The Reproductive Behaviour of α-, β-, and γ-Male Morphs in Paracerceis Sculpta, a Marine Isopod Crustacean

Abstract

[Males occur as three distinct morphotypes in Paracerceis sculpta, a marine isopod inhabiting the Gulf of California. Alpha-males are largest in body size (x ± SD = 6.55 ± 0.72mm, N = 63) and defend breeding sites in intertidal sponges. Beta-males are smaller (x ± SD = 4.34 ± 0.43 mm, N = 59) and resemble sexually receptive females in behaviour and body form. Gamma-males are tiny (x ± SD = 2.65 ± 0.73 mm, N = 23) and are capable of rapid movements. All three male morphs are sexually mature and successfully sire young in the field (SHUSTER, 1989a). Using artificial sponges to simulate natural breeding habitats, I examined (a) competitive interactions among α-males for access to breeding sites and sexually receptive females, (b) behaviours used by β- and y-males to enter spongocoels, and (c) the tendency for β- and γ-males to discriminate breeding site quality in terms of the availability of sexually receptive females. I found that: 1. Resident and intruder α-males used their walking legs, uropods and body positioning to retain or gain access to breeding sites. 2. Intruders were consistently more aggressive than residents in contests, except when breeding sites containing sexually receptive females were limited. 3. Despite their relative quiescence, residents retained their spongocoels in most contests, apparently due to the positional advantage residents gain when situated in the spongocoel. 4. When interacting with α-males, β-mates imitated female courtship behaviour, and, Males occur as three distinct morphotypes in Paracerceis sculpta, a marine isopod inhabiting the Gulf of California. Alpha-males are largest in body size (x ± SD = 6.55 ± 0.72mm, N = 63) and defend breeding sites in intertidal sponges. Beta-males are smaller (x ± SD = 4.34 ± 0.43 mm, N = 59) and resemble sexually receptive females in behaviour and body form. Gamma-males are tiny (x ± SD = 2.65 ± 0.73 mm, N = 23) and are capable of rapid movements. All three male morphs are sexually mature and successfully sire young in the field (SHUSTER, 1989a). Using artificial sponges to simulate natural breeding habitats, I examined (a) competitive interactions among α-males for access to breeding sites and sexually receptive females, (b) behaviours used by β- and y-males to enter spongocoels, and (c) the tendency for β- and γ-males to discriminate breeding site quality in terms of the availability of sexually receptive females. I found that: 1. Resident and intruder α-males used their walking legs, uropods and body positioning to retain or gain access to breeding sites. 2. Intruders were consistently more aggressive than residents in contests, except when breeding sites containing sexually receptive females were limited. 3. Despite their relative quiescence, residents retained their spongocoels in most contests, apparently due to the positional advantage residents gain when situated in the spongocoel. 4. When interacting with α-males, β-mates imitated female courtship behaviour, and]