Male reproductive system in stenopodidean shrimps: Unveiling basal reproductive traits in shrimp‐like Pleocyemata (Stenopodidae and Spongicolidae)

Stenopodidea represents one of the basal lineages within Pleocyemata, yet the male reproductive system (MRS) of this group remains poorly understood, with limited information available regarding its morphology and function. This study provides the first detailed description of the MRS in four stenopodidean shrimp species from two families: Stenopodidae (Stenopus hispidus, S. scutellatus, and S. spinosus) and Spongicolidae (Microprosthema semilaeve). We analyzed the anatomy, histology, and histochemistry of the testes and vas deferens (VD), as well as spermatozoal ultrastructure, and compared these findings with data from more derived pleocyematans to identify potentially ancestral reproductive traits. Our analyses revealed two principal types of secretion in the VD of Stenopus species and three in M. semilaeve, which together form the presumptive spermatophore. Some secretions occur in small amounts and are restricted to specific VD regions. The layers surrounding the spermatozoa are relatively simple, consisting primarily of a sperm cord enclosed by thin secretory layers, suggesting a plesiomorphic reproductive condition in Stenopodidea. Spermatozoa are elliptical and characterized by a large nucleus in direct contact with the cytoplasm, numerous peripheral vesicles, and a large vesicle containing concentric membrane whorls. This structure, previously described as a lamellar body, is reinterpreted here as a putative acrosome vesicle and differs markedly from acrosome vesicles described in other Pleocyemata. Taken together, the comparatively simple spermatophore architecture and distinctive spermatozoa ultrastructure highlight Stenopodidea as an important lineage for understanding the early evolution of reproductive traits in Pleocyemata.