Abstract
An ultrastructural investigation of spermatogenesis was conducted on the reef-building polychaete Phragmatopoma lapidosa. Spermatogonia are released into the coelomic cavity from small testes which are attached to the walls of intersegmental blood vessels. Spermatocytes and spermatids are connected by intercellular bridges and develop into mature sperm which are of the “primitive” type but which possess highly modified acrosomes. The mature sperm has a long, curved acrosome which consists of three morphological regions including an unusual anterior portion containing a series of parallel, transverse striations, a central collar, and a bulbous posterior region. The proximal and distal centrioles and associated flagellum are laterally displaced, giving the spermatozoon a rotational asymmetry. Fertilization in this species occurs freely in seawater so the mode of sperm transfer does not appear to be related to the observed alteration in acrosome morphology. It is suggested that structural alterations in the acrosome have occurred in response to modifications of the egg envelope.
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