Functional morphology of the seminal receptacle of Mithrax, Mithraculus and Omalacantha spider crabs (Brachyura: Mithracidae)

Abstract

Most Majoidea crabs display high sperm competition rates due to the development of spermatic strata within the seminal receptacle (SR). To verify the occurrence of sperm competition, SR anatomy and histochemistry analyses were performed in Mithrax hispidus, Omalacantha bicornuta, and Mithraculus forceps. The SR of the three Mithracidae species is classified into two regions, one of mesodermal origin (dorsal), consisting of a stratified epithelium with desquamation cells that produce a holocrine secretion, and the other, an ectodermal region (ventral) comprising a simple cubic epithelium covered by a cuticle. The oviduct opens in a ventral position near the transition region, which exhibits more pronounced folds on the opposite face of the oviduct, which may aid fertilization due to the presence of an external musculature. Sperm masses in a circular format were observed in the SR lumen, reminiscent of coenospermic spermatophores, and no morphological evidence strata of sperm packets were observed in any of the three studied species. The ventral SR followed the most common pattern observed in Majoidea. The secretion produced in the receptacle is composed of glycoproteins, with neutral polysaccharides. Acidic polysaccharides probably play a role against microorganisms from male seminal fluid. Due to the absence sperm packets, we were unable to determine whether the investigated females receive material from more than one male nor (if this does, in fact, occur) whether any preference for the spermatozoa of one male over another takes place during fertilization, that can may indicate the absence of sperm competition in the investigated species.