Impact of Environment and Ontogeny on Relative Fecundity and Egg Quality of Female Oysters (Crassostrea virginica) from Four Sites in Northern Chesapeake Bay.

Resource allocation to reproduction is a primary physiological concern for individuals, and can vary with age, environment, or a combination of both factors. In this study we quantified the impact of environment and individual age on the reproductive output of female oysters Crassostrea virginica. We determined the relative fecundity, egg total lipid content, and overall and omega-3/omega-6 (ω3/ω6) fatty acid signatures (FAS) of eggs spawned by female oysters over a 2-year period (n = 32 and n = 64). Variation was quantified spatially and ontogenetically by sampling young and old oyster populations from two rivers in Chesapeake Bay, totaling four collection sites. During Year 1, when oysters underwent oogenesis in different locations, overall and ω3/ω6 egg FAS varied significantly by river, with no significant differences observed in the FAS of oysters by age in Year 1. In Year 2, when oysters from different sites underwent oogenesis in a single location, no significant differences in the overall egg FAS or ω3/ω6 egg FAS by river or age were observed. These findings suggest that oysters integrate environment into their reproductive output, but that time spent growing at a specific location (in this case, represented by oyster age) plays a relatively minor role in the biochemical composition of oyster eggs. These results have consequences for our understanding of how resources are allocated from the female oyster to eggs and, more generally, the impact of environment and ontogeny on reproductive physiology.

Composition, Shell Strength, and Metabolizable Energy of Mulinia lateralis and Ischadium recurvum as Food for Wintering Surf Scoters (Melanitta perspicillata).

Decline in surf scoter (Melanitta perspicillata) waterfowl populations wintering in the Chesapeake Bay has been associated with changes in the availability of benthic bivalves. The Bay has become more eutrophic, causing changes in the benthos available to surf scoters. The subsequent decline in oyster beds (Crassostrea virginica) has reduced the hard substrate needed by the hooked mussel (Ischadium recurvum), one of the primary prey items for surf scoters, causing the surf scoter to switch to a more opportune species, the dwarf surfclam (Mulinia lateralis). The composition (macronutrients, minerals, and amino acids), shell strength (N), and metabolizable energy (kJ) of these prey items were quantified to determine the relative foraging values for wintering scoters. Pooled samples of each prey item were analyzed to determine composition. Shell strength (N) was measured using a shell crack compression test. Total collection digestibility trials were conducted on eight captive surf scoters. For the prey size range commonly consumed by surf scoters (6-12 mm for M. lateralis and 18-24 mm for I. recurvum), I. recurvum contained higher ash, protein, lipid, and energy per individual organism than M. lateralis. I. recurvum required significantly greater force to crack the shell relative to M. lateralis. No difference in metabolized energy was observed for these prey items in wintering surf scoters, despite I. recurvum's higher ash content and harder shell than M. lateralis. Therefore, wintering surf scoters were able to obtain the same amount of energy from each prey item, implying that they can sustain themselves if forced to switch prey.

Shell hardness and compressive strength of the Eastern oyster, Crassostrea virginica, and the Asian oyster, Crassostrea ariakensis.

The valves of oysters act as a physical barrier between tissues and the external environment, thereby protecting the oyster from environmental stress and predation. To better understand differences in shell properties and predation susceptibilities of two physiologically and morphologically similar oysters, Crassostrea virginica and Crassostrea ariakensis, we quantified and compared two mechanical properties of shells: hardness (resistance to irreversible deformation; GPa) and compressive strength (force necessary to produce a crack; N). We found no differences in the hardness values between foliated layers (innermost and outermost foliated layers), age class (C. virginica: 1, 4, 6, 9 years; C. ariakensis: 4, 6 years), or species. This suggests that the foliated layers have similar properties and are likely composed of the same material. The compressive force required to break wet and dry shells was also not different. However, the shells of both six- and nine-year-old C. virginica withstood higher compressive force than C. virginica shells aged either one or four, and the shells of C. ariakensis at both ages studied (4- and 6-years-old). Differences in ability to withstand compressive force are likely explained by differences in thickness and density between age classes and species. Further, we compared the compressive strength of differing ages of these two species to the crushing force of common oyster predators in the Chesapeake Bay. By studying the physical properties of shells, this work may contribute to a better understanding of the mechanical defenses of oysters as well as of their predation vulnerabilities.

Isolation by distance in the eastern oyster, Crassostrea virginica, in Chesapeake Bay.

Intensive efforts are underway to restore depleted stocks of Crassostrea virginica in Chesapeake Bay. However, the extent of gene flow among local populations, an important force mediating the success of these endeavors, is poorly understood. Spatial and temporal population structures were examined in C. virginica from Chesapeake Bay using eight microsatellite loci. Deficits in heterozygosity relative to Hardy-Weinberg expectations were seen at all loci and were best explained by null alleles. Permutation tests indicated that heterozygote deficiency reduced power in tests of differentiation. Nonetheless, genotypic exact tests demonstrated significant levels of geographic differentiation overall, and a subtle pattern of isolation by distance (IBD) was observed. Comparisons between age classes failed to show differences in genotype frequencies, allelic richness, gene diversity, or differentiation as measured by F(ST), contrary to predictions made by the sweepstakes hypothesis. The IBD pattern could reflect an evolutionary equilibrium established because local gene flow predominates, or be influenced in either direction by recent anthropogenic activities. An evolutionary interpretation appears justified as more parsimonious, implying that local efforts to restore oyster populations will have local demographic payoffs, perhaps at the scale of tributaries or regional subestuaries within Chesapeake Bay.