Similar offspring production by normal and intersex females in two populations of Gammarus minus (Malacostraca, Amphipoda) with high levels of intersexuality

Two Virginia (U.S.A.) populations of the freshwater amphipod Gammarus minus (Say, 1818) exhibit high frequencies of females with male genital papillae (60 to 75% in one population and 100% in the other) that have been observed for over 30 years. This observation is surprising because sex-allocation theory predicts that such intersex individuals should have lower evolutionary fitness than ‘normal’ males or females. Here we tested this theory by comparing a major component of reproductive fitness (offspring production, as estimated by number and total mass of offspring in a brood) between intersex and normal females. Although intersex females tend to brood slightly fewer, but larger embryos than those of normal females, these differences were not significant overall, and moreover they resulted in nearly identical mean production of total offspring mass per brood. Maternal somatic investment (body mass and body condition) during brooding also did not differ significantly between intersex and normal females. These results suggest that intersex females may not have lower reproductive fitness than that of normal females, which may help explain their high frequencies in the Virginia populations. Consistent with this hypothesis, available literature data on four amphipod species show a positive association between intersex frequency and the offspring production of intersex females relative to that of normal females. However, this hypothesis needs to be tested further by examining other reproductive and survival traits related to fitness, and by examining other amphipod populations with varying intersex frequencies. Moreover, the cause(s) of the intersex condition in the Virginia populations (and most other amphipod populations) remains largely unknown.