Nesting and Reproductive Behavior of the Sand-Dwelling Goby Hazeus ammophilus (Gobiidae) with Radial Ditches Around Its Nest

The reproductive behavior and nest-building activity of the sand-dwelling goby Hazeus ammophilus were investigated to examine its nesting characteristics and to determine how and why this species builds radial structures around its nests. Field observations revealed that males spawned with multiple females in open muddy-sand bottoms, using bivalve shells or fallen leaves as spawning substrates. Males cared for eggs after spawning and repeatedly mated with multiple females, suggesting a male-territory-visiting polygamous mating system. A distinctive feature of this species was the presence of radial ditches extending from the nest. These ditches developed through repeated male behaviors of digging from the nest toward the surrounding area and sweeping accumulated sand out of the nest, resulting in a crater-like structure around the nest. These behaviors may contribute to cleaning and stabilizing the spawning substrate, and the resulting structures themselves may also be involved in female mate choice. Taken together, these findings indicate that H. ammophilus has evolved a flexible reproductive strategy, and nest-building behavior possibly adapted to unstable open sandy environments, highlighting the behavioral diversity and ecological plasticity within gobiid fishes.