Sexual dimorphism and reproductive strategy of the kelp-dwelling isopod Amphoroidea typa (Isopoda, Sphaeromatidae)

Abstract Sexual dimorphism and the reproductive strategy of the isopod Amphoroidea typa were examined and compared with other species from the isopod family Sphaeromatidae. We analyzed populations of A. typa occurring on the kelps Lessonia spp. and Macrocystis pyrifera at six locations on the coast of northern-central Chile. Males could be clearly distinguished from females starting at total body lengths (TL) ⩾ 14 mm, taking as criteria the presence of reproductive appendages and the development of gonopods in males. Females reached substantially larger sizes (up to 22.20 mm) than males (up to 18.25 mm), confirming the existence of sexual size dimorphism in A. typa . Although females had developed oostegites at lengths ⩾ 14 mm TL, only the largest females (⩾17 mm TL) were ovigerous. This suggests that female A. typa reproduce once before dying. At most study sites there were more adult females than males occurring on Lessonia spp., while males dominated on M. pyrifera . Some A. typa individuals had white stripes present on the dorsal part of the pereon, a colour polymorphism apparently independent of the sex of the isopods but becoming more frequent in large individuals, suggesting high predation pressure and that individuals with these stripes have higher survival rates. The results of this study confirm sexual size dimorphism in A. typa , which appears to be a semelparous species. A comparison with other sphaeromatid species indicates that the evolution of semelparity is unrelated to body size, but might be more common in species from habitats with high predation pressure.