Ultrastructure of the body wall, body cavity, nephridia and spermatozoa in four species of the Chrysopetalidae (Annelida, ``Polychaeta'')

Abstract

The Chrysopetalidae are a small taxon of polychaetous Annelida with a controversial position in the phylogenetic system. Although they most likely belong to the Phyllodocida, their position is difficult to infer because they share some characters with most taxa of this group. Little information is available about the internal anatomy of the Chrysopetalidae. In order to provide additional data for clarifying the systematic position of the group, we here describe the structure of the body wall, coelomic organization, nephridia, oogenesis, spermatozoa and copulatory organs in four species. The Chrysopetalidae comprise a group of relatively large, epibenthic shallow-water species and a group of small endobenthic forms; two representatives of each group have been chosen, two Chrysopetalum spp. and two Dysponetus spp. All species have comparatively short segments, no circular body wall muscles, a highly developed system of transverse, oblique, dorsoventral and parapodial muscles, metanephridia and a closed circulatory system without a heart. Spermatozoa of the primitive type and gonochorism were observed in both Chrysopetalum species, whereas Dysponetus pygmaeus is a protandric hermaphrodite possessing filiform spermatozoa showing mitochondrial interpolation and male copulatory organs. These features of the reproductive system and a relatively simple organization of the other structures investigated are regarded as apomorphies and adaptations typical of meiobenthic polychaetes. The results of the present investigation are indicative of a fairly basal position of the Chrysopetalidae within the Phyllodocida.