Synalpheus paraneptunus (Crustacea : Decapoda : Caridea) populations with intersex gonopores: a sexual enigma among sponge-dwelling snapping shrimps

Caridean shrimp populations are usually gonochoristic, i.e., composed of separate sexes; however, some carideans are protandric sex changers or simultaneous hermaphrodites. In this contribution we investigated the sexual composition and its determination in two sponge-dwelling Synalpheus species with different degrees of sociality. Because the usual external sexing methods (presence or absence of appendices masculinae in males) can not be used in Synalpheus spp., we sexed individuals by observation of male and female gonopores using scanning electron microscopy (SEM). We found that populations of two species of the Synalpheus paraneptunus complex consist almost entirely of individuals with both male and female gonopores (termed herein "intersexes"). Such a high incidence of intersexes has not been previously reported in other decapods. We performed morphological and behavioral observations to test various hypotheses explaining the presence of intersex gonopores. Whereas protandry and simultaneous hermaphroditism are not likely to occur in these two species, capacity of immature helpers to mature as either sex, or developmental anomaly without adaptive significance could explain the presence of both female and male gonopores in these shrimps.