On the Success of the Hadal Snailfishes

Abstract

Synopsis Deep-sea trenches, depths 6000–11,000 m, are characterized by high pressures, low temperatures, and absence of sunlight. These features make up the majority of the deepest marine habitat—the hadal zone—home to distinct communities from those in the surrounding abyssal plains. The snailfishes, family Liparidae (Scorpaeniformes), have found notable success in the hadal zone from ∼6000 to 8200 m, comprising the dominant ichthyofauna in at least six trenches worldwide. The hadal fish community is distinct from the abyssal community where elongate, scavenging fishes such as rattails (Macrouridae), cutthroat eels (Synaphobranchidae), tripodfishes (Ipnopidae), eelpouts (Zoarcidae), and cusk eels (Ophidiidae) are most common. Until recently, little was known about the biology of these deepest-living fishes, or the factors that drive their success at hadal depths. Here, I review recent investigations spanning the abyssal–hadal boundary and discuss the factors structuring these communities, including the roles of pressure adaptation, feeding ecology, and life history. Hadal fishes show specialized adaptation to hydrostatic pressure both in accumulation of the pressure-counteractant trimethylamine n-oxide and in intrinsic changes to enzymes. Stomach content and amino acid isotope analyses, and jaw morphology suggest that suction-feeding predatory fishes like hadal liparids may find an advantage to descending into the trench where amphipods are increasingly abundant. Analysis of otolith growth zones suggest that snailfishes may be adapted to a seismically active, high-disturbance hadal environment by having relatively short life-spans. This review synthesizes the known literature on the planet’s deepest-living fishes and informs new understanding of adaptations to life in the trenches.

Methylmercury produced in upper oceans accumulates in deep Mariana Trench fauna

“…Total Hg concentrations in trench amphipods have an average of 547 ± 230 ng g −1 (1 SD, 235 to 1070 ng g −1 , dry weight, Supplementary Table 1), which is comparable to their counterparts from the abyssal Arctic Ocean (250-1000 ng g −1 ) 33 , but considerably higher than benthic organisms in freshwater and coastal ecosystems (mostly <400 ng g −1 on average) 34,35 . The relatively high Hg concentrations in trench amphipods likely relate to their low tissue turnover rates and long life spans 36,37 . Total Hg concentrations of amphipods show no correlation with their depths of occurrence (R 2 = 0.00, P = 0.85), and an insignificantly positive correlation with their body length, a surrogate of life span (R 2 = 0.12, P = 0.08, Supplementary Fig.…”

“…Also shown are typical depth profiles of NPO dissolved MMHg concentrations 10,12 , Mariana dissolved oxygen [O 2 ] 59 , and Mariana seawater salinity and temperature measured in this study. The hadal food web is based on Gerringer 37 .…”

Geology, environment, and life in the deepest part of the world’s oceans

“…117,118 The strong pharyngeal jaw apparatus and inflated stomachs found in hadal snailfishes have been speculated to increase their ability to feed on the large amphipod biomass. 119,120 Although hadal invertebrates do not seem to be significantly different from shallow-living animals in morphology, there are more species showing strong mobility, which might help them to obtain the limited food in the hadal zone. For instance, deep-sea sea cucumbers from the families Pelagothuriidae, Laetmogonidae, Psychropotina, and Elpidiidae often show higher abilities to float and commonly swim faster.…”

Giant protists (xenophyophores) function as fish nurseries

“…e), attached deep in xenophyophores collected from two sites on the Costa Rican slope at 1,902 m (Mound Jaguar) and 1,866 m (Jaco Scar). Liparidae, a group highly adapted to the deep sea (Gerringer ) and well known for their brood‐hiding relationships with invertebrates (Chernova ), have never been reported to lay their eggs in any protozoan tests.…”

“…Snailfishes (Liparidae) are one of the few groups of vertebrates that have become deep‐sea specialists, and are the only fish group to commonly occur in trenches below 7,000 m (Gerringer ). Thus, it is interesting to note that they oviposit their embryos into the test structures formed by another group of deep‐sea specialists, Xenophyophoroidea.…”