Unusual spermatozoa and reproductive modalities of Xenodasys eknomios (Gastrotricha: Xenodasyidae)

The rare macrodasyidan Xenodasys eknomios is the first member of the genus to be found in the Mediterranean Sea and the fourth species known worldwide. As Xenodasys has proved to be at the base of the Macrodasyida clade, we provide new data on the reproductive system and spermatozoa to try and shed light on the ground pattern of gastrotrich reproduction. The hermaphroditic system of X. eknomios consists of two testes with ventrolateral pores and two caudal ovaries. A sac-like frontal organ, generally containing a spermatophore, is enveloped by a basal lamina and attached to the body wall by muscular fibres, appearing as a permanent structure. The spermatophore contains mature, filiform, spermatozoa, each composed of acrosome, spiralized nucleus, connecting piece and flagellum. The complex acrosome is the predominant element and forms the axis of the sperm. Most of the acrosome, is surrounded by two helixes, the external one is the nucleus and the internal one is a crystalline-like ribbon structure. The peculiar acrosome–nucleus complex, and the long connecting piece appear as autapomorphies. The structural plans of the reproductive system and the spermatozoa support the current systematization of Xenodasyidae and provide evidence for a possible sperm transfer modality in these species.