Spermatogenesis in the deep-sea brachiopod Pelagodiscus atlanticus and the phylogenetic significance of spermatozoon structure.

Details of spermatogenesis and sperm organization are often useful for reconstructing the phylogeny of closely related groups of invertebrates. Development in general and gametogenesis in particular usually differ in shallow water and deep-sea invertebrates. Here, the spermatogenesis and ultrastructure of sperm were studied in the deep-sea brachiopod Pelagodiscus atlanticus. The testes of P. atlanticus are voluminous sacs located along the lateral sides of the body. Germ cells develop around the blood capillaries, contact the basal lamina, and contain germ plasm, numerous mitochondria, Golgi apparatus, lipid droplets, and centrioles of the rudimentary cilium. During spermatogenesis, several proacrosomal vesicles appear at the posterior pole of the cell; these vesicles then fuse and migrate to the anterior pole. The spermatozoon has a head with an acrosome, nucleus, eight mitochondria, proximal and distal centrioles orthogonally arranged, and a long tail. Comparative analysis suggests that the spermatozoon of P. atlanticus can be considered the most ancestral among all brachiopods. Such an organization indicates that fertilization is external in this deep-sea species. Spermatozoa of other brachiopods should be regarded as derived from this ancestral type. The transformation of brachiopod spermatozoa might have occurred in three different ways that correspond to the three main clades of recent brachiopods: Linguliformea, Craniiformea, and Rhynchonelliformea.