Diversity of spermatozoan morphology in two families of Branchiobdellida (Annelida: Clitellata) from North America

Abstract

The spermatozoa of 13 species belonging to seven different genera of the branchiobdellidan families Cambarincolidae and Bdellodrilidae (Annelida: Clitellata) were examined and compared with previously described members of the family Branchiobdellidae. Based on an examination of the spermatozoa in this relatively large number of species within the order, the variation in sperm morphology is now known to be much greater than was previously expected. Considerable variation was found in nearly all characters examined: the acrosome varied in length from 3 to 60 µm and in shape from straight for most of its length to twisted for its whole length. The nuclei also varied in size (from 4 to 22 µm) and in shape from straight to twisted. Four parallel but twisted mitochondria were present in all species examined, and differed only in length. The marginal fiber that coiled around the flagellum varied in size and pitch. By comparing the present data with those from other branchiobdellidans, we found that the apical concavity of the nucleus, the presence of the marginal fiber of the tail, and the absence of a true basal body with the central apparatus of the flagellum penetrating the midpiece mitochondria are the only characters common to all members of the Branchiobdellida. Sperm characters in this taxon are extremely variable, even within a single genus, thus making the definition of "a single" sperm model for the whole group highly problematic. However, the new data will be of significant value in future phylogenetic work.