First report of a genus <i>Gymnapogon</i> utilizing subterranean chambers in the upper intertidal zone of an estuary for shelter and reproduction in Shizugawa Bay, Miyagi Prefecture, Japan

Estuaries and coastal areas, including the upper intertidal zone, serve many ecosystem functions and provide habitat and reproductive sites for fish and benthic species. In addition to being widely distributed in marine, brackish, and freshwater areas around the world, the family Apogonidae is characterized by having developed male mouthbrooding. Species in the genus Gymnapogon are seldom seen, and little is known about their ecology. In June and July 2022, we collected Gymnapogon japonicus specimens (n=13, including four mouthbrooders), in sediment-covered chambers connected to sympatric shrimp burrows in the upper intertidal zone of the estuary in Shizugawa Bay, Miyagi Prefecture, Japan. Dissection of the gonads revealed that the mouthbrooding individuals were male. The eggs were oval and connected by bundles of adhesive filaments. The developmental stages of the eggs at the time of collection ranged from the cleavage stage to immediately before hatching. In conclusion, G. japonicus is unique in that it exhibits mouthbrooding in subterranean chambers connected to shrimp burrows in the upper intertidal zone of estuaries. This is the first report of mouthbrooding in the genus Gymnapogon. Moreover, no other species in the Apogonidae have been reported to brood in subterranean chambers, making this a surprising discovery.