Is sexual selection operating in the androdioecious clam shrimp,Eulimnadia texana (Crustacea: Conchostraca)?

Experiments were performed to document the existence of intersexual or intrasexual selection in the clam shrimp,Eulimnadia texana. Individuals within this species are either males or hermaphrodites. Hermaphrodites can self their own eggs or outcross with a male, but they cannot outcross with other hermaphrodites. Theoretical considerations suggest that both intrasexual and intersexual selection could be occurring on the part of the hermaphrodites and the males. When males were given a choice between two non-gravid hermaphrodites of different sizes, they did not exhibit a mating preference based upon size. When two males of different sizes were isolated with a single nongravid receptive hermaphrodite, the hermaphrodite showed no preference between the two males. There was evidence, however, of male-male competition for receptive hermaphrodites and of mate guarding on the part of the males. During aggressive encounters between twp males, the larger of the two had a significant advantage over the smaller, and larger males were always the victors hermaphrodite takeovers occurred as a result of male-male conflict. Hermaphrodites appear to control the mating process both by struglling with males when they are not receptive to them and by selfing in the presence of males. This suggests that hermaphrodites withhold receptivity cues from males, or produce non-receptivity cues, when they are going to self. Though hermaphrodites do not appear to select males based upon size, they make a selection between selfing and outcrossing by controlling the use of receptivity signals.