FISHERY BULLETIN: VOL. 81, NO. 4 



maturity" procedure, first used by Chipperfield 

 (1953). One problem with such a subjective approach 

 is that it does not recognize intermediate stages of 

 development. However, the stereology technique 

 described above is also subject to criticism, since dif- 

 ferent gametogenic stages may have similar GVF 

 values, as is the case withM. edulis (Fig. 6). It is only 

 when the two methods are used together that a 

 meaningful description of the gametogenic develop- 

 ment of an animal can be constructed 



O-INACTIVE 

 I- EARLY DEVELOPING 



H-LATE DEVELOPING 

 HI-RIPE 



IV-SPAWNING 

 V-SPENT 



1.0 - 



O °- 8 



< 



"" 0.6 



UJ 



go.4 



< 0.2 



o 



o.o 



II 



II 



II 



I 



i 



Din hi iv 



DEVELOPMENTAL STAGES 



FIGURE 6. — Mean values of gamete volume fraction for each de- 

 velopmental stage of Mytilus edulis. Values for males and females 

 are combined. 



Sex Ratios and Gonad Color 



Oocyte diameter of ripe females at the time of 

 spawning was 0.065-0.070 mm. It is possible to 

 determine the sex of mussels from the gonad color 

 once the animal has reached stage III. At this time, 

 the female gonad (mantle) is a definite apricot hue, 

 while the male gonad is cream or yellow. During the 

 other developmental stages, however, gonad color 

 does not serve as a reliable indicator of the 

 animal's sex. 



In the population studied, the proportion of females 

 in all size classes (N= 235) did not differ significantly 

 from one-half. Male and female gonads were dis- 

 tinguishable in all size-classes studied (>26 mm). 

 Although no protandry was observed, there was 

 evidence of a simultaneous hermaphroditism in some 

 individuals. One mussel contained both male and 

 female gametes in the mantle, and 7 of 360 mussels 



(2%) contained one type of sex cell in the mantle and 

 the other in the visceral mass. Trematode sporocysts 

 (species undetermined) were found in the digestive 

 gland and gonadal tissue of eight individuals collect- 

 ed from June to November. 



DISCUSSION 



Mytilus edulis is dioecious, the sexes of which are 

 distinguishable either by examining the sex products 

 or from inspection of gravid individuals. Female M. 

 edulis are characterized by a bright orange to apricot 

 gonad, whereas the males have a cream- colored 

 gonad. This is due to the accumulation of carotenoids 

 in the gonads at maturation (Campbell 1969). Few 

 species of bivalves can be sexed in this manner. The 

 low incidence of hermaphroditism exhibited by this 

 species suggests M. edulis possesses stable gono- 

 chorism, a condition characterized by the presence of 

 some hermaphrodites in a normally gonochoristic 

 species. 



Gonad examinations indicate that M. edulis from 

 Fairfield, Conn., spawn once annually during May 

 and June; however, the presence of ripe and partially 

 spawned mussels during the winter months in 1982 

 (January and February) suggests that the major re- 

 productive effort in the spring may have been pre- 

 ceded by a less synchronous release of gametes. It is 

 interesting to note that Newell et al. (1982), in their 

 study of two M. edulis populations on Long Island, 

 reported that the one from Stony Brook exhibited 

 one spring spawning peak, whereas the Shinnecock 

 population spawned 3 mo later and over a more pro- 

 longed period. The spawning pattern of the Fairfield 

 population is more similar to that observed for the 

 mussels from Stony Brook than that of the Shin- 

 necock population. This is not surprising; although 

 all three populations are located at approximately 

 the same latitude, only the Stony Brook and Fairfield 

 populations are in Long Island Sound (Fig. 1). This 

 finding, therefore, reinforces the interpretation by 

 Newell et al. (1982) that latitudinal effects on the re- 

 productive cycle of M. edulis are secondary to effects 

 of habitat- specific differences in the time and dura- 

 tion of maximum food availability. 



As more information on bivalves is gathered, it 

 becomes clear that the traditional view of a single, 

 fixed pattern of spawning for a population is inade- 

 quate. Instead, a certain degree of flexibility is pos- 

 sible, depending on variation in environmental 

 factors. This flexibility can be manifested either as 

 geographic variation among populations or as annual 

 variation within a population. Existence of the for- 

 mer is well documented (see Bayne 1976; Sastry 



738 



