Field evidence of polygamous mating system in the moray eel Gymnothorax eurostus

Moray eels (Muraenidae), belonging to the order Anguilliformes, are resident apex predators in reef environments and are suggested to play important ecological roles in fish assemblages. However, information on the reproductive ecology of moray eels is limited. To document the reproductive behaviors and mating systems of moray eels, we conducted field observations of the spawning behaviors of Gymnothorax eurostus in the coastal waters of Kagoshima, southern Japan. We recorded the spawning behaviors of G. eurostus 168 times over two breeding seasons in 2024 and 2025. Male G. eurostus mating territories were concentrated along the reef edge, and females visited these territories to spawn. Spawning of G. eurostus occurred in pairs after sunset and followed a semilunar cycle around the full and new moons. After courtship behavior, including the male touching the female’s head with his snout, the male grasped the female’s upper jaw. The mating pair then rushed upward approximately 1.0–2.5 m into the water column and released gametes at the peak of the ascent. Because both males and females spawned with multiple mates during each breeding season, the mating system of G. eurostus was concluded to be male-territory-visiting polygamy. Subordinate males of G. eurostus conducted sneaking, that is, pair spawning during temporal intrusion into the dominant males’ territories, as seen in various polygamous fishes. This study provides the first quantitative field data on mating behaviors in muraenids, gaining new insights into the reproductive ecology of anguilliform fishes.