Sperm morphology and some aspects of acrosomal complex development in four species of Heteronemertea (Pilidiophora, Nemertea)

Abstract

The ultrastructure of sperm morphology and some aspects of acrosomal complex development were examined in Kulikovia alborostrata, K. torquata, Kulikovia sp., and Micrura bella using transmission and scanning electron microscopy. The studied Kulikovia species have spermatozoa with a bullet-shaped head consisting of an acrosomal complex with a single cup-like acrosomal vesicle, a cone-shaped nucleus, and a short midpiece that are characteristic of the ect-aquasperm type. Despite the general similarity in spermatozoon organization, an ultrastructural analysis has revealed some species-specific size differences. M. bella is distinguished by an elongated curved sperm head with an acrosomal complex containing one large electron-dense and a few small, moderately electron-dense acrosomal vesicles. As a rule, five mitochondria are located in the midpiece in all the studied species. According to data published in literature and the present findings, all Pilidiophora have spermatozoa with several (three, four or five) mitochondria, and they never contain a single mitochondrion. We suggest this trait to be a synapomorphy for the class Pilidiophora, while the posterior acrosomal ring component, an annular structure formed by the basal part of the acrosomal vesicle, to be a synapomorphy for Heteronemertea. Acrosomal vesicles develop from numerous proacrosomal vesicles that are synthesized in spermatocytes I and fuse (or partly fuse in M. bella) at the spermatid stage. We regard the separate small acrosomal vesicles in the acrosomal complex of M. bella spermatozoa as the retention of an ancient trait in the sperm structure via “progenetic spermatogenesis” during the sperm cell development.