Abstract
The reproductive biology of a Northumberland population of Trichobranchus glacialis was studied between July 1978 and June 1979. The gonads of both sexes surround the ventral blood vessel in three anterior segments of the body. The onset of oogenesis in the population occurs after January with a prolonged period of oocyte prolifeiation lasting at least seven months. During this periodprevitellogenic oocytes are released from the ovaries into the coelomic fluid where they complete their vitellogenic development. Mature oocytes measuring 150–200 ,um in diameter and containing large quantities of yolk granules, are spawned during a restricted breeding season in December. Active spermatozoa were only observed in abundance in November and December; they possess a small, round head measuring 2 ,urn in diameter, and a long filamentous flagellum. Fertilization is external and the larvae have a direct, lecithotrophic development.
