Female copying and sexual selection in a marine isopod crustacean, Paracerceis sculpta

The opportunity for sexual selection on males can be influenced by 'female copying', a form of female mate selection in which females imitate the mate choices of other females. Female copying always increases the opportunity for sexual selection on males because it increases the number of males obtaining no mates, as well as the number of males obtaining many mates. Paracerceis sculpta, a marine isopod crustacean, breeds in intertidal sponges in the Gulf of California. In the field and in laboratory choice tests, females in this species do not exhibit male or sponge preferences, but do prefer to breed in spongocoels containing other females. This tendency for females to aggregate is a form of female copying. Males in this species occur as three genetically distinct morphotypes: large α-males defend breeding sites in intertidal sponges; small β-males resemble sexually receptive females in behaviour and body form; and tiny γ-males that are effective at entering spongocoels and engaging in sperm competition with other males. α-Males prefer to guard large aggregations of females. β-Males and γ-males prefer to invade spongocoels containing large female aggregations. This paper reports the degree of female copying that is sufficient to account for the observed distribution of females in spongocoels in the field. These results also indicate that while female copying increases variance in harem size among α-males, it also increases the opportunity for β- and γ-males to invade large breeding aggregations. Female aggregation behaviour thus creates a 'mating niche' for β-males and γ-males. The availability of this niche may partially explain the persistence of multiple male morphs in this species.