Oyster (Crassostrea virginica,Gmelin) spat settlement and copper in aufwuchs

Oysters from paired sites of high and low spatfall at three mid-Chesapeake Bay locations were analyzed for tissue copper and tissue zinc as well as copper in aufwuchs scraped from the upper (right valve) shell surface. Paired sites had no significant differences in oyster tissue copper, aufwuchs copper or oyster tissue copper-zinc ratio. In the laboratory, natural aufwuchs material on oyster shell rapidly concentrated copper up to 10× from copper-enriched estuarine water, with a partition coefficient (Ka) of 0.640 I kg^?1. Aufwuchs-sorbed copper resisted depuration, suggesting a strong metal-binding substance in aufwuchs material. Eyed veliger oyster larvae (setting stage) exposed to oyster shell fragments having copper-enriched aufwuchs showed normal setting preference for bottoms and edges of shell surfaces but a slight decrease in total set with increasing aufwuchs copper concentration. Settled oyster spat died or failed metamorphosis with LD50=534 μg Cu g^?1 aufwuchs. Since mid-Bay aufwuchs copper concentrations averaged 35.5 μg^?1, ranging up to 103 μg g^?1, oyster spatfall probably is not affected solely by present natural aufwuchs copper concentrations, but the potential for problems exists.     

Viability of American oyster,Crassostrea virginica,spermatozoa exposed to stress

Viability was determined for oyster spermatozoa maintained in salt water for extended periods after activation and for spermatozoa exposed to thermal and chemical stress. Determinations were based on methods that employ motile gametes and gametes in which eosin-nigrosin stain is repelled as estimates of viability. Motility of spermatozoa maintained in salt water at 25C diminished from 90% to 10% at 173 h after activation. Number of unstained gametes decreased from 95% to 26% at 10 h after activation, and further declined to 6% at 173 h. Spermatozoa exposed to temperatures of ?20, 50, and 60 C, and immersed in a phenol-formaldehyde-alcohol solution exhibited motilities of 43, 38, 11, and 7%, respectively, as compared to 90% for unstressed spermatozoa. Percent of unstained gametes after exposure to the above-described stresses was 59, 31, 7, and 0%, respectively, in contrast to 93% for unstressed controls. Correlation between results generated by the motility and staining methods of estimating viability is not significant at the 0.95 probability level.     

Bioaccumulation of heavy metals in oyster (Crassostrea virginica) tissue and shell

Oysters and sediment have been collected from most major US Gulf of Mexico bays and estuaries each year since 1986. Selected samples of oyster soft tissue, shell and sediments were analyzed for Cd, Cr, Cu, Fe, Mn, Pb, and Zn for this study. Concentrations varied considerably from place to place but ratios of metals remained relatively constant. Cu and Zn are greatly enriched in oyster tissues, which is related to their physiological function. Cd is enriched in oyster shell because of the easy substitution between Cd and Ca. The concentrations of Pb and Cr in oysters are significantly lower than that in sediment, suggesting a good discrimination against these metals by oysters. Metal variations are a result of both nature and human activity. Received: 13 September 1999 · Accepted: 8 December 1999     

Variations in trace metal concentrations in American oyster (Crassostrea virginica) collected from Galveston Bay, Texas

Nearly one thousand oysters (Crassostrea virginica) were collected at 15 sites on four sampling trips to Galveston Bay during 1992–1993. Iron, silver, arsenic, cadmium, copper, lead, and zinc were determined in the whole soft part of the individual oysters. Trace metal concentrations were found to vary among individuals within a site on a given trip, and spatially and temporally around the Galveston Bay estuarine system. Differences in metal concentrations from site to site were a factor of 5 or more for some metals, and similar differences were found from time to time at a given site. Zinc was found in anomalously high concentrations at several sites, mainly along the west side of Galveston Bay. Isolated high values of other metals were found scattered throughout the bay; however, only one site (GBSL, near Swan Lake in lower Galveston Bay) was found to have significantly higher than average concentrations for several metals (Ag, Cu, Pb, and Zn). One obvious trend found in this study was that metal concentrations, except for As, were highest in the summer, followed by the fall, and lowest in the winter and spring. The temporal changes in metal concentrations in oysters cannot be explained by changes in body weight in different seasons, and are apparently caused by changes in environmental conditions (e.g., metal input, salinity) or biological activity (e.g., metabolism, depuration).     

Selective feeding and grazing rates of oyster (Crassostrea virginica) larvae on natural phytoplankton assemblages

Results of laboratory feeding experiments suggest that oyster (Crassostrea virginica) larvae select small phytoplanktonic organisms (<10 μm) over larger-celled forms from natural estuarine assemblages, but that little selection occurs within the small phytoplankton fraction. Laboratory-reared larvae grazed each of five small-celled phytoplankton groups enumerated (coccoids, centrate diatoms, pennate diatoms, dinoflagellates, and flagellates) at rates proportional to phytoplankton group densities in controls (no larvae added). Larval grazing rates (number of cells removed per larva per hour) were inversely proportional to both larval density and experimental duration.     

Faunal utilization of created intertidal eastern oyster (Crassostrea virginica) reefs in the southeastern United States

Oyster cultch was added to the lower intertidal marsh-sandflat fringe of three previously createdSpartina alterniflora salt marshes. Colonization of these created reefs by oysters and other select taxa was examined. Created reefs supported numerous oyster reef-associated faunas at equivalent or greater densities than adjacent natural reefs. Eastern oyster (Crassostrea virginica) settlement at one site of created reef exceeded that of the adjacent natural reefs within 9 mo of reef creation. After only 2 yr, harvestable-sizeC. virginica (>75 mm) were present in the created reefs along with substantial numbers ofC. virginica clusters. The created reefs also had a higher number of molluscan, fish, and decapod species than the adjacent natural reefs. After 2 yr the densities ofC. virginica, striped barnacle (Balanus amphitrite), scorched mussel (Brachidontes exustus), Atlantic ribbed mussel (Geukensia demissa), common mud crab (Panopeus herbstii), and flat mud crab (Eurypanopeus depressus) within the created reefs were equivalent to that of adjacent natural reefs. From these data it is evident that created oyster reefs can quickly acquire functional ecological attributes of their natural counterparts. Because the demand for oysters continues to increase in the face of dwindling natural resources, habitat creation techniques need to evolve and these approaches need to consider the ancillary ecological benefits reef creation may provide. Reef function as well as physical and ecological linkages of oyster reefs to other habitats (marsh, submerged aquatic vegetation, and bare bottom) should be considered when reefs are created in order to provide the best use of resources to maintain the integrity of estuarine systems.     

Young-of-the-year oyster,Crassostrea virginica,reproduction in coastal Georgia

Young-of-the-year (YOY) oysters (Crassostrea virginica) in the Gulf of Mexico mature sexually and spawn in their first year. This study determined whether YOY oysters in the southeastern United States also mature and spawn in their first year. In 1991 and 1993, 300 YOY oysters were marked in May and 20–30 were sampled in subsequent months along with 20 adult controls. Two sites were chosen in 1991 (House Creek and Skidaway River) and one in 1993 (Skidaway River). At House Creek, YOY oysters were mature and spawned in September. YOY oysters at Skidaway spawned in October. Adult oysters did not appear to spawn in 1991. In 1993, the adult oysters spawned in August and September whereas, the YOY oysters spawned heavily in October. Oyster recruitment in coastal Georgia extends through October. The potential contribution of YOY oysters to this late season set is substantial, given that the YOY oysters are relatively large (≈4 cm) and are found in greater numbers than the adults. Oysters are capable of having two, if not more, generations within one year, prior to exposure of disease-causing organisms. Also, this particular reproductive strategy would increase the oysters suitability as a test subject in population genetic studies.     

The effects of hydrocarbons on the setting of the American oyster,Crassostrea virginica,in intertidal habitats in Southeastern North Carolina

Effects of petroleum covered substrate on intertidal oyster spat (Crassostrea virginica) set were measured at three intertidal elevations in a southeastern North Carolina estuary.Mercenaria mercenaria shells were coated with Bunker C crude oil or a 40∶1 mixture of gasoline: 2-cycle engine oil and placed intertidally for seven 13-d periods. Spat densities were significantly lower on oil treatments versus control and gas-treated shells in the high intertidal zone. This was principally attributed to an increased sediment coat on oiled shells. Maximum spat size was smaller on oil-treated shells at all elevations when compared to gas and control shells, indicating that setting may be delayed on oiled shell. For all experimental 13-d periods in the low intertidal zone and for three periods in the mid-tidal zone, barnacle densities (primarilyBalanus improvisus andB. eburneus) were significantly greater on oiled shells than on control shells.     

Genetic variation in the oyster,Crassostrea virginica (Gmelin), in relation to environmental variation

Four polymorphic loci in seven natural oyster (Crassostrea virginica Gmelin) populations of the Chesapeake Bay were analyzed by protein-gel electrophoresis for genetic variation. Samples of about 100 each were taken in 1977 along environmental gradients from stations in the James and Potomac rivers. The distribution of genotypes at four loci for each of the stations was nog significantly different from Hardy-Weinberg expectations. At two loci (Pgi andLap-2), a pool of all seven stations was also in equilibrium. Significant genetic differentiation between rivers was detected for two other loci (Pgm-1 andLap-1), and frequency of one genotype (Lap-1 ^95/95) was significantly correlated with mean temperature over the water monitoring period (1972–1977).     

Habitat partitioning between the xanthid crabsPanopeus herbstii andEurypanopeus depressus on intertidal oyster reefs (Crassostrea virginica) in southeastern North Carolina

The abundances of the xanthid crabsPanopeus herbstii andEurypanopeus depressus were examined relative to surface oyster shell cover, surface oyster cluster volume, subsurface shell content, substrate sand and silt composition, and oyster reef elevation. During August 1986 through July 1987, xanthid crabs were collected monthly from twelve 0.25 m²×15 cm deep quadrats, during low tide, from intertidal oyster reefs in Mill Creek, Pender County, North Carolina, USA, with respective quadrat details recorded. The abundance ofP. herbstii, and to a lesser degreeE. depressus, was positively correlated with surface shell cover. The abundance ofE. depressus, and to a lesser degreeP. herbstii, was positively correlated with surface cluster volume. The majority ofP. herbstii inhabited the subsurface stratum of the oyster reef, whereas the majority ofE. depressus inhabited the cluster stratum. Seasonality (i.e., temperature) appeared to influence the strata habitation of both species, with a higher incidence of cluster habitation during warmer months and a lower incidence during colder months. Crab abundance was not related to other factors examined, such as subsurface shell, substrate sand and silt composition, or elevation within the oyster reef. The analyses show thatP. herbstii andE. depressus have partitioned the intertidal oyster reef habitat, withE. depressus exploiting surface shell clusters andP. herbstii the subsurface stratum.     

Recruitment and growth of the Eastern oyster,Crassostrea virginica,in North Carolina

The effects of location, salinity, and depth on recruitment and growth of the eastern oyster Crassostrea virginica in Pamlico and Core sounds, North Carolina, were investigated from 1988 to 1990. We measured length and density of spat settling on oyster cultch deployed at deep (～3 m) and shallow (～1 m) depths at six sites in areas with low salinity and six sites in areas with high salinity. These data were compared with similar data taken at some of these sites by the North Carolina Division of Marine Fisheries since 1981 as part of their cultch planting program. Recruitment was generally greater in the high salinity sites, compared to the low salinity sites. Recruitment was less at shallow depths compared to deeper depths. In all three years the highest recruitment occurred in August and September, corresponding to the months of maximum water temperature. Recruitment was highly variable in space and time, but appeared to diminish from 1988 to 1990. Recruitment was reduced by sedimentation and a variety of sessile organisms. All sites appeared to have a similar potential for growth.     

The relationship between trends in a condition index of the American oyster,Crassostrea virginica,and environmental parameters in three Virginia estuaries

Oyster Condition Index (CI) was partitioned, using a moving average filter, into seasonal cycles and long-term trends in the James, York, and Rappahannock rivers for the period 1970–1983. Seasonal cyclic fluctuations in CI could be explained partially by changes in salinity and number of days within various temperature regimes. Long-term trends in the James River show a steady increase in CI over the study period, while a concurrent decline was noted in the Rappahannock River. Superimposed on these trends is a 4 to 5 yr cycle that is in synchrony with river discharge (salinity). In the York River, CI peaked in 1975–1976 at all stations. Measured environmental parameters do not sufficiently explain the trends. We speculate that the differences in the Rappahannock and James rivers may be due to a decline in bottom oxygen as a result of gravitational circulation differences.     

Silver and copper accumulation in two estuarine bivalves,the eastern oyster (Crassostrea virginica) and the hooked mussel (Ischadium recurvum) in the Patuxent River estuary,Maryland

In order to assess the source of trace elements and to compare uptake between different bivalve species, oysters and mussels were transplanted to five sites in the upper Patuxent River estuary during 1988. Transplant sites were located above and below the discharge of Chalk Point Steam Electric Station (CPSES)—a historic point source for copper. Organisms were sampled approximately monthly for 1 yr and analyzed for copper and silver. During spring and summer, concentrations of silver and copper increased in oysters at the upstream stations. There was very little change in silver and copper concentrations in the mussels with either time or among stations. Copper concentrations accumulated by oysters approached those found during previous periods when CPSES used Cu?Ni alloy condenser tubes (1966–1987), suggesting that corrosion from the Cu?Ni condenser tubes was only a minor contributor to the copper burdens of oysters in the river nearby. The lack of accumulation by mussels at the same sites suggests that part of the reason for the accumulation by oysters may be a taxonomically specific physiological effect caused by the salinity regime in the upper Patuxent.     

Biological Assessment of Eastern Oysters (Crassostrea virginica) Inhabiting Reef,Mangrove, Seawall,and Restoration Substrates

The eastern oyster, Crassostrea virginica, plays an essential functional role in many estuarine ecosystems on the east and Gulf coasts of the USA. Oysters form biogenic reefs but also live on alternative intertidal substrates such as artificial surfaces and mangrove prop roots. The hypothesis tested in this study was that non-reef-dwelling oysters (i.e., those inhabiting mangrove, seawall, or restoration substrates) were similar to their reef-dwelling counterparts based upon a suite of biological parameters. The study was carried out at six sites in three zones in Tampa Bay on the west coast of Florida using monthly samples collected from October 2008–September 2009. The timing of gametogenesis and spawning, fecundity, and juvenile recruitment were the same for oysters in all four habitats. Oyster size (measured as shell height), density, and Perkinsus marinus infection intensity and prevalence varied among habitats. This study indicates that oysters on mangroves, seawalls, and oyster restoration substrates contribute larvae, habitats for other species, and likely other ecosystem benefits similar to those of intertidal oyster reefs in Tampa Bay. Oysters from alternative intertidal substrates should be included in any system wide studies of oyster abundance, clearance rates, and the provision of alternate habitats, especially in highly developed estuaries.     

The effect of shell infestation byPolydora sp. and infection byHaplosporidium nelsoni (MSX) on the tissue condition of oysters,Crassostrea virginica

A number of stressors, including the endoparasiteHaplosporodium nelsoni (MSX), reduce metabolic condition of oysters; however, previous studies of the effect of the shell-boring polychaetePolydora sp. on condition have produced conflicting results. To assess the effects of both symbionts, singly and in combination, we examined 214 oysters for shell infestation byPolydora sp. mud blisters (by image analysis), intensity of infection byH. nelsoni (by histology), and nutritional state (estimated by soft tissue gravimetric condition index). Among all oysters tested, condition index was negatively (p <0.05) correlated withPolydora sp. infestation, although the explained variance (r²) was only about 5%. When oysters withoutH. nelsoni were considered alone, the variance explained byPolydora sp. rose to approximately 30%, indicating that the presence of one deleterious symbiont can confound the effect of another. In oysters withoutH. nelsoni, a 36% loss in tissue condition was associated withPolydora sp. blister infestation covering 45% of the shell. After a single season's exposure to both symbionts, oysters withH. nelsoni had significantly (p<0.05) greaterPolydora sp. infestation than did animals withoutH. nelsoni, but the difference disappeared after a second year, when nearly all shells were infested. The mean condition index for oysters with bothPolydora sp. andH. nelsoni was 13% lower than that for individuals with neither; oysters with only one symbiont had intermediate values. The interaction between percent blister coverage andH. nelsoni intensity on tissue condition was significant (p<0.002), verifying that the effect of the two symbionts was not additive. We conclude thatPolydora sp. reduces the ability of oysters to accumulate nutritional reserves, but that this effect may be obscured by other deleterious influences.     

Swimming speeds of oyster larvaeCrassostrea virginica in different salinities and temperatures

Swimming speeds of oyster larvaeCrassostrea virginica were determined in constant and increasing salinities to learn more about the oyster larval “salinity response”. “Normal” non-directed swimming speeds ranged from less than 1 cm/min for early veligers to 5 cm/min for “eyed” veligers with temperature an important variable. When subjected to hourly salinity increases of 0.5‰, most larvae swam upward or downward at approximately 3 times the above speeds. Larvae with values closed in response to traces of formalin sank at speeds of 5 to 50 cm/min depending on larval stage. The results may explain the differential vertical position of larval stages in estuaries and suggest the presence of a taxic component to the salinity response.     

Habitat associations of estuarine species: Comparisons of intertidal mudflat, seagrass (Zostera marina), and oyster (Crassostrea gigas) habitats

The complexity of habitat structure created by aquatic vegetation is an important factor determining the diversity and composition of soft-sediment coastal communities. The introduction of estuarine organisms, such as oysters or other forms of aquaculture, that compete with existing forms of habitat structure, such as seagrass, may affect the availability of important habitat refugia and foraging resources for mobile estuarine fish and decapods. Fish and invertebrate communities were compared between adjacent patches of native seagrass (Zostera marina), nonnative cultured oyster (Crassostrea gigas), and unvegetated mudflat within a northeastern Pacific estuary. The composition of epibenthic meiofauna and small macrofaunal organisms, including known prey of fish and decapods, was significantly related to habitat type. Densities of these epifauna were significantly higher in structured habitat compared to unstructured mudflat. Benthic invertebrate densities were highest in seagrass. Since oyster aquaculture may provide a structural substitute for seagrass being associated with increased density and altered composition of fish and decapod prey resources relative to mudflat, it was hypothesized that this habitat might also alter habitat preferences of foraging fish and decapods. The species composition of fish and decapods was more strongly related to location within the estuary than to habitat, and fish and decapod species composition responded on a larger landscape scale than invertebrate assemblages. Fish and decapod species richness and the size of ecologically and commercially important species, such as Dungeness crab (Cancer magister), English sole (Parophrys vetulus), or lingcod (Ophiodon elongatus), were not significantly related to habitat type.     

Boccardia hamata (Polychaeta:spionidae): A potential pest of the American oyster in the James River,Virginia

During a comprehensive survey of fauna associated with James River oyster reefs in 1971–72, a large population of the spionid polychaeteBoccardia hamata was found. The abundance of this boring species was greater than that of the well-known oyster pestPolydora websteri. B. hamata has not been reported previously in the Chesapeake Bay system and its occurrence on the east coast of the United States is not common. The large numbers in 1971–72 and continued presence ofB. hamata in 1975 suggests that the species may be well established in the James River. This may have some implications to the oyster industry if the species is as harmful as other boring spionids.