On the fertilization problem in the gymnolaematous bryozoa

Abstract

Since the commonly accepted thesis that the gymnolaematous bryozoans reproduce sexually only by self-fertilization is unsatisfactory for genetical reasons, a study of the development and destiny of the sex cells in Electra posidoniae Gautier and E. crustulenta (Pallas) has been undertaken. In both species many zoids may simultaneously contain morphologically mature sperm and egg cells although there is a distinct tendency towards proterandry. As previously known the ova are discharged through the intertentacular organ. The fertilization membrane appears within 1 hour after liberation. The sperm are discharged through the two dorsomedial tentacles serving as vasa deferentJia and drift away through the water. In E. posidoniae they are sucked in by the tentacle crown of another zoid, and adhere to the tentacles. When the zoid starts discharging ova, the sperm detach themselves and sweep down to the ova, which are presumably fertilized there and then. In E. crustulenta they are probably collected by the intertentacular organ which thus serves as a receptaculum seminis. Thus, in Electra a sperm mass with its contained gene pool is released into the free water. Cross-fertilization is, therefore, possible and self-fertilization may even for reasons discussed in the text be prevented by mechanical or (and) chemical means. The fertilization is external in E. posidoniae, whereas, in E. crustulenta, it probably takes place in the intertentacular organ. Certain facts indicate that cross-fertilization occurs in at least the vast majority of the Gymnolaemata, both in those which deposit their ova in free water and those which deposit them in ooecia, tentacle sheaths, or the like. However, the morphological conditions are shown to be such as to theoretically allow for external as well as internal fertilization in different species, the intertentacular organ (coelomopore) serving as an oviduct which possibly also functions as a vagina in some forms. Further research on different types of Gymnolaemata is required to answer these questions fully.