The male reproductive system of the sea spider Phoxichilidium femoratum (Rathke, 1799).

Sea spiders (Pycnogonida) are marine chelicerates. As a sister clade to Euchelicerata, Pycnogonida are an interesting group for comparative anatomy, however data on pycnogonid anatomy and biology remain scarce. This research provides a detailed account of the complete male reproductive system, gametogenesis, and sperm structure of a sea spider at the ultrastructural level. The male reproductive system of P. femoratum includes the testis, femoral, and ovigeral glands. The testis is typical of Pycnogonida: U-shaped with pedal outgrowths, opening with gonopores on legs 2-4. The testis lays within the horizontal septum, separated from it by ECM. The reproductive sinus is reduced. The ventral wall of the testis is germinative, spermatogenesis proceeds in cysts, all stages are evenly distributed throughout the whole testis. Sperm of P. femoratum is a typical sperm of animals with fertilization in mucus but without an acrosome. It lacks apomorphic euchelicerate features such as an acrosomal filament and implantation fossa. Femoral and ovigeral glands are sex-specific and likely related to reproduction. Ovigeral glands possibly secrete a fungicide substance, while the function of femoral glands remains obscure.