THE COMPARATIVE ULTRASTRUCTURE OF SPERMATOZOA FROM 5 BRANCHIOBDELLIDANS (ANNELIDA, CLITELLATA)

The ultrastructure of mature spermatozoa from five species of branchiobdellidans is described. The species come from four genera, and the three major families in the Branchiobdellida (Annelida: Clitellata) are all represented. The spermatozoa of Branchiobdella pentodonta and B. astaci (Branchiobdellidae) have a very long helical acrosome and a helical marginal fiber surrounding the flagellum, whereas those of Xironogiton instabilis show a twisted acrosome similar to that found in some oligochaetes. Bdellodrilus illuminatus (Bdellodrilidae) is extremely atypical, with no true acrosome, and with a long helical end-piece. Cambarincola fallax (Cambarincolidae) has an acrosome with a helical ridge on the acrosome tube, as in hirudineans, but shares with Branchiobdella spp. a helical marginal fiber around the flagellum. The spermatozoan characters in these species display a mixture of oligochaete and hirudinean features, as well as some autapomorphies, thus dispelling the idea that branchiobdellidan spermatozoa are very similar. Although a unique model cannot yet be defined for branchiobdellidan spermatozoa, a number of characters do show promise. These observations are still too few to allow any inference of evolutionary polarity in the Branchiobdellida or Clitellata, but they do suggest the possibility of using the comparative ultrastructure of the spermatozoa as an additional tool for studying branchiobdellidan phylogeny.