Spermatozoon ultrastructure of Xenoturbella bocki (Westblad 1949)

Obst, M., Nakano, H., Bourlat, S.J., Thorndyke, M.C., Telford, M.J., Nyengaard, J.R. and Funch, P. 2011. Spermatozoon ultrastructure of Xenoturbella bocki (Westblad 1949). —Acta Zoologica (Stockholm) 92: 109–115. Here, we report on the sperm ultrastructure of Xenoturbella bocki (Westblad 1949), which we studied for the first time in detail using light, scanning and transmission electron microscopy. The mature spermatozoa are of the bilaterian primitive type, also called aquasperm and develop as uniflagellate sperm consisting of a round head with distinct mitochondria at the base and a 9+2 flagellum of approximately 42 μm in length. The acrosomal complex consists of a small, round electron translucent acrosomal vesicle and a subacrosomal base. There is no separate midpiece, and the mitochondria surround the proximal and distal centriole in the posterior part of the head. The primitive structure of the spermatozoa suggests that these fertilize the egg by free spawning, probably the ancestral mode of fertilization in early bilaterians. When compared to the spermatozoa of other metazoans, we find that the arrangement of organelles in the Xenoturbella sperm shows similarities to a wide range of protostome and deuterostome taxa and does not seem to indicate any particular phylogenetic relationship.