The reproductive biology of the gymnolaemate bryozoan Bowerbankia gracilis (Ctenostomata: Vesiculariidae)

Abstract

Various aspects of the reproductive biology of the stoloniferan ctenostome bryozoan Botoerbankia gracilis have been studied by laboratory observations and by light and electron microscopy. Biennial peaks of larval release indicate a biphasic reproductive season characteristic of other short-lived bryozoans with more than one generation each year. The tubular autozooids are protandrous hermaphrodites that brood embryos singly in the vacated tentacle sheath after the degeneration of the lophophore. The ovary develops on the lateral cystid wall and is supplied with a strand of the funiculus. The ovary ogenesis the follicular epithelium increases in thickness and the follicle cells exhibit a dramatic increase in rough endoplasmic reticulum, basal vesicles, and basolateral interdigitations. At the same time the apposing oolemma exhibits coated vesicles indicative of pinocytosis. The oocyte also contains rough endoplasmic reticulum and annulate lamellae. During the vitellogenic period the oocyte increases in diameter from 50 to 160 /-1m and accumulates numerous yolk granules. The ultrastructural evidence is indicative of both heterosynthetic and autosynthetic origins of yolk. Coincident with the degeneration of the lophophore that precedes brooding, there is restructuring of the coelomic mesothelium on the neural side of the pharynx to form a novel organ, the ciliated gutter, that facilitates ovulation and oviposition. The ciliary plates of the gutter become active at the time of ovulation and transport the egg into the tentacle sheath, presumably via the supraneural pore. Fertilization is believed to occur upon the subsequent partial eversion of the tentacle sheath, which exposes the egg to the ambient seawater. Upon the subsequent retraction of the introvert surrounded by a fertilization envelope. The embryonic period lasts 13 to 14days at 12.5°;C, after which the tentacle sheath is everted again to allow larval release. Additional observations on the apparent adaptations of a sympatric subspecies, B. gracilis var. aggregata, to an intertidal existence are discussed with regard to the systematics of the taxa.