. Responses to thermoclines varied with larval age and time of day, and 28 to 30 d veligers passed in both directions through a 1.5 °C thermocline. We conclude that larval behaviour is a major determinant of whether veligers pass through a thermocline. Kinematic viscosity may play a role in perception of temperature changes. Two potential consequences of such behaviour are (1) remaining in more productive upper water layers, where feeding opportunities are enhanced, and (2) increased horizontal transport in the region of the thermocline, which may enhance recruitment. Received: 15 May 1996 / Accepted: 11 May 2000     

Lipid composition of eggs and adductor muscle in giant scallops (Placopecten magellanicus) from different habitats

Scallops (Placopecten magellanicus Gmelin) were collected during August 1989 from shallow water (10 m) and deep water (31 m) habitats at Sunnyside, Trinity Bay, Newfoundland, to compare the lipid composition of eggs and adductor muscle tissue. Less favorable food levels and lower temperature conditions associated with deeper water have previously been shown to produce slower growth and reduced fecundity in individuals from this habitat. Triacylglycerol reserves consistently accounted for 60% of the total lipids present in both groups. The total lipid content of the eggs and the composition of their triacylglycerol fatty acid pools were similar in shallow water and deep water scallops, indicating very little if any nutritional difference between the two groups. Relative to their counterparts from shallow water, individuals from deeper water contained higher proportions of docosahexaenoic acid [22:6(n-3)] in the egg phospholipids and higher levels of 24-methylenecholesterol (a phytosterol commonly found in diatoms) in the adductor muscle. Differences in fatty acid composition are interpreted as biochemical adjustments of cell membranes to increase membrane fluidity, thereby compensating for the lower temperatures prevailing at the greater depth.     

Exfoliation of epithelial cells by the scallop Placopecten magellanicus : seasonal variation and the effects of elevated water temperatures

Previous studies have shown that large numbers of ciliated and nonciliated epithelial cells (diam.: 6 to 15 μm) are released by adult sea scallops, Placopecten magellanicus (Gmelin), during summer months in Newfoundland when water temperatures are at a maximum and gonads are well developed. Such exfoliation of cells could be a response to stresses associated with elevated water temperatures and/or spawning activity. In the present study an electronic particle counter/sizer was used to further investigate the factors that influence exfoliation of epithelial cells by juvenile and adult scallops throughout the year. We observed release of epithelial cells from juveniles, and from adults collected in months when gametogenic activity was minimal, indicating that exfoliation does not occur as a result of reproductive activity alone. SEM analysis revealed little difference in surface characteristics of the gills, mantle and gonad between scallops that had released cells and those that had not, suggesting that exfoliation of small numbers of cells may be a consequence of cellular turnover and normal physiological function. Adult scallops were monitored in a second experiment to determine the effects of raising water temperatures from 8.5 to 14.7 and to 21.0 °C on the frequency (proportion) and rate of cellular exfoliation. Only at the highest experimental temperature (21.0 °C) were exfoliation rates significantly higher than rates recorded at 8.5 or 14.7 °C. SEM analyses revealed some damage to gill, mantle and gonad tissues when scallops were exposed to 14.7 and then to 21.0 °C for a total of 8 d. Received: 21 August 1996 / Accepted: 13 September 1996     

Uptake and effects of copper and cadmium in the gonad of the scallop Placopecten magellanicus: concurrent metal exposure

Sea scallops Placopecten magellanicus in early gametogenesis from the southern shelf of Hudson Canyon, New Jersey, USA, were exposed in late winter 1984 to sublethal levels of Cu and Cd in a flowing seawater system at the NMFS Milford Laboratory. Exposure was to copper (10 and 20 gl^-1: low-Cu and high-Cu groups) or to a combination of copper and equimolar cadmium (10g Cu+17.7 g Cdl^-1: low-Cu/Cd group) for eight weeks, with sampling at 2-wk intervals. Copper had a strongly inhibitory effect on gamete production and maturation, which in some respects was partially moderated in the presence of cadmium in the female gonad only. Total gamete weight per scallop doubled in control individuals but dropped by 60% in both high-metal exposure groups over the 8-wk exposure period, with a smaller, temporary decrease in the low-Cu group. Cadmium did not add to the inhibition by copper of gamete development in the low-Cu/Cd group, but there was no partial recovery at 8 wk, as was seen in female scallops exposed to low-Cu alone. Gonadal RNA, higher in the females, decreased proportionately more in that sex than in the males of the metal exposure groups. Conversely, DNA levels were higher in the male than in the female gonad, and decreased sharply in all metal-exposed males. Gonadal protein concentration also dropped in all metal-exposed scallops with time and degree of metal exposure. Copper uptake in the gonad increased with time and metal exposure concentration, and cadmium increased in the low-Cu/Cd group with time. In contrast, manganese decreased significantly in the gonads of Cu-exposed scallops, especially in the high-Cu group. In the low-Cu/Cd group, manganese concentrations stabilized after an initial sharp drop at 2 wk in the low-Cu/Cd group, then rose at 6 and 8 wk. We ascribe this phenomenon to the induction by cadmium of metal-binding proteins (Fowler and Megginson 1986). In both sexes, gonadal magnesium concentrations did not change with increasing tissue burdens of copper and cadmium, but instead rose initially in proportion to the degree of metal exposure, homeostasis being maintained thereafter.     

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.     

Quantitative genetic analysis of growth in larval scallops (Placopecten magellanicus)

We estimated the broad-sense heritability of larval size in 20 full-sib families of the giant scallop, Placopecten magellanicus (Gmelin, 1791) grown in laboratory culture in August and September 1991. The goal was to compare scallops with other bivalves which have been shown to have significant heritabilities for larval growth. Secondly, we estimated the lipid content of occytes from female parents, since this is hypothesized to affect larval growth and survival. Finally, we estimated the among-family variation in mortality from 4 to 21 d as a test of possible genetic variation for viability among larval scallops. Significant genetic variation (h²=1.10 to 1.24) was estimated for larval shell length at 4, 14, and 21 d. There was a significant correlation (r=0.66) between larval size at 4 d and lipid content of oocytes, but only when two females with high levels of lipid oocyte^–1 were excluded as outliers. There was no correlation between larval size at 14 and 21 d and lipid content of oocytes. Mortality among families from 4 to 21 d was high (69 to 97%), and was significantly different among families. These results indicate that there is significant heritability for larval growth which is largely independent of the lipid content of the oocytes. A high heritability for larval growth may indicate that this trait is only weakly correlated with fitness.     

Settlement of larvae of the giant scallop,Placopecten magellanicus,in 9-m deep mesocosms as a function of temperature stratification,depth, food,and substratum

In January and February 1992 an experiment was conducted in a 10.5-m deep tank (diameter: 3.7 m, volume: 117 m³) to examine the effects of food distribution with respect to a stable thermocline, depth, and substratum type on the settlement and metamorphosis of larvae of the giant scallop, Placopecten magellanicus (Gmelin). Polyethylene tube bags (diameter: 0.60 m) were used to enclose 9-m deep columns of seawater which were then used as treatment replicates. A sharp thermocline (i.e. 7 to 11°C gradient) was created between a depth of 4.0 and 5.0 m. At the beginning of the experiment, one million 6-d old larvae were added to the surface of each tube. Two or three replicate tubes of each of four feeding treatments were established: (1) food (Isochrysis galbana) added to the top 1 m of the water column (top-fed, n=3); (2) food added to the bottom 1 m of the water column (bottom-fed, n=3); (3) food added throughout the water column (mixed, n=3); and (4) no food added (unfed, n=2). Settlement collectors were placed in two replicate tubes of each treatment at depths of 0.1, 4, 5, and 9 m and contained two different substrata, Polysiphonia lanosa (a red filamentous alga) and aquarium filter-wool as an algal mimic. Spat settlement in the different feeding treatments was a function of larval growth rate. Most spat were collected in the mixed tubes. Fewer individuals were collected in the top-fed treatment and fewer still in the bottom-fed treatment; minimal numbers of spat were found in the unfed tubes. Filter-wool collected more spat than P. lanosa, but this was evident only in the 4-m deep collectors in the mixed tubes. Most spat were found in the 0.1-m or 4-m deep collectors; generally few were located below the thermocline in collectors at 5 or 9 m. We suggest that, in areas of intense stable stratification, spat collection of the giant scallop may be enhanced by the placement of collectors with appropriate substratum material at or above the zone of stratification, rather than near the bottom. Furthermore, we propose that natural settlement may be increased in areas where a stratification layer intersects with the sea floor or where the layer is disrupted by turbulent mixing.Contribution to the programs of OPEN (Ocean Production Enhancement Network, one of the 15 Networks of Centres of Excellence supported by the Government of Canada) and GIROQ (Group Interuniversitaire de Recherches Océanographiques du Québec)     

Recruitment,growth and production of giant scallops (Placopecten magellanicus) along an environmental gradient in Baie des Chaleurs,eastern Canada

We measured population structure, growth and production of four beds of the giant scallop Placopecten magellanicus (Gmelin, 1791) in Baie des Chaleurs, southwestern Gulf of St. Lawrence, Canada in July 1992. Differences in the relative strength of the younger cohorts in each of the four beds, indicated marked differences in recruitment and/or survival in individual years over distances of 10 to 40 km. Part of this variability appeared to be related to environmental conditions prevailing during and after spawning. The growth rate of small individuals (<50 mm) was correlated with the gradient of increased temperature and food availability going up the bay. In all beds, both total production and reproductive effort increased with increasing age and then leveled off. Somatic production reached a maximum after 7 to 10 yr and then decreased for older scallops. Reproductive effort (percentage of production dedicated to reproduction) increased monotonically with age and, for any given age, followed the increase in temperature and food availability going from the mouth into Baie des Chaleurs. Marked differences in the contribution of different beds to the total reproductive output for Baie des Chaleurs are inferred from inter-bed differences in the lipid content of the oocytes, in the proportion of non-reproductive juveniles, and in individual reproductive output.     

Gill function and particle transport in Placopecten magellanicus (Mollusca: Bivalvia) as revealed using video endoscopy

The technique of endoscopic video observation was used to study feeding processes of Placopecten magellanicus (Gmelin), collected from Bull Arm, Newfoundland in August 1991 and 1992, under near-natural feeding conditions. The fate of captured particles depended on the extent of ingestive or handling capacity saturation. Under low (1 to 10 particles l^-1) to medium (10 to 20 particles l^-1) particle concentrations, most particles were incorporated in continuous anteriorly directed slurries in the dorsal ciliated tracts of the gill arch and dorsal bends. As particle concentration or exposure time to the lower particle concentrations increased, four endogenous mechanisms of ingestion volume control were increasingly observed: (1) rejection of dense mucus-particle masses from the principal filament troughs onto the ventrally beating cilia and associated currents of the ordinary filament plicae, counter to and below the incoming pallial current maintained by the principal filament cilia; (2) intermittent stopping of the anteriorward flow in the dorsal ciliated tracts; (3) reduction or cessation of input from the principal filaments to the dorsal ciliated tracts; (4) detachment of the dorsal bends from the mantle to establish a shunt from the infrabranchial to the suprabranchial cavity. Chemical and histochemical tests of purified fluid withdrawn from the dorsal ciliated tracts indicate that mucus is present at all particle concentrations. Mucus therefore participates both in normal feeding and in ingestion volume control on the bivalve gill, although different mechanisms, and types of mucus, effect transport of material in the dorsal (feeding) and ventral (cleaning) ciliated tracts.     

New observations of the gills of Placopecten magellanicus (Mollusca: Bivalvia), and implications for nutrition

The internal anatomy and microanatomy of the gill of Placopecten magellanicus Gmelin collected in May and November 1985 from Chamcook Bay, New Brunswick, Canada, was studied using thin-section light microscopy and transmission electron microscopy. Most of the spurs show no evidence of organic union, and hence do not participate in vascular exchange. However, the dorsal bend shows both ciliary and organic interfilamentar union. The internal structure and the hemocytes of the dorsal respiratory expansion are presented. The epithelium consists of three distinct cell types, bounded by apical microvilli. All regions of the gill contain an epithelial basal membrane, which is greatly convoluted in the interconnecting vessels of the dorsal respiratory expansion. The significance of these observations is discussed in relation to possible roles in respiration, transmembrane transport and nutrition. The apical surface of all ciliated cells is covered with an acellular matrix composed of clear spherical vesicles, which may serve a mechanical function for which mucus would be unsuited. The significance of the abundance of mucocytes on the abfrontal surface of the principal filaments is discussed in terms of the escape response of pectinid bivalves.     

New observations of the gills of Placopecten magellanicus (Mollusca: Bivalvia), and implications for nutrition

The organization, general anatomy, and surface microanatomy of all regions of the gills of a representive bivalve mollusc, Placopecten magellanicus Gmelin, were studied using stereo-microscopic, histological, and scanning electron microscopic techniques. Individuals were collected in May and November 1985 from Chamcook Bay, New Brunswick, Canada. In addition to correcting earlier accounts of this structure, a number of new observations are reported. The orientation of the ciliated spurs appears to be responsible for the sinusoidal arrangement of the gill filaments. Micrographs showing the structure of the dorsal respiratory expansion are presented. The entire abfrontal surface of the principal filament, including the dorsal respiratory expansion, is densely ciliated and mucosecretory. These characteristics may aid in the establishment of a respiratory current and in the prevention of gill damage during escape responses. All nonciliated regions of the gill filaments are covered with microvilli, thus greatly increasing the surface area of the gill. The feeding mechanism is discussed in relation to the dorsal and ventral ciliated tracts. Symbiotic ciliate protozoans are constantly dislodged from the gill filaments and transported via the ventral mucus string to the buccal region. The nutritional implications of these observations are discussed.     

Ontogenetic changes in the vertical distribution of giant scallop larvae,Placopecten magellanicus,in 9-m deep mesocosms as a function of light,food, and temperature stratification

To understand how thermal stratification and food abundance affects the vertical distribution of giant scallop larvae Placopecten magellanicus (Gmelin), a mesocosm study was conducted in January and February 1992. The position of larvae was followed over 55 d in replicated 9-m deep tanks in relation to a sharp thermocline and the presence or absence of phytoplankton. Growth and vertical position of larvae were monitored in separate treatments which included phytoplankton added above the thermocline, below the thermocline, throughout the mesocosm, or absent from the mesocosm. Changes in the vertical position of larvae over time were quantified with a new, profiling, video-optical instrument capable of semi-automatically identifying, counting and sizing larvae. The strong diurnal migration of scallop larvae resulted in aggregations at two interfaces: the air/water interface during the night, and at the thermocline during the day. At times, the concentration of larvae within cm of the surface was > 100 times that in the remaining water column. The formation of bioconvective cells of swimming larvae at the air/water interface allowed larval aggregations to persist throughout the period of darkness. Regardless of the distribution of food, larvae remained above the thermocline during most of the experiment. Therefore, only in those treatments where food was also present above the thermocline was larval growth relatively high. Larger larvae penetrated the thermocline only after reaching a shell length of about 200 m; thus larval size, rather than chronological age, was more important in describing their vertical distribution. The rapid increase in kinematic viscosity with decreasing water temperature at the thermocline may retard the movement of larvae and contribute to aggregation at this interface. The influence of larval size on their vertical distribution, and the resulting potential for horizontal transport to settlement sites, points to the importance of persistent hydrographic features as critical factors contributing to settlement variance in scallops.     

Allozyme and physiological variation in the scallop Placopecten magellanicus and a general model for the effects of heterozygosity on fitness in marine molluscs

Heterozygosity and growth rate have been correlated in many molluscs, although the phenomenon is not universal. Enhanced growth of heterozygous molluscs has been attributed to lower energetic requirements for basal metabolism. We have investigated heterozygote deficiency, and the correlation between single-locus and multiple-locus heterozygosity and phenotype in juveniles of the scallop Placopecten magellanicus (Gmelin). Six samples were collected between 22 November 1984 and 11 December 1985 at Passamaquoddy Bay, New Brunswick, Canada. On average, heterozygote deficiency was small, although somewhat higher at the octopine dehydrogenase (Odh) locus, and decreased with age. No correlation was observed between genotype and growth rate. This result and published records indicate that allozyme heterozygosity of pectinids does not influence growth to the same degree as in other bivalves. However, we have observed in P. magellanicus a correlation between allozyme heterozygosity and octopine accumulation, a trait that relates to functional anaerobiosis, and may be related to the scallop's scope for movement. We combine these results and results from the literature into a model that relates the hypothesis of associative overdominance (at the population genetics level) with the hypothesis of energetic efficiency (at the physiological level). The model assumes that energy savings attributed to heterozygosity are used to maximize fitness. In freely moving molluscs this results in enhanced activity (such as searching for prey or swimming away from a predator), and in sessile molluscs either in enhanced somatic growth in juveniles or gonadal growth in adults.