Crassostrea virginica , eastern

Background

The inhabits waters from the Gulf of St. Lawrence to the Gulf of Mexico and West Indies. DNA evidence suggests there are genetic differences in over very broad spatial scales (Hoover and Gaffney 2005, Reeb and Avise 1990). A profile for the eastern oyster was provided by Stanley and Sellers 1986 and by http://www.dfo-mpo.gc.ca/zone/underwater_sous-marin/oyster/oyster-huitre_e.htm . Oysters live on firm, stable bottom in brackish waters of sheltered bays and estuaries, usually at depths of two to three meters. This species form aggregations in the form of reefs which can be very large. Adults are sessile. Oysters are filter feeders. In the Maritime provinces the eastern oyster grows from May to late November with a recess in July for spawning. Over the species range the spawning season extends from spring to fall with fertilization occurring in the water column. Approximate early life stage durations are eggs six to nine hours, larvae two to three weeks after which they settle to the bottom and crawl around searching for suitable substrate of shells or stones to which they soon affix. The juvenile stage lasts for a few weeks only. Oysters are fished and cultured commercially in Prince Edward Island, New Brunswick, and Nova Scotia waters of the southwest Gulf of St. Lawrence, and in U.S. waters from Maine to Texas.

Temperature limits, critical thresholds, vulnerability, and barriers to adaptation

The southern distribution limit of the eastern oyster is well south of 25 oN latitude, the southern margin of our study area. Thus, the lower tolerance limit for this species of - 2.1 oC was its February minimum in our study area determined from AVHRR data, and an estimated upper thermal tolerance limit of 35 oC was chosen based on the literature.

Thermal criteria for eastern oysters are from Burrell (1986) and references therein, Stanley and Sellers (1986) and references therein, http://www.tpwd.state.tx.us/huntwild/wild/species/easternoyster/ , and http://www.dfo-mpo.gc.ca/zone/underwater_sous-marin/oyster/oyster-huitre_e.htm . Eastern oysters from different areas have different thermal requirements related to their thermal history, but perhaps with a genetic component over a very large spatial scale (Gulf of St. Lawrence, mid-Atlantic, Gulf of Mexico). Reported natural temperature limits for the eastern oyster are -2o-36 oC. In Canada, other than the warm, shallow bays and estuaries of the southwestern Gulf of St. Lawrence and coves of the Bras d’Or Lakes, water temperatures are too cold for reproduction and suitable growth. Below 6 o-8oC feeding and growth are greatly reduced or cease. Below about 4 oC oysters close their shells until the water warms again. Though the critical thermal maximum has been reported as 48 oC, water temperatures above 35 oC for a complete tidal cycle have caused mortality. Adults grow at temperatures of 10 o-30 oC or higher though growth, reproduction, and survival are optimized from 20 o-30 oC (to 26 oC in Canadian waters). Spawning temperatures differ across large regions: 16 oC in New England, 20 oC in the mid-Altantic states, and 25 oC in the Gulf of Mexico. In its warmest habitats oyster

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spawning may be limited by temperatures exceeding 35 oC. Reported temperature ranges for development of Atlantic populations in the laboratory vary. Normal embryonic development occurs at 20 o-25 oC or 30 oC, and abnormalities increase progressively below 15 oC and above 25 o- 35 oC. Larval development is normal above 20 oC; growth and survival are best from 27.5 o-30 oC or 32.5 oC. A serious pathogen of the eastern oyster from Delaware to Mexico is the protozoan Perkinsus marinus . Infections are lethal during periods of elevated summer water temperatures.

Eastern oysters were somewhat above average rank in the sensitivity analysis. This is attributable to their mobility as planktonic eggs and larvae which may aid dispersal and their very eurythermal nature. There appears to be little difference among life stages in potential vulnerability to global warming as their thermal criteria are similar.

Impacts

A 4 oC rise in global temperature should have no negative impact on the future distribution of eastern oysters in our study area since predicted August maximum SSTs will reach only about 32oC (Chapter 2), well below the estimated upper thermal tolerance limit of 35 oC. However, there is potential for a positive impact. Currently Canadian Atlantic waters in addition to those of the southern Gulf of St. Lawrence and Bras d’Or Lakes are suitable for oysters based on February minimum and the upper tolerance limit temperatures, but oysters don’t occupy these waters. This likely is because on an annual basis those waters are too cold for reproduction and necessary growth. With global warming February minimum and August maximum temperatures in Canadian Atlantic waters will remain suitable for oysters, and there is potential for temperatures on an annual basis to increase sufficiently for oysters to expand their Canadian range. This situation would allow growth in the Canadian oyster industry. No negative impact on commercial fishing in U.S. waters in our study area is predicted. Elevated temperatures due to global warming may increase the susceptibility of oysters to trace metal pollution (Sokolova 2004).

References

Burrell, V.G., Jr., 1986. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (South Atlantic) – American oyster. U.S. Fish and Wildlife Service Biological Report 82(11.57). U.S. Army Corps of Engineers TR EL- 82-4. 17 pp. Reeb, C.A., and J.C. Avise. 1990. A genetic discontinuity in a continuously distributed species: mitochondrial DNA in the American oyster, virginica . Genetics 124: 397-406. Hoover, C.A., and P.M. Gaffney. 2005. Geographic variation in nuclear genes of the eastern oyster, Crassostrea virginica Gmelin. Journal of Shellfisheries Research January 2005.

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Sokolova, I.M. 2004. Cadmium effects on mitochondrial function are enhanced by elevated temperatures in a marine poikilotherm, Crassostrea virginica Gmelin (: Ostreidae). Journal of Experimental Biology 207: 2639-2648. Stanley, J.G., and M.A. Sellers. 1986. species profiles: life histories and environmental requirements of coastal fishes and invertebrates (Gulf of Mexico) – American oyster. U.S. Fish and Wildlife Service Biological Report 82(11.64). U.S. Army Corps of Engineers, TR EL-82-4. 25 pp.

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