Coiled Filamentous Structure in the Sperm Acrosome of Gastropoda, Sulculus aquatilis and Turbo cornutus

A helically coiled filamentous structure, termed the truncated cone originally identified in Haliotis discus , was demonstrated to exist in the apex of the acrosome subjacent to the outer acrosomal membrane of the sperm in two species of Gastropoda, Sulculus aquatilis and Turbo cornutus . Thin section and whole mount preparations revealed that in intact sperm this filamentous structure consisted of about 12 helically arranged filaments with a diameter of 10–12 nm which were tightly packed in a truncated shape. This truncated cone elongated anteriorly from the acrosomal opening and transformed into a cylinder which closely surrounds the acrosomal process during the acrosome reaction. In S. aquatilis and T. cornutus , the truncated cone elongated more than 3 and 1.5 times as long as the original height, respectively. The elongated truncated cone was characterized by striations with increase in its periodic spacing and inclinations in thin sections. The truncated cone in both the species was fundamentally analogous to that of Haliotis discus , further suggesting that the truncated cone plays a role in fertilization as a common cytoskeletal structure among the species of Gastropoda.