First Description of Reproductive Behavior and Alternative Reproductive Strategy in the Deep-sea Fish, Careproctus pellucidus.

Abstract 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. To the best of our knowledge, this is the first report to observe and describe the reproductive behavior of deep-sea fish under free-living conditions. Fishes were collected at a depth of approximately 300 m (36°70′ N; 141°00′ E) on July and August 2018 and kept for observation until March 2019. We observed that almost all spawnings 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 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 unique reproductive biology of the deep-sea fish, C. pellucidus, which does not depend on visual information and uses other sensory modalities.