Spermiogenesis and seminal receptacles in Aeolosoma singulare (Annelida, Polychaeta, Aeolosomatidae)

Spermatogenesis, spermatozoa, seminal receptacles and the transformations of sperm after mating have been examined for the first time in the polychaete annelid Aeolosoma singulare from Lake Baikal. The structure of the spermatozoon in the coelomic cavity is very similar to that already described for A. litorale, in particular in the presence of a cytoplasmic droplet containing lysosome - like vesicles around the nuclear basis. However, a comparison of coelomic spermatozoa with those stored in the sperm receptacles reveals a progressive reduction of the cytoplasmic droplet. The cytoplasmic droplet can thus be considered as a sort of remnant of the spermiogenetic process in the mature spermatozoon. The seminal receptacles of A. singulare, like those of the other aeolosomatids, are epidermal formations; but in this species they have a multicellular organisation and penetrate the muscular layers to end deep inside the coelomic cavity. The spermatozoon of A. singulare was compared with those of polychaetes considered close to aeolosomatids and found to have characters in common only with potamodrilids