Same sexual system but variable sociobiology: evolution of protandric simultaneous hermaphroditism in Lysmata shrimps

The sexual system of the decapod (caridean) shrimp Lysmata is protandric simultaneous hermaphroditism (PSH). Individuals first mature as males (male phase = MP) and then when older (larger) change to the external phenotype of female carideans (female phase = FP). However, unlike purely protandric carideans, Lysmata FPs retain reduced male gonadal tissues and ducts, and are able to mate nonreciprocally as males as well as to reproduce as females. Thus, FPs of Lysmata species are functional simultaneous hermaphrodites although most reproductive effort is devoted to embryo production and incubation. The question explored here is, given the propensity of carideans to protandry, the apparent low cost, and high reproductive advantage of PSH, why has not PSH evolved more frequently? The mating systems and sexual selection of caridean shrimps, the original sex of protandric individuals, the cost of maleness, and sex allocation theory are discussed in relation to protandry and PSH. None of these factors adequately explains the evolution of PSH of Lysmata species. Lysmata has at least 2 species groups with very different sociobiologies; these groups do not appear to share current selective pressures that would explain PSH in both. A historical contingency hypothesis, testable in part with a phylogenetic analysis, may explain the evolution of PSH in Lysmata.