BAUER, R.T.; Univ. Louisiana, Lafayette: Same Sexual System but Variable Sociobiology: Evolution of Protandric Simultaneous Hermaphroditism in Lysmata Shrimps

Hermaphroditism is rare in decapod crustaceans except for protandry (male to female sex change) in over 40 caridean shrimp species. In Lysmata spp., individuals go through a male phase (MP) before changing to the female phase (FP), but male gonadal elements are not lost after sex change as in other protandric shrimps. In spite of female external morphology, FPs mate quite successfully as males. Lysmata FPs are thus functional simultaneous hermaphrodites that mate non-reciprocally as male or female; they do not self-fertilize. The size (age) of sex change is variable. Abiotic factors related to breeding seasonality explain some of this variation. However, the speed of sex change is influenced by the demographic environment of an MP sex-change candidate (SCC), presumably mediated by social interactions. SCCs reared with abundant FPs changed from MP to FP more slowly than those reared with fewer or no FPs. Mating experiments showed no MP male mating advantage; the simultaneous hermaphrodite FPs, mating as males, are fully competitive with MPs. Male function in FPs reduces brood production but this cost appears minor compared to the benefits of simultaneous hermaphroditism. A major question posed is: why has not this highly adaptive �protandric simultaneous hermaphroditism� (PSH) evolved more frequently in the Caridea? A search of socioecological traits in Lysmata spp. fails to show a common selective pressure. There is a strong dichotomy in the sociobiology of Lysmata spp. �Pairs� species are tropical, pair-living, specialized fish cleaners, while �Crowd� species are subtropical, group-living generalists with unspecialized, facultative fish cleaning. A historical contingency hypothesis is proposed to explain the apparently unique evolution of PSH in Lysmata shrimps.