Ultrastructure of the sperm of Catostylus mosaicus and Phyllorhiza punctata (Scyphozoa, Cnidaria): Implications for sperm terminology and the inference of reproductive mechanisms

Sperm of marine organisms are usually classified as either "primitive" or " modified". Primitive sperm, with heads comprised of a simple acrosome, sphe rical nuclei and mitochondria and a free flagellum, are thought to be assoc iated with the occurrence of external fertilization. The implication is, of course, that external fertilization is a primitive condition wherever it o ccurs. Here the sperm of two Scyphozoa, Catostylus mosaicus (Quoy and Gaima rd 1824) and Phyllorhiza punctata von Ledenfeld 1884, are described. Both o f these taxa are known to brood planulae and are likely have some sort of i nternal fertilization. Catostylus mosaicus has a typical "primitive" sperm with a small head (2 mum long) comprised of a short cylindrical nucleus, fo ur spherical mitochondria and a some small spherical electron dense spheres that are may be acrosomal in function. The anchoring apparatus is comprise d of two centrioles, a complex of satellite fibres and a striated "spur". T he flagellum contains a typical 9+2 axoneme. Phyllorhiza punctata has sperm with a head 6 mum long. The nucleus is a narrow cylinder, 4 mum long and c urved in one plane. There are some anterior electron dense spheres, and fiv e ellipsoidal mitochondria abut the nucleus posteriorly. Behind the mitocho ndria an anchoring apparatus comprised of the distal centriole and a satell ite array is connected to the bases of the mitochondria. The flagellum cont ains a typical 9+2 axoneme. Sperm structure and the various reproductive mo des occurring in Scyphozoa are reviewed and placed in a phylogenetic contex t based on the current classification of the group. It is concluded that ex ternal fertilization is likely to be the plesiomorphic condition in Scyphoz oa, but there are various possible scenarios about the transformation to an d from external fertilization within the group. Sperm structure is shown to be a poor predictor of reproductive mode. It is recommended that the terms "primitive sperm" and "modified sperm" be abandoned and replaced with term s that describe sperm in terms of function.