Gamete spawning and fertilization in the gymnolaemate bryozoan Membranipora membranacea

The simultaneously hermaphroditic zooids of Membranipora membranacea colonies spawn primary oocytes and spermatozeugmata (aggregates of 32 or 64 spermatozoa) into ambient seawater. Eggs are released through the intertentacular organ (ITO) whereas spermatozeugmata are spawned through the tips of the two distomedial tentacles. Fertilized eggs undergo planktotrophic development to form long-lived cyphonautes larvae. Examination of ovarian, coelomic, and spawned oocytes for sperm nuclei, using either Bisbenzimide H33342 or aceto-orcein staining, revealed that a single sperm fuses with primary oocytes during or shortly after ovulation. In M. membranacea, egg activation does not immediately follow sperm-egg fusion, but occurs after oocytes are spawned through the ITO. The period between sperm-egg fusion and egg activation may last up to four days. Once zygotes begin to develop, they follow an Ascaris-type of fertilization pattern (Wilson, 1925). Internal sperm-egg fusion does not preclude cross-fertilization in M. membranacea, because spawned spermatozeugmata enter maternal coeloms through ITOs after being drawn into lophophores. The ITO actively regulates the entrance of spermatozeugmata and the release of oocytes by the closure of the distal pore. The ITO does not act as a filter to prevent self-fertilization, so that the paternal colony may also function as the maternal colony. Self-fertilization may be reduced in M. membranacea via increasing sperm dispersal away from the paternal colony, which is accomplished by the bending of the distomedial tentacles such that they release spermatozeugmata into the exhalent feeding current of the colony.