Fertilization in Callochiton castaneus (Mollusca)

A fine-structural study of fertilization in Callochiton castaneus has revea led that the mechanism of sperm penetration into the egg is intermediate be tween the primitive condition found in members of the order Lepidopleurida and the more derived condition found in the Chitonida. C. castaneus sperm h ave the long needlelike nuclear filament and reduced acrosome that characte rizes all Chitonida, but they have retained several plesiomorphic features such as an unspecialized mid-piece and a lack of flagellar reinforcement. A s in some Lepidopleurida but unlike any Chitonida, the egg hull in this spe cies comprises a thick, smooth jelly coat permeated by pores that permit sp erm rapid access to the vitelline layer. The jelly coat is delicate and qui ckly dissolves when a sperm concentrate is used, suggesting that excess acr osomal enzymes may be responsible. Once the sperm have penetrated the vitel line layer, the long nuclear filament bridges the gap to cups in the egg me mbrane. However, once the fertilization membrane is raised, the perivitelli ne space exceeds the length of the nuclear filament, preventing other sperm from penetrating the egg. A fertilization cone forms around the nuclear fi lament of the penetrating sperm, but it does not appear to engulf the body of the sperm. Rather, the nuclear chromatin is injected into the egg as a l ong thread. The remaining sperm organelles are apparently abandoned on the egg surface. If this is the case, it would be a significant departure from fertilization in other molluscs and many other metazoans, in which sperm or ganelles, such as centrioles and mitochondria, enter the egg. New sperm and egg characters, as well as significant differences in fertili zation, indicate that Callochitonidae are basal to all other members of the order Chitonida and may warrant separation as the sister taxon to the subo rders Chitonina and Acanthochitonina.