-L 



Centimeters 



Figure 3. — O. (Alectryonia) megodon. Pearl Islands, Panama. Dimensions: height 17 cm., length 16 cm. 



(hermaphroditic). The gonad of a bisexual oyster 

 produces eggs and sperm simuUaneously, but the 

 relative cjuantity of sex cells of one or another 

 type alternates periodically from male to female 

 and vice versa. 



The sexes of nonincubatory (oviparous) oj'sters 

 {C. nrginica, C. gigas, C. angulata) are separate. 

 Instances of hermaphroditism in this group are 

 very rare. The sexes are, however, unstable and 

 once a year a certain percentage of oysters change 

 their sex. This change takes place after spawning 

 during the indifferent pliase of gonad development. 

 Alternation of sex (discussed in ch. XV) has been 

 studied in detail only for a few species. 



HABITAT 



Salinity, turbidity, and depth of water are 

 frecjuently mentioned in the brief statements 

 that accompanj' the descriptions of various 



species. Ecological data are, however, of no 

 help to classification. With the exception of a 

 few commercial species, which have been more 

 adequately studied, little is known about the 

 environmental requirements of the populations 

 of other species. In a general way it can be 

 stated that C. virginica, C. gigas, and probably C. 

 angulata are more tolerant to diluted sea water 

 than are 0. lurida and 0. edulis. The two latter 

 species survive better in more saline and less 

 turbid environment. Geographical distribution of 

 0. equestris suggests the preference of this species 

 to the waters of full oceanic salinity (about 35°/oo). 

 The same is true for many tropical oysters living 

 along the continental shores and in the lagoons 

 of oceanic islands where the salinity of water 

 changes but little or remains constant throughout 

 the year. Tolerances of these tropical species to 

 lowered salinity have not been studied. 



TAXONOMY 



