Male genital organs, spermatogenesis and spermatozoa in the enigmatic terrestrial polychaete Parergodrilus heideri (Annelida, Parergodrilidae)

Abstract

The terrestrial polychaete species Parergodrilus heideri (Parergodrilidae) has many features in common with the Clitellata and with Hrabeiella periglandulata, another terrestrial species of the Annelida. An ultrastructural investigation of the male genital organs, spermatogenesis and spermatozoa was undertaken in R heideri to elucidate whether similarities to these taxa might exhibit adaptive characters typical of terrestrial annelids in general or synapomorphies. In P heideri the male organs consist of a pair of seminal vesicles situated in chaetigers 6-9 and two pairs of ciliated sperm ducts opening into a genital atrium together with numerous accessory gland cells in chaetiger 10. In addition, the enlarged chaetae of this last chaetiger are probably copulatory chaetae. The seminal vesicles consist of coelomic spaces supplied with blood vessels. The vesicles originate from the mesentery, where the pair of testes are also found. Mitotic divisions within the testes produce spermatogonia, which are released into the vesicles where they undergo further divisions. The spermatids develop on typical cytophores although only four cells are present on each cytophore. In contrast to previous reports, the spermatozoa are of the so-called modified type; the head comprises the elongated acrosome and nucleus, and in the midpiece eight ovoid mitochondria surround the distal centriole, which gives rise to the axoneme. A small proximal centriole is present, which is oriented parallel to the distal one. Acrosome and tail are set off by two prominent infoldings of the cell membrane. The specific characters observed in P heideri are typical of species with direct transfer of sperm and internal fertilisation. However, there are no synapomorphies with either the Clitellata or H. periglandulata; the similarities between these terrestrial Annelida are very likely due to a corresponding reproductive biology in similar habitats and thus do not reflect a common ancestor.