ARNOLD, Emily M.; VUTURO, Steven A.; SHUSTER, Stepehn M.; Northern Arizona University: Sexual Selection in a Gulf of California Mud Shrimp, Neotrypaea uncinata

Sexual selection occurs when receptive females are clumped in space and dispersed in time. Under these circumstances few males mate, characters that enhance male mating success are favored, and sexual dimorphism evolves. Mud shrimp (Thalassinidea) are among the most dimorphic of decapod crustaceans, yet little is known of their sexual behavior. We investigated the potential for sexual selection in a breeding population of Neotrypaea uncinata, a mud shrimp inhabiting Estero Morua near Puerto Pe�asco, Sonora, M�xico. To examine the morphology as well as the spatial and temporal distribution of males and females in two different locations, we took 30, 10 cm cores every meter along two transects separated by 400 m. We collected 35 males, 46 receptive females, 29 gravid females, and eight juveniles. Our collections were highly dimorphic; males possessed proportionately larger dominant chelae than females. Females were spatially aggregated within the cores, and males were closer to burrow entrances than receptive females, while gravid females were deepest. Both receptive and gravid females were collected in each transect, thus females were clearly asynchronous in their sexual receptivity. The sex ratio was female-biased in both locations, and we collected fewer, larger males where receptive females were most spatially concentrated. Our results suggest that males defend and mate sequentially with aggregations of receptive females in burrows, and support the hypothesis that sexual dimorphism has evolved in mud shrimp through sexual selection.