SEXUAL PHASES IN WOOD-BORING MOLLUSKS

Abstract

1. The three species of wood-boring mollusks Teredo navalis, T. diegensis and Bankia setacea, are all protandric, with a strong tendency toward rhythmical changes of functional male and female phases. 2. Each species differs as to the degree of ambisexuality characteristic of the primary male phase and of the subsequent sexual phases. 3. The primary gonad in all three species develops from branching follicles filled with large vacuolated follicle cells and having the primary gonia scattered along walls of the follicles. 4. In each of these species the gonads of young animals indicate that there are two types of primary male-phase individuals: (1) ambisexual males or protandric females, characterized by many ovocytes on the walls of the spermatic follicles, and (2) true males with few ovocytes. In those of the former type the male phase is of short duration, while true males retain the male phase longer or in some cases indefinitely. 5. In T. navalis the first female phase does not usually become functional until nearly all the sperm of the primary male phase have been discharged. Functional hermaphroditism is not usual, although the gonad is histologically ambisexual during the change of sexual phase in both directions. 6. In T. diegensis, on the other hand, functional hermaphroditism is of usual occurrence and the sexual phases are not sharply demarcated. 7. In B. setacea functional hermaphroditism occurs only occasionally in the primary male phase; the subsequent sexual phases are clearly differentiated, often with a resting stage intervening between two sexual phases. The sexual phases are of the alternative type in that any sexual phase, after the first, may be followed by either a male or female phase if the length of life suffices. The relatively short life of many individuals, however, allows but a single change of sex, from male to female, in the genetic females, and none at all in true males. 8. In all of the three species the eggs begin development after artificial fertilization. In Bankia the larvae may be reared to the free-swimming veliger stage, but in the other two species the larval stages require the peculiar environmental conditions of the maternal gill chambers. Under experimental conditions self-fertilization and apparently normal cleavage occurs readily in the two species of Teredo and occasionally in Bankia.