|Internal fertilisation in hermaphroditic colonial invertebrates|
Ryland, J.S.; Bishop, J.D.D. (1993). Internal fertilisation in hermaphroditic colonial invertebrates. Oceanogr. Mar. Biol. Ann. Rev. 31: 445-477
In: Oceanography and Marine Biology: An Annual Review. Aberdeen University Press/Allen & Unwin: London. ISSN 0078-3218, more
Biological fertilization; Colonies; Hermaphroditism; Marine
|Authors|| || Top |
- Ryland, J.S., more
- Bishop, J.D.D.
Theories for the significance of hermaphroditism, and factors influencing the transfer of free sperm through water between mates, are briefly considered. The occurrence of hermaphroditism in different groups of sessile colonial marine invertebrates is summarised. For two such taxa, the Bryozoa and the Ascidiacea, evidence is reviewed for the occurrence of self- or cross-fertilisation, and the mechanisms of sperm transfer are discussed. Obligate or routine self-fertilisation does not appear to be commonplace in either group.In Bryozoa, direct observations suggest that internal cross-fertilisation is widespread, despite earlier confusion concerning possible routes for sperm release and uptake. Population genetic evidence also indicates the occurrence of outcrossing. Nevertheless, recent experimental work suggests that selfing may take place in reproductively isolated material in at least some species. The occurrence in a range of gymnolaemates of precocious sperm penetration of the undischarged oocyte, with activation of the egg delayed until exposure to sea water occurs, is noteworthy. Unitary Ascidiacea are in general externally fertilised, but most colonial ones are internally fertilised and larviparous. It is concluded from experimental and genetic evidence that outcrossing may be widespread in colonial forms (and in unitary brooding species), despite the apparent difficulties of sperm transfer between mates. The route of sperm through the functional female to the site of fertilisation remains to be clarified. The occurrence of storage, in the broad sense, of exogenous sperm (conceivably involving precocious insemination of early oocytes, as in bryozoans) could solve various apparent problems concerning the mechanism of cross-fertilisation.