TL;DR: A review of the literature on records of the deepest living bony fishes can be found in this paper, where the authors discuss the ecological and physiological significance of these two major hadal families and present recommendations for future research.
Abstract: Bony fishes are extremely successful in the marine environment, having evolved into nearly every ocean habitat. However, bony fishes do not seem to inhabit the ocean’s deepest depths, likely due to constraints of pressure adaptation. How deep do bony fishes live? Relatively few studies have examined the deepest living vertebrates, because sampling in hadal environments, depths 6000–11,000 m, is technologically challenging. Here, we review the literature on records of the deepest living bony fishes. Current depth records are held by the hadal snailfish Pseudoliparis swirei (family Liparidae) in the Mariana Trench, collection depth 7966 m, filmed to 8178 m, and the cusk eel Abyssobrotula galatheae (family Ophidiidae) in the Puerto Rico Trench, collection depth 7965 m. Observations of abyssal and hadal fish communities suggest that hadal snailfishes are endemic to trenches but occasionally cross into abyssal areas. On the other hand, cusk eels dwell on the abyssal plains, but can extend their ranges into the trenches. These habitat differences allow both snailfishes and cusk eels to occupy distinct niches in the greatest ocean depths. We then comment on the ecological and physiological significance of these two major hadal families and present recommendations for future research.
TL;DR: This result shows that the larvae of viviparous Barathronus maculatus undertake an ontogenetic vertical migration after a period of larval drift that may facilitate their wide distribution on the sea floor.
Abstract: Otolith stable-oxygen-isotope composition and microstructure were analysed in order to investigate the vertical habitat shift of deep-sea cusk eels (Ophidiiformes). Otolith δ18 O profiles suggested that both viviparous blind cusk eels and oviparous cusk eels experienced a pelagic larval stage and then settled to the deep-sea floor over a vertical distance that ranged among individuals from 200 to >1000 m. This result shows that the larvae of viviparous Barathronus maculatus undertake an ontogenetic vertical migration after a period of larval drift that may facilitate their wide distribution on the sea floor.
TL;DR: It is suggested that such associations provide juvenile cusk eels with shelter from predators or flow (or both) and access to prey far from other forms of refugia.
Abstract: We describe a commensal relationship between small (approximately 6 to 9 cm) deep-sea cusk eels, one tentatively identified as Barathrites sp. (Osteichthyes: Ophidiidae), and pancake urchins, both Hygrosoma petersi and Phorosoma placenta (Echinodermata: Echinothuriidae). These interactions were observed in situ using a remotely operated vehicle (ROV), deployed at depths of 1410 to 1775 m on three seamounts in the western North Atlantic (Bear and Rehoboth seamounts in the New England Seamounts and Yakutat Seamount in the Corner Rise Seamounts). Full motion video and still frames documented individual cusk eels associated with urchins in open fine-grained sediment and coral rubble habitats. Cusk eels foraged around the periphery of urchins and took refuge at the base and among the long spines on the aboral surface. We suggest that such associations provide juvenile cusk eels with shelter from predators or flow (or both) and access to prey far from other forms of refugia.