Occurrence of intersexuality in the amphipod Corophium volutator (Pallas) in the upper Bay of Fundy, canada

Corophium volutator (Pallas) is a common amphipod in the intertidal mudflats of the Bay of Fundy, Canada. During a population study of C. volutator at four sites in the upper Bay (May 2000-May 2001), we observed several adults (217 of 11,148) that were intersex. Intersex individuals have both male and female sexual appendages (penial papillae and oostegites, respectively). Oostegites of intersex. individuals were without setae. No intersex individual carried eggs. Intersex individuals displayed a mix of male and female secondary characteristics. For example, the second antenna, which is longer and sturdier-looking in males than in females, was of intermediate appearance in intersex individuals. Also, the first article of the first antenna of most intersex. individuals had a serrated ventral edge (which is male-like) and two, long, sharp spines (which are female-like). The occurrence of intersexuality varied significantly for different combinations of site and month (0 to 8.5% of all adults), but a seasonal or site pattern could not be identified. Given the high density of C. volutator, it is apparent that large numbers of intersex individuals occur in the Bay. However, the origin and biological significance of intersexuality in C. volutator (a dioecious species) remain unknown.