The sexual system of the caridean genus Lysmata has been described until recently as protandric hermaphroditism, in which individuals change sex from

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.

Hypotheses on delayed sex change in the protandric simultaneous hermaphrodite Lysmata wurdemanni were tested with observations from population samples, mating experiments, and experiments on sex change under optimal and suboptimal breeding conditions. Male-phase individuals (MPs) much larger than the minimum size of sex change were most frequent in a natural population from fall through early spring. The hypothesis was tested that some MPs delay sex change to the simultaneous hermaphrodite female-phase (FP) because MPs are more competitive in obtaining copulations with parturial FPs than are FPs mating as males (MFPs). In different experiments, parturial FPs were maintained with two potential male mating partners (large MP and MFP, small MP and MFP, large MP and small MP) through the parturial molt and spawning; activities were recorded with time-lapse video. MFPs gained the single copulation with the parturial FP as frequently as MPs, large or small, but large MPs copulated with more FPs than small MPs. The hypothesis of FP reversion to large MP was tested experimentally and rejected. Rate of change of MP to FP was much lower in large MPs maintained under suboptimal (fall/winter) than optimal (spring/summer) breeding conditions. The results presented here suggest that the occurrence of large MPs from the fall to early spring is better explained by abiotic proximate factors related to breeding than by socially mediated sex change in different demographic environments.