Male reproductive system in Aegla (Decapoda: Aeglidae) and its anatomic-histological and ultrastructural relationship with other anomuran crabs.

The male reproductive system (MRS) of decapods in the genus Aegla remains poorly understood from both histological and ultrastructural perspectives. This study provides a comparative description of the anatomy, histology, and ultrastructure of the MRS in multiple Aegla species, with the aim of exploring their phylogenetic relationships with representatives of the superfamilies Lomisoidea and Chirostyloidea. Anatomically, the MRS of Aegla is located in the cephalothorax and consists of a bilateral structure. The testes are connected by a central commissure and independently open into each vas deferens. The vas deferens is a translucent tube subdivided into proximal, medial, and distal regions. Across all regions, the seminal fluid contains few free spermatozoa, and spermatophores are absent. This fluid comprises two types of secretion: type I (basophilic) and type II (strongly basophilic), both composed mainly of proteins and acidic polysaccharides, with interspecific variation in acidic polysaccharide content. Aegla spermatozoa exhibit a standard organization, organized into two hemispheres-cytoplasmic and nuclear-and possess an acrosome vesicle with two concentric layers. Comparatively, their ultrastructure closely resembles that of Lomis hirta, whereas members of Chirostyloidea lack similar features, indicating an evolutionary divergence. These findings contribute important insights into the evolutionary history of Anomura, highlighting the absence of spermatophores in Aegla-a condition typical of this infraorder-and underscoring the similarity in spermatozoa ultrastructure between Aegla and Lomis, likely reflecting a shared ancestral trait.