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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 mereenaria), 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 californi-
anus, Septifer bifurcatus, Anomia sivtplex, Mytihis edtilis, Fetricola pholadiforfnis,
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. Needless
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