Mating behavior and sex change of the anemonefish, Amphiprion clarkii, in the temperate waters of southern Japan

Abstract

In the main habitat of the anemonefishes Amphiprion, their movements between host sea anemones are generally restricted because of the low population density of hosts and high predation pressures. On the contrary, movements of A. clarkii between hosts were usual in the present study area (temperate waters of southern Japan), where host anemones are abundant. The general social unit of the anemonefishes is an isolated group consisting of a monogamous breeding pair and a varying number of nonbreeders. In the present study area, however, monogamous pairs established territories almost contiguous to others and nonbreeders had home ranges on the outskirts of the pairs' territories. The high host population density allows A. clarkii to move between hosts for searching for mates and acquiring additional mates. Most pair bonds lasted for more than 6 months, but 13% of the pairs separated because of migration of a mate to another territory. Bigamy occasionally originated from a penetration into a territory of a breeder by a mated neighbor of the opposite sex after the former's mate loss. Among 18 males who had lost their mates, only 3 changed sex and others re-paired with immigrant females, migrated to unmated females' territories or invaded pairs' territories. In the present study area, sex change to female is not the best way for an unmated male to increase his future reproductive success because of a loss of time spent on sex change and an opportunity to re-pair with new mates larger than himself, but is adaptively maintained as the best of a bad situation for the unmated male.