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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.

Females are semelparous in Paracerceis sculpta, a harem polygynous isopod that breeds in intertidal sponges in the northern Gulf of California. Reproductive females locate males established in spongocoels, and initiate courtship, which involves oral contact of males by females, and lifting and shaking of females by males. Despite this apparent assessment, females do not discriminate among males that differ by 10% in body length, by 30 days in age, or that lack appendages used in courtship. In most cases, females pair with the first male they encounter. Males, moreover, show no reluctance to pair with any female that approaches their spongocoel. Females as well as males are attracted to spongocoels occupied by multiple gravid females. Males attempt to control such sites, and body size confers an advantage in spongocoel takeover. Predation risks incurred by females while searching for spongocoels may favour individuals responding to chemical cues that reliably indicate breeding site quality, e.g. those emanating from established breeding aggregations. That sponges containing gravid females attract unmated females may explain why males attempt to monopolize these sites.

Females and parental males commonly discriminate among potential mates. Male discrimination is often assumed to be lacking in species with non-parental males. However, male competition in these species may favour male discrimination since indiscriminate matings may waste time and energy. Males in such species should attempt to maximize their fertilization rates; females in such species should mate only with males able to enhance female reproductive success. Males of the Socorro isopod, Thermosphaeroma thermophilum , engage in precopulatory guarding, preferring larger, more fecund females and females near a reproductive moult. Males also guard post-moult females. Large males prevail when usurping or resisting usurpation, and guard large females. Females may choose mates by selective resistance to insemination attempts.