LAUSR

The coral-dwelling damselfish Dascyllus reticulatus shows heterosexual cohabitation on branching corals and has been considered to maintain a haremic mating system, where a few males monopolize mating within the group. However, details of the group structure have not been investigated. To clarify the individual-level group structure of D. reticulatus, we conducted field observational surveys on a damselfish population on reefs of Kuchierabu-jima Island, southern Japan. Relatively large D. reticulatus inhabited corals with long branches and wide gaps with a female-biased sex ratio, and they maintained haremic groups where the largest male monopolized mating. In contrast, small adults and juveniles cohabited in higher individual densities on short-branch corals, with no bias in individual sex ratio. Only nine of 26 adult males in the short-branch coral groups showed mating activities. Nineteen of 37 adult females in the short-branch coral groups spawned, and their spawning frequency was lower than that of the females on the long-branch coral. Thus, we observed two contrasting social compositions and mating activities within harem-like cohabitation groups that depended on body size and sheltering coral structures. We observed inter-harem moves by large non-breeding individuals from the short-branch corals to the long-branch corals, implying a conditional use of the two types of groups related to body size. Our observations present a new example of multiple forms of groups in haremic reef fishes.