Reproductive behavior and alternative reproductive strategy in the deep-sea snailfish, Careproctus pellucidus

The reproductive biology of deep-sea fishes is largely unknown because of the difficulty of behavioral observations in this environment. In this study, the reproductive behavior of the deep-sea snailfish, Careproctus pellucidus , which lives at depths > 200 m, was observed in an aquarium. Fishes were collected at a depth of approximately 300 m in the North Pacific Ocean (36°70′N; 141°00′E) in July and August 2018 and kept for observation until March 2019. We observed that spawning events usually took place between one male and one female. Males defended a spawning substrate against other males and exhibited body-wiggling behavior for both courtship and aggressive display. Females visited the male’s territory and spawned a demersal adhesive egg mass on the substrate. The territorial male then sought the spawned eggs using the filamentous rays in the lower pectoral-fin lobe and released sperm when he located the eggs. Males remained near the spawning substrate after spawning but did not perform any parental care. Notably, sneaking behavior by a non-territorial male was observed in one case; this is the first report of this alternative reproductive strategy (or tactic) in a deep-sea fish. Our study reveals the spawning of C. pellucidus appear to use the mechanosensory lateral line and the chemical sense heavily to adapt to the dim sunlight deep-sea environment.