American oyster, continued 



Salinity - Juveniles and Adults: The salinity require- 

 ments of oysters vary depending on geographic loca- 

 tion, life cycle stage, and environmental parameters 

 (Killam et al. 1992). Adults are euryhaline, tolerating 

 meso- to euhaline waters (Galtsoff 1 964, Burrell 1 986). 

 In Gulf of Mexico estuaries, they normally occur at 

 salinities from 10 to 30% o , tolerating a range from 2 to 

 43.5%o (Gunter and Geyer 1 955, Copeland and Hoese 

 1966). Low salinities (0%o) may be tolerated for short 

 periods of time (Loosanoff 1965) with optimum adult 

 growth occurring from 14 to 30%o (Castagna and 

 Chanley 1973). Gunter (1953) reported high mortali- 

 ties during spring floods in Mississippi Sound and 

 Louisiana. This has also been reported for Mobile Bay 

 (May 1972) and the Santee River, South Carolina 

 (Burrell 1977). Oysters from the Laguna Madre of 

 Texas tolerate higher salinities, growing and spawning 

 in salinities greater than 40%o (Breuer 1 962). Eleuterius 

 (1977) found salinities from 2 to 22%o from areas of 

 productive reefs. Salinity tolerance is inversely corre- 

 lated to the surrounding water temperature (Berrigan 

 et al. 1991). Higher water temperatures generally 

 result in reduced tolerance to salinity. At temperatures 

 below 5° C, oysters are tolerant of low salinity condi- 

 tions, but will die after only a few days at the same 

 salinity when the temperature is 15° C. 



pH: pH can influence oyster reproduction and develop- 

 ment (Berrigan et al. 1 991 ). Normal egg development 

 and larval growth occur between a pH of 6.75 to 8.75, 

 with an optimum pH for larval growth between 8.25 to 

 8.50 (Calabrese and Davis 1966, Calabrese 1972). 

 Optimum pH for spawning is 7.80, and the pH must be 

 greater than 6.75 for successful recruitment to occur. 



Dissolved oxygen (DO): Information on the DO re- 

 quirements for the American oyster is limited (Killam et 

 al. 1 992). Oysters are facultative anaerobes, enabling 

 them to withstand daily periods of low or no oxygen, but 

 an oxygen debt builds up (Berrigan et al. 1991). In a 

 laboratory experiment, the hourly oxygen consumption 

 for six whole animals (including shell) was 39 ml/kg or 

 303 ml/kg of wet tissue weight (Hammen 1969). Sur- 

 vival for up to five days has been noted in oysters kept 

 in water with <1 ppm DO content (Sparks et al. 1 958). 

 Larvae appear able to cope well aerobically with most 

 low oxygen conditions through simple diffusive. pro- 

 cesses (Mann and Rainer 1990). The consumption 

 rate of oxygen is a function of water salinity and 

 temperature (Berrigan etal. 1991). In Mobile Bay, low 

 oxygen conditions killed oysters and reduced the set- 

 ting of spat in 1971 (May 1972). 



Migrations and Movements : Since adults are sessile, 

 their distribution is determined by settlement of larvae 

 and subsequent survival of the spat. The planktonic 

 larval stages are transported by tides and migrate 



vertically through the water column. Larvae aggregate 

 near the surface on rising tides and near the bottom on 

 falling tides, thus ensuring their wide dispersion and 

 diminishing their chances of being swept out to sea. 

 Plantigrade larvae are capable of crawling on sub- 

 strates to determine suitability (Burrell 1986, Stanley 

 and Sellers 1986). Spat and adults from restricted 

 waters are often moved to leased lots in approved 

 waters for depuration and/or to increase the abun- 

 dance in that area for future harvests. 



Reproduction 



Mode : Adults exhibit protandry and protogyny, but are 

 gonochoristic (Andrews 1979). True functional her- 

 maphrodites occur in less than 1% of a given popula- 

 tion. Young oysters are predominantly male; subse- 

 quent sex inversion with age increases the proportion 

 of females (Loosanoff 1965, Bahr and Lanier 1981, 

 Burrell 1 986). The male releases sperm and a phero- 

 mone into the water column that can be detected by the 

 females at the inhalent siphon, triggering the release of 

 eggs for external fertilization (Andrews 1979). 



Spawning : The reproductive state is dependent upon 

 a number of factors, the most important of which is 

 water temperature. Water temperature triggers the 

 time of spawning, and the critical temperature varies 

 with geographical location (Burrell 1 986, Gauthier and 

 Soniat 1989). In the Gulf of Mexico, the temperature 

 must be constantly above 20°C for spawning, and 

 above 25°C for mass spawning (Hopkins 1931, Ingle 

 1 951 , Bahr and Lanier 1 981 , Burrell 1 986, Stanley and 

 Sellers 1986, Gauthier and Soniat 1989). Along the 

 lower part of Florida's west coast, spawning probably 

 occur during all months except during periods of high 

 orlowtemperatureextremes(Killametal. 1992). Peak 

 spawning in this area probably occurs in the spring and 

 fall months, with the fall being the more successful. In 

 the northern Gulf of Mexico, spawning occurs from 

 March to November (Butler 1954). Peaks occur in 

 Louisiana in late May-early June and September- 

 October (Pollard 1 973, Gauthier and Soniat 1 989). In 

 Mississippi, spawning occurs from May to October with 

 a peak in June (MacKenzie 1977). In south Texas, 

 spawning occurs in all months except July and August 

 because of high temperature (Copeland and Hoese 

 1966). 



Fecundity : A single female can produce 15 to 114.8 

 million eggs in a single spawn; fecundity is generally 

 proportional to the size of the female. Females may 

 spawn several times within a season (Davis and Chanley 

 1955, Galtsoff 1964, Loosanoff 1965, Gauthier and 

 Soniat 1989). 



24 



