Life of the Clams 41 



the eggs are fertilized within the mantle cavity, and the young complete 

 development to the adult form in brood pouches on the gills of the parent. 

 Usually self-fertilization does not occur, for in the majority of these species 

 the sperm is discharged before the eggs are mature in the same individual 

 (protandric hermaphroditism). 



Other kinds of bivalves are accustomed to practicing sex reversal in 

 which the early part of their lives is spent as males and their "adulthood" 

 as females. In the Quahog {Mercenaria mere en aria), nearly all individuals 

 experience a male phase in which functional sperm is produced while the 

 clam is only a few months old. Following this initial male phase, about half 

 of the population turns female to produce eggs, while the other half remains 

 male. No further sex change takes place. 



Sex reversal is apparently very popular among some of the oysters, 

 such as our native Pacific Coast Ostrea liirida. In this species there is a series 

 of male and female phases. There may be three changes within a single 

 year. Usually the male phase comes on first. Alternating sexuality also 

 occurs in our Atlantic Oyster {Crassostrea virginica), but the early sex organs 

 are capable of turning toward either male- or femaleness. It is not known, at 

 present, to what extent environmental conditions determine the direction 

 of sex chan^-e. It has been shown, however, that under unfavorable circum- 

 stances, when circulation of water is poor and the food supply low, there are 

 more female oysters in a colony. When conditions improve, the percentage 

 of males increases considerably. 



Thorough studies have now been made to show that normally no sex 

 reversal occurs and that the sexes are separate and of equal numbers in a 

 given colony in the following species: Modiolus deviissiis, Mytihis calif orni- 

 anus, Septifer bifurcatus, Anomia sivtplex, Mytihis edtilis, Fetricola pholadi- 

 forfnis, Donax goiildi, Mya arenaria (Soft-shell Clam) and the Angel Wing, 

 Barnea costata. 



The number of eggs produced by the female bivalve may vary consider- 

 ably depending upon the species and environmental conditions. Species 

 which retain the fertilized eggs within their bodies for further development 

 invariably produce fewer eggs than those species which discharge them into 

 the water. The oysters are probably among the greatest molluscan producers 

 of eggs. C. R. Elsey estimates that one female Crassostrea gigas of Japan 

 and our northwest Pacific Coast may discharge into the water each year 

 eggs numbering looo to the eighth power. If all survived in five generations, 

 the aggregate would be large enough to make eight worlds like ours. Need- 

 less to say, enemies and unfavorable conditions kill off most of the young. 



In contrast to this prodigious eflFort on the part of the oyster, the Dwarf 

 Turton Clam (Turtonia iimmta) deposits only 12 to 20 eggs which are 

 neatly encased in oval tgg masses of gelatinous material. While most species 



