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

The sexual system of the decapod (caridean) shrimp Lysmata isprotandric simultaneous hermaphroditism (PSH). Individuals firstmature as males (male phase = MP) and then when older (larger)change to the external phenotype of female carideans (femalephase = FP). However, unlike purely protandric carideans, LysmataFPs retain reduced male gonadal tissues and ducts, and are ableto mate nonreciprocally as males as well as to reproduce asfemales. Thus, FPs of Lysmata species are functional simultaneoushermaphrodites although most reproductive effort is devotedto embryo production and incubation. The question explored hereis, given the propensity of carideans to protandry, the apparentlow cost, and high reproductive advantage of PSH, why has notPSH evolved more frequently? The mating systems and sexual selectionof caridean shrimps, the original sex of protandric individuals,the cost of maleness, and sex allocation theory are discussedin relation to protandry and PSH. None of these factors adequatelyexplains the evolution of PSH of Lysmata species. Lysmata hasat least 2 species groups with very different sociobiologies;these groups do not appear to share current selective pressuresthat would explain PSH in both. A historical contingency hypothesis,testable in part with a phylogenetic analysis, may explain theevolution of PSH in Lysmata.