Male‐territory‐visiting polygamy in the sand‐dwelling goby <scp> <i>Fusigobius inframaculatus</i> </scp> (Gobiidae) inhabiting reef caves

We conducted an observational field survey of the innerspotted sandgoby Fusigobius inframaculatus on reefs of Kuchierabu-jima Island, southern Japan, to investigate its mating system. Individuals of the goby maintained home ranges on sandy bottoms within rocky caves or beneath overhangs that served as shelters. Of 34 identified individuals, 32% disappeared during the 4-month reproductive season, and nearly all had disappeared by the next reproductive season, indicating a high mortality rate. Territorial males constructed spawning nests on the ceilings of these cavities and mated with multiple females. The mating groups were isolated from one another and intergroup movements were rare, suggesting restricted movement under a strong predation pressure. Although facultative harem polygyny was observed in some local groups, most females visited male territories from outside for spawning and often changed mates. Based on the segregated spatial relationships of home ranges between sexes and unstable mating relationships, the mating system was characterized as male-territory-visiting polygamy. The goby exhibited sexual dimorphism in body size, with males being the largest individuals within local groups, and exhibited subtle and transient coloration changes in males during courtship, which may function as female choice traits. Our results suggest that the polygamous mating system in F. inframaculatus is maintained even under constraints of mate availability due to limited mobility within the narrow sheltering habitat at the boundary between sandy bottoms and rocky substrates.