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Previous studies on two species of the genus Rhynchocinetes (“hingebeak” shrimps) have described mating systems in which large dominant “robustus” males, with hypertrophied cheliped and third maxilliped weaponry, guard and defend smaller females during copulation. The sexual system of these species is gonochoric (separate sexes). In this preliminary study on the sexual biology of R. uritai, we report observations indicating a very different sexual and mating system for Rhynchocinetes. A sample of R. uritai taken off Shimoda, Japan, revealed a population structure with small males and large females with no overlap in size. Two intermediate-sized individuals showed transitional sexual characteristics indicating sex change from male to female (protandrous sequential hermaphroditism). Transitional individuals had male sexual appendices but also were developing female “breeding dress” (incubatory) characters. Furthermore, these individuals had large vitellogenic oocytes in the gonads, typical of maturing ovaries. Mating observations were made on three receptive females to examine whether the smaller males employ the “pure searching” mating strategy. Matings occurred at night after female molting. Mating behavior was brief and simple: males approached and attempted to grasp the newly molted female, followed by a brief (∼ 2 sec) copulation, with no evidence of the complex “caging” (guarding) behavior described in two other Rhynchocinetes species. These observations on R. uritai suggest a “pure searching” mating system, in which mating is brief with little pre- or postcopulatory interaction between male and female and little or no agonistic behavior among males. Such a mating system is characteristic of decapod shrimp species with small males and larger females, i.e., the population structure observed in our sample.

First Description of a Pure-Search Mating System and Protandry in the Shrimp Rhynchocinetes uritai (Decapoda: Caridea)