Female Sexual Receptivity Associated with Molting and Differences in Copulatory Behavior Among the Three Male Morphs inParacerceis sculpta(Crustacea: Isopoda)

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Abstract
Paracerceis sculpta, a sphaeromatid isopod crustacean inhabiting the northern gulf of California, forms harem polygynous breeding aggregations in spongocoels of intertidal sponges. Males in this species occur as three distinct morphs; .alpha.-males are large and possess modified uropods and telsons, .beta.-males resemble females, and .gamma.-males are small and inconspicuous. Females are semelparous, and sexual receptivity is associated with a terminal molt; the half-molted (sexually receptive) condition lasts 6-50 h. Field-collected premolt females do not contain sperm. Half-molted females possess sperm masses in both oviducts, and postmolt females contain sperm tails in their spent ovaries. The presence of an .alpha.-males does not affect the duration of female receptivity, but females can delay initiation of their reproductive molt if males are absent. Isolated premolt females are incapable of resorbing uninseminated ova. Such females molt, but do not transport ova into their marsupium and die without reproducing. All three male morphs complete similar behavioral sequences during intromission. However, .beta.- and .gamma.-males copulate quickly and abandon females immediately after copulation, while .alpha.-males copulate longer and retain females after mating. The duration of female receptivity may encourage multiple matng and thus influence relative fertilization success among the three male morphs.