Multi-male group and bidirectional sex change in the gobiid fish, Trimma caudomaculatum

The gobiid fish, Trimma caudomaculatum, lives in groups. We investigated group structure, mating system and bidirectional sex change of this species. Four groups examined contained more than two males. The males were significantly larger than the females. By the observations in aquarium, the males occupied a nest and the females visited the nest for spawning. While there was no specific pair bond, the males mated with multiple females. Hence, this species establishes multi-male groups. In the experiments, four larger females had changed sex to male among 25 females. The smallest male changed to female in the group of four males.