west Florida. These researchers concluded 

 that the two populations were only 82% 

 genetically similar, approximately the 

 level of similarity between £. virginica 

 and £. rhizophorae . The latter two spe- 

 cies are genetically close enough to have 

 been successfully hybridized in the labor- 

 atory (Menzel 1968). Stauber (1950) pos- 

 tulated that C. virginica was discontin- 

 uously distributed on the east coast dur- 

 ing prehistoric times, and that speciation 

 was occurring before oyster culture activ- 

 ities by man removed the barriers to gene 

 transfer. 



N. E. Buroker (University of Maryland 

 Marine Products Laboratory, Crisfield, 

 Maryland; pers. comm. ) indicates a single, 

 large panmictic (genetically homogeneous) 

 population exists between Cape Cod, Mary- 

 land, and Corpus Christi, Texas, with 96% 

 to 99% genetic similarity. Levinton (1973) 

 reported that six species of bivalve mol- 

 lusks (not including oysters) showed an 

 increase in genetic variability with an 

 increase in intertidal elevation, corre- 

 sponding to increasing environmental vari- 

 ability. This would be an interesting 

 parameter to study in intertidal reef oys- 

 ter populations. 



Without further consideration of the 

 evolutionary origins of the oyster, we 

 will concentrate on the functional (eco- 

 logical) classification of C. virginica 

 between Cape Fear, North Carolina, and 

 Cape Canaveral, Florida. From this point 

 on, the generic term "oyster" will mean £. 

 virginica , and "oyster reef" will refer to 

 oyster reefs in the study area unless spe- 

 cified otherwise. 



The general anatomy of the adult oys- 

 ter appears in Figures 7 and 8 (adapted 

 from Galtsoff 1964). Note the insert dia- 

 gram in Figure 7 showing the proper way to 

 describe oyster size. 



2.2 OYSTER REPRODUCTION AND DEVELOPMENT 



oyster is dioecious (with sepa- 

 0. but once a year some members 



The 

 rate sexes] 



of a given local population change their 

 gender from male to female (protandry) or 

 female to male (protogyny). This sexual 

 lability is possible partly because of the 

 simplicity of the oyster reproductive 



system, which lacks ducts, glands, or sec- 

 ondary sexual structures (Yonge 1960). 

 Oysters develop functional gonads at a 

 young age (2 to 3 months) and small size 

 (less than 1 cm in height). Usually they 

 tend to develop as males during their 

 first season, with subsequent protandric 

 change (to females) in following seasons 

 (Menzel 1955). A small percentage of any 

 given population ( <1%) functions as true 

 hermaphrodites (Kennedy and Battle 1963), 

 and this pattern seems to hold for other 

 species in the genus Crassostrea (Asif 

 1979). 



Some preliminary evidence indicates 

 that populations of oysters under certain 

 kinds of stress tend to develop a higher 

 proportion of males than females, but this 

 remains to be conclusively demonstrated 

 (Amemiya 1936; Loosanoff and Nomejko 1955; 

 Kennedy and Battle 1963; Bahr and Hillman 

 1964). It is interesting to speculate, 

 however, that the stress encountered in 

 the higher portions of the oysters' verti- 

 cal range in the intertidal zone (the 

 upper reef zone) could produce androgenous 

 (predominantly male) colonies that would 

 contribute little to the reproductive suc- 

 cess of the population. 



After oyster gonads reach maturity in 

 a local population, a temperature (or sa- 

 linity) shock triggers the emission of 

 sperm from one or more males. The temper- 

 ature at which oyster populations in dif- 

 ferent regions begin to spawn has been 

 used in the past to distinguish physiolog- 

 ical races. Atlantic coast and gulf coast 

 oysters have thus been separated into 

 17° C, 20° C, and 25° C spawners (Yonge 

 1960). Reef oysters subject to very high 

 summer temperatures are probably members 

 of the last group. 



The emission of sperm from male oys- 

 ters occurs via the exhalent chamber of 

 the mantle. A chemical constituent of the 

 sperm (a protein pheromone) stimulates the 

 females in the area to release eggs, and a 

 spawning chain reaction can sweep dramati- 

 cally over a dense population, turning the 

 water white. Females expel eggs from the 

 inhalent chamber rather than through the 

 exhalent chamber. This process involves a 

 preparatory contraction in portions of the 

 mantle margins to reduce the size of the 

 exhalent opening. Eggs then pass through 



20 



