American oyster, continued 



U.S. Egg, larval, juvenile, and adult stages all occur in 

 mesohaline to euhaline environments in depths up to 

 10 m (Galtsoff 1964, Bahr and Lanier 1981, Burrell 

 1986). Price (1954) discusses the various develop- 

 ment, shapes and location of oyster reefs with respect 

 to shoreline, channels and distance from the Gulf. 

 Reefs grow from the shoreline out; as a current is 

 encountered the reef turns to a right angle and parallels 

 the current, eventually turning back on itself. Other 

 reefs grow parallel to channels. Oysters can grow and 

 survive over a wide range of environmental conditions, 

 but they are most successful when attached to firm 

 substrate in areas where water circulation provides 

 sufficient food (Berrigan et al. 1991). The preferred 

 habitats are estuarine intertidal areas, shallow bays, 

 other oyster shell and hard surfaces, mud flats and 

 offshore sand bars (Butler 1954, Marshall 1954, 

 Copeland and Hoese 1966, Menzel et al. 1966). The 

 intertidal zone affords oysters some protection from 

 predation by carnivorous gastropods and other com- 

 mon oyster predators (Marshall 1954). Wild popula- 

 tions of oysters need to be in the vicinity of freshwater 

 discharges such as rivers, creeks, and bayous (Berrigan 

 et al. 1 991 ). These discharges provide food and dilute 

 the higher salinity waters of the Gulf of Mexico. The 

 resulting moderate salinity habitats that are created 

 are necessary forsuccessful oyster settling and growth, 

 and provide protection from high salinity predators and 

 disease. 



Substrate : Hard, elevated substrates provide increased 

 surface area on the bottom to help support oysters as 

 they grow and prevent them from sinking into the 

 sediment and smothering (Marshall 1954, Berrigan et 

 al. 1991). Any type of hard substrate such as glass, 

 rock, concrete, metal, wood, rubber, or shell is suitable 

 for settlement of oyster spat (Burrell 1 986, Berrigan et 

 al. 1991). Oyster reefs are typically on hard bottoms, 

 but individuals are also abundant on surrounding mud 

 bottoms. Maximum setting occurs on horizontal sur- 

 faces (Clime 1 976). Larvae do, however, show prefer- 

 ence for established oyster beds, responding perhaps 

 to pheromones, ammonia, or other metabolites re- 

 leased by adult oysters or to proteins on the surface of 

 oyster shells (Hidu and Haskin 1 971 , Bahr and Lanier 

 1 981 , Fitt and Coon 1 992). Harry (1 976) demonstrated 

 that the American oyster can thrive on bottoms consist- 

 ing of 17 to 100% sand. 



Physical/Chemical Characteristics : The American oys- 

 ter is typically exposed to wide variation in environmen- 

 tal parameters (salinity, temperature, dissolved oxy- 

 gen, etc.) in its estuarine habitat (Killam et al. 1992). 

 Because of the oyster's tolerance of these fluctuations, 

 the environmental requirements of this species are not 

 readily defined with precision. 



Temperature - Eggs and Larvae: Normal egg develop- 

 ment occurs between approximately 18° and 30°C 

 (Loosanoff 1965). Larval development occurs gener- 

 ally at >20°C (Burrell 1 986) with maximal growth occur- 

 ring between 30° to 32.5°C at salinities ranging from 

 7.5 to 27%o (Davis and Calabrese 1964, Loosanoff 

 1965). 



Temperature - Juveniles and Adults: Adults exist 

 within the range of -2°C in New England to 36°C in the 

 Gulf of Mexico. During low tide, the American oyster 

 can withstand temperatures below freezing and above 

 49°C, but it typically stops feeding at 6°-7°C, and at 

 42°C most bodily functions cease or are greatly re- 

 duced (Galtsoff 1964). Normal growth occurs at tem- 

 peratures ranging from 10° to 30°C or greater (Burrell 

 1986). There may be as many as three races of 

 American oyster based on temperature regimes (Ahmed 

 1975). Buroker et al. (1979) found all oysters to be 

 genetically equivalent, and Groue and Lester (1982) 

 found the Laguna Madre oysters to be genetically 

 distinct from four other Gulf populations. These racial 

 distinctions may be reflected in spawning tempera- 

 tures determined by Stauber (1950): Gulf of Mexico 

 oysters spawn around 25°C (water temperatures must 

 be consistently over 20°C and above 25°C for mass 

 spawnings); there are two races on the East Coast that 

 spawn at 16 and 20°C. Cake (1983) reports that Gulf 

 oysters are not as tolerant of freezing as the East Coast 

 race. 



Salinity - Eggs and Larvae: Normal egg cleavage in 

 Virginia waters occurs between 7.5 and 34%o (meso- 

 euhaline) with optimum development between 10 and 

 22%o (Castagna and Chanley 1973). The optimum 

 salinity for proper egg and larval development may be 

 related to the salinity at which the adult gonads com- 

 plete gametogenesis (Davis 1958, Loosanoff 1965). 

 Egg and larval development from mesohaline adult 

 populations (9-1 0%>) are optimum at approximately 10 

 to 1 5%o (Davis 1 958), with an upper limit of about 22%o 

 (Loosanoff 1965). Development of spawn from adults 

 in polyhaline areas (26-27%o) is best at 23%o for the 

 eggs and 18%o for the larvae (Davis 1958) with a 

 tolerance of 15 to 35%o. In general, larvae are meso- 

 to euhaline tolerating salinities between 5 and 39%o 

 (Castagna and Chanley 1973). Larval growth is usu- 

 ally limited at lower salinities (10%o) (Chanley 1957) 

 with optimums, in most cases, at higher salinities (25- 

 29%o) (Castagna and Chanley 1973). Spat setting is 

 usually less at low salinities, with consistent settling 

 occurring from 16% to 22%o, and peaking at 20%o to 

 22%o (Menzel etal.1 966, Chatryetal. 1983). Metamor- 

 phosis occurs between 5.6%o and 35%o, with best spat 

 growth between 13 to 30%o (Chanley 1957, Castagna 

 and Chanley 1973). 



23 



