Spermatozoon Ultrastructure in the Symbiotic Hoplonemertean, <i>Malacobdella japonica</i> Takakura, 1897

The sperm morphology of Malacobdella japonica, a symbiotic nemertean living in the clam Spisula sachalinensis, has been examined using light and electron microscopy. In this species, the structure of the elongated spermatozoon, consisting of a straight head and a posteriorly oriented flagellum, is similar to that of Malacobdella grossa, as previously studied. In both species, the sperm head has an acrosomal complex, an elongated nucleus, and a neck region. The small acrosomal complex, including a thimble-like acrosomal vesicle, a post-acrosomal ring of electron-dense material, and a subacrosomal space, is located asymmetrically at the anterior end of the nucleus. However, there are some differences between the species in the structure and organization of the mitochondrial compartment. In M. grossa, the mitochondria are elongated along the nucleus and remain separate all along their length. In M. japonica, the mitochondria merge at the base of the neck region, forming a ring around the distal centriole. Above this area, they remain separate, similarly to the pattern observed in M. grossa. This finding suggests M. japonica to be an intermediate stage between M. grossa, distinguished by its unique separate mitochondria, and other hoplonemerteans that have a single, ring-like mitochondrion. We assume that the ultrastructure of spermatozoa in symbiotic nemerteans is shaped rather by the mode of fertilization than by their lifestyle. With external fertilization, without forming clutches, the lifestyle does not lead to the development of highly modified spermatozoa.