Abstract
The last common ancestor of Chitonida, the largest order of chitons, evolved unique mechanisms of fertilizing elaborate eggs with dart-like sperm that are typical of the group. In contrast Leptochiton asellus and other Lepidopleurida [sensu Sirenko 1993], have retained the plesiomorphic condition with smooth-hulled eggs and sperm with prominent acrosomes. The mechanism of fertilization in L. asellus is expected to be similar to scaphopods and some other mollusks, because of basic similarities in sperm and egg design. By this mechanism, the acrosome reaction releases enzymes that digest a large hole in the jelly layer and vitelline layer. An acrosomal process polymerizes and extends the inner acrosomal membrane down to fuse with an egg microvillus. A fertilization cone is raised up through the vitelline layer and engulfs the sperm including the nucleus, centrioles, mitochondria and part of the flagellum. The transition to elaborate egg hulls and reduced acrosomes, which characterize all Chitonida, may be evident as intermediate stages among certain lepidopleurids, such as Deshayesiella curvata and Hanleya hanleyi, which have eggs with smooth hulls but sperm with smaller acrosomes on short nuclear filaments. Furthermore, Callochiton dentatus is of special interest, as it has retained a similar egg to D. curvata but its sperm is derived, like all other chitonids. Fertilization follows the chitonid pattern, in which a tiny acrosome digests a hole in the vitelline layer that permits only the injection of chromatin into the egg. The fertilization cone remains below the vitelline layer and so does not engulf the body of the sperm. Thus sperm organelles seem to be abandoned on the egg surface in a bag of sperm membrane. This suggests that centrioles, as well as mitochondria, of Chitonida are maternally inherited. New characters developed for Callochiton dentatus, Deshayesiella curvata and Hanleya hanleyi, have enabled a revision of previous phylogenies. The revised analysis suggests that Callochitonidae is the sister taxon to all of Chitonida and is not part of the order Lepidopleurida [sensu Sirenko, 1993]). Furthermore, Lepidopleurida appear to be a paraphyletic grouping. The Chitonida are still clearly divisible into the two suborders, Chitonina and Acanthochitonina.
