On the Success of the Hadal Snailfishes: The Influence of Trophic Ecology, Life History, and Pressure Adaptation on Depth Zonation in the Planet's Deepest-Living Fishe

The snailfishes, family Liparidae (Scorpaeniformes), have found notable success in the hadal zone from ~6,000--8,200 m, comprising the dominant ichthyofauna in at least five trenches worldwide. The hadal fish community is distinct from the surrounding abyss where solitary, scavenging fishes such as rattails (Macrouridae), cutthroat eels (Synaphobranchidae), eelpouts (Zoarcidae), and cusk eels (Ophidiidae) are most common. Little is known about the biology of these deepest-living fishes, or the factors that drive their success at hadal depths. Using recent collections from the Mariana Trench, Kermadec Trench, and neighboring abyssal plains, this dissertation investigates the role of trophic ecology, pressure adaptation, and life history in structuring fish communities at the abyssal-hadal boundary. Stomach content and amino acid isotope analyses suggest that suctionfeeding predatory fishes like hadal liparids may find an advantage to descending into the trench -- where amphipods are abundant. More generalist feeders and scavengers relying on carrion, such as macrourids, might not benefit from this nutritional advantage at hadal depths. Hadal fishes also show specialized adaptation to hydrostatic pressure, as seen in metabolic enzyme activities. Maximum reaction rate of lactate dehydrogenases from hadal liparids increased under pressures of 600 bar, while in shallow-living fishes, this enzyme was pressure-inhibited. These types of pressure adaptation are necessary for fishes to thrive at hadal depths. Intraspecific activities of tricarboxylic acid cycle enzymes, considered proxies of metabolic rate and nutritional condition, increased with depth of capture in hadal snailfishes, further suggesting an advantage to snailfishes living deeper in the trench where food availability may be higher. Analysis of otolith growth zones support an additional hypothesis -- snailfishes may be adapted to a seismically active, high-disturbance hadal environment by having relatively short life-spans that are on the order of fifteen years when compared to other deep-sea fishes. Additional aspects of hadal snailfish biology, including thermal histories, reproduction, swimming kinematics, and buoyancy strategies are explored and discussed. The taxonomic description of a newly-discovered hadal liparid from the Mariana Trench is also included. This study provides insight into the ecology and physiology of deep-dwelling fishes and provides new understanding of adaptations to life in the trenches.