Maintenance of androdioecy in the freshwater shrimp Eulimnadia texana: sexual encounter rates and outcrossing success

The clam shrimp Eulimnadia texana has a rare mating system known as androdioecy, in which males and hermaphrodites cooccur but there are no pure females. In this species, reproduction takes place by outcrossing between males and hermaphrodites, or by selfing within a hermaphrodite; this system provides a unique opportunity to examine the adaptive significance of out-crossing and selfing in animals. Our study examined mating behavior in hermaphrodites and males from two populations to understand the propensity of these shrimp to mate and to estimate a parameter of a model developed by Otto et al. (American Naturalist 141:329-337), which predicts the conditions for stability of the mixed mating system in E. texana. Here we present evidence that mating frequency is environmentally sensitive, with greater numbers of encounters and matings per male when males are rare and in younger males. However, the effects of shrimp density, relative male frequency, and shrimp age interact in a complex way to determine male mating success. Overall, mating frequency was determined by a combination of encounter rates between the sexes and the proportion of encounters resulting in mating. The mating rates were then used to estimate one of four parameters of the Otto et al. model, and these estimates were combined with previous estimates of the other three parameters to examine the fit of the predicted to the observed sex ratios in the two populations. Copyright 2002.