Ultrastructure of mature sperm and eggs of the brooding hermaphroditic chiton, Lepidochitona fernaldi Eernisse 1986, with special reference to the mechanism of fertilization

Abstract

AbstractMost chitons are gonochoric and free spawning with fertilization occurring in seawater. Brooding hermaphroditic chitons retain their embryos in the pallial groove but whether the eggs are fertilized internally in the female genital tract or externally in the pallial groove was not previously known. Furthermore, the transition to this mode of reproduction could be expected to involve other changes in the functional morphology of eggs and sperm. We describe for the first time observations on live spawning, as well as the ultrastructure of sperm and eggs, in the brooding hermaphrodite, Lepidochitona fernaldi.Sperm and eggs are spawned through common gonoducts and the eggs are self‐fertilized in the pallial groove. Sperm structure is unmodified from that typical of other Lepidochitonidae, which suggests that the medium filling the pallial groove is seawater and not some more viscous fluid. The egg hull is reduced in this species to a series of flattened hexagonal plates that block access to sperm. Sperm must, therefore, penetrate the egg in between hull plates. In this region are located numerous micropores that provide more direct access to the vitelline layer beneath the hull. This region also overlies an area of the egg membrane that is rich in microvilli. The biological significance of these results is discussed with reference to the evolution of self‐fertilization. © 1993 Wiley‐Liss, Inc.