The occurrence of protandrous hermaphroditism is demonstrated in the alpheid shrimp Athanas indicus (Coutiere), a species which lives in symbiosis with the sea urchin Echinometra mathaei (de Blainville) in Indo-Pacific coral systems. Protandry is partial (true males persist in the population), and male sexual types (true males and sex-changers) do not differ in their external morphological characteristics. The female reproductive success is enhanced by increased body size (clutch size is positively correlated with body mass). In males, however, the effects of body dimension on reproductive success are contradictory. On the one hand, size seems to be uninfluential, because sperm production commences when the males are still very small, female-male pair formation was not observed, and mating occurs during a brief encounter; on the other hand, fighting, where the largest individual is more likely to win, is frequent. One scenario based on the socioecological milieu of A. indicus suggests that the smallest male in a group must exhibit great plasticity in his behavior to achieve reproductive output. Several options, including leaving the group, changing sex, waiting for some misfortune to strike the dominant males, or else sneaking copulations, are possible, but none are totally rewarding. The resulting behavior will therefore depend on ''making the best of a bad situation.''
