WILDEY, H.C.*; SHUSTER, S.M.: Measures of Sexual Selection in a Northern Gulf of California Mud Shrimp (Decapoda: Thalassinidea)

Thalassinidean shrimp (Decapoda: Crustacea) are abundant infaunal organisms in mud flat and estuarine habitats. Very little is known about mating behavior or reproduction in mud shrimp, however, they are know to exhibit sexual dimorphism, with males possesing larger dominant chela than females. It is widely held that the enlarged chelae of males are used in agonistic interactions, presumably for access to females and during mating attempts, although this behavior has never been observed. We collected the mud shrimp Neotrypaea uncinata once a month for 11 months at Estero Morua in the northern Gulf of California. Shrimp were collected along 30 meter transects coring every meter using a “shrimp gun,” a 90 x 10cm PVC tube. For each of these months we calculated R, the sex ratio within cores, m*, the mean crowding of all shrimp and females, and P, the patchiness of all shrimp and females. We found no significant positive relationship between the density of mud shrimp and R, suggesting that males were not defending harems. Mean crowding and patchiness of females were low, indicating that females were overdispersed in space. Females in this population were gravid only once a year, suggesting that the temporal crowding of females at the time of mating is probably high. Despite extreme sexual dimorphism in this species, all of our measurements pointed to a low opportunity for sexual selection. These results suggest that if mating in this population occurs over an extremely brief period of time, spatial clumping of females must increase considerably. Our results also suggest that alternative explanations may be required to explain sexual dimorphism in mud shrimp.