Abstract
Shrimps of the genus Synalpheus (Caridea) are commonly found in subtropical and tropical marine habitats around the world. The present work aims to provide the first description of the reproductive system of this genus using histological, histochemical, and morphological perspectives to elucidate the gonochoric or hermaphrodite sexual system in this genus. Males have a slender and very translucent reproductive system that is challenging to observe. The entire cephalothorax was histologically sectioned to ensure that all regions of the reproductive system could be identified. This non-gambarelloides group shows a gonochoric condition. The caridean pattern of the reproductive system was observed where the male has a pair of testes and a pair of very thin vasa deferentia, ending at the opening of the gonopore. Spermatophore structure is absent, without any layers surrounding the spermatozoa that are aggregated in a sperm mass. In females, the reproductive system consists of a pair of ovaries that extend to the end of the pleon. The extension of the ovary has a particular pattern compared to other caridean shrimps. Here, we hypothesize that males invest energy on somatic morphological structures to protect the host and female, using the larger pistol-like cheliped, while females invest energy in reproduction since all the members of this group are typical "pair-bond" examples. The observed pattern of the reproductive male morphology could also be evidence of a possible synapomorphy for this genus within the infraorder.