California Sea Grant College Program

Nonflagellated sperm of M. rosenbergii resemble an in­verted umbrella. A 10 µm(base) houses the nucleus and a single 12 µm convex surface of the base. Radial fibrils in the base anastamose to form the spike. These structures contain 6 nm filaments with a 32 nm cross-striated periodicity. Centrioles are closely associated with the radial fibrils at the spike base. Eggs, covered by a bilayered investment coat composed of a 0.5 µm protein outer layer and a 2.5 µm mucopolysaccharide inner layer, are spawned through an externally held spermatophore following a female's ovigerous molt and mating. Sperm (<5/egg) attach base-first with the spike oriented perpendicularly to the investment coat. Within 15 seconds, the spike bends at the base and contacts the investment coat. The spike penetrates the investment coat, the base is inverted, and fertilization occurs within 2 minutes. The spike remains briefly as a central core in a fertilization cone. Fertilization is not inhibited by cytochalasin B and sperm isolated with and without protease inhibitors have no actin-comigrating band on SDS polyacrylamide gel electrophoresis. Flourescein-tagged DNAase I does not label the spike or radial fibrils. Spike bending can be induced in vitro with ATP or GTP treatment on glycerinated sperm. Investment coat penetration is inhibited by 0.01% soybean trypsin inhibitor or tosyl-L-lysine chloromethyl ketone but not by L-1-tosyl­-amide-2-phenylethyl chloromethyl ketone. These data sug­ gest spike bending is a nonactin contractile process and investment coat penetration is facilitated by a serine pro­tease(s).