The double sperm line in the tubificid Clitellio arenarius (Annelida, Oligochaeta)

Abstract

The tubificid oligochaete Clitellio arenarius produces two kinds of spermatozoa which group to form spermatozeugmata in the spermathecae. The fertilizing (typical) spermatozoa, forming the core of the spermatozeugmata have: an acrosome with a deeply withdrawn acrosome vesicle and an acrosome rod; a long nucleus apically twisted and basally straight; five small mitochondria forming the middle piece; a fully transformed centriole; a flagellum with a central axonemal apparatus showing a ≪prominent central sheath≫ tract, followed by a ≪tetragon fibre≫ tract, and by a conventional (two singlets) tract; plasma membrane at a short distance from the axoneme. The atypical spermatozoa, forming the cortex of the spermatozeugmata, show a small acrosome, in which the acrosome vesicle is external to the tube, and lacks a rod; a small thread‐like nucleus; a middle piece formed by three large mitochondria; a partly untransformed centriole; a flagellum with conventional axoneme and plasma membrane largely separated from the axoneme. It is possible to distinguish morphologically the two sperm lines in early spermiohistogenesis: pycnotic nuclei are present in the cytophores of the atypical line and the microtubular manchette surrounding the early spermatids is incomplete. It is proposed that the spermatozeugmata of Clitellio are more plesiomorphic than those of Tubifex. A heterochronic mechanism may be responsible for the differentiation of the two sperm lines.

Key words:

• Spermatozoa
• Spermiogenesis
• Dichotomous sper‐matogenesis
• Spermatozeugmata
• Oligochaeta