When organisms (any thing living) are faced with an adverse environment, or when they cope with adversity over long periods of time, their genomes constantly change in order to adapt, improve their survival success (i.e. finding food, avoiding predators, and reproducing). The deepest areas of the ocean (i.e. between 6,000 and 11,000 m) are commonly referred to as the hadal zone, representing about 1–2% of Earth’s benthic region (i.e. ecological zone at the lowest level of any body of water such as an ocean, lake, or stream). These deep-ocean areas are among the most hostile environments on Earth, due to their high hydrostatic pressure, darkness, limited food supply, low temperatures, and hypoxia (low oxygen content). The most conspicuous environmental constraint in the hadal zone is hydrostatic pressure, which increases by 10 atmospheres (atm) per 100 m of depth (thereby reaching ~1,000 atm of pressure in the deepest ocean trenches).
Nevertheless, amazingly, life thrives in these poorly explored realms, which might mimic hostile environments on other planets in our universe. Recent technological advances have prompted a renewed wave of hadal exploration — resulting in discovery of hundreds of deep-dwelling species — including microbes, protists (single-celled organisms like protozoa or simple algae), worms, Porifera (e.g. sponges), Mollusca (e.g. mussel, clam), Echinodermata (coelenterates) Crustacea (e.g. crabs, lobsters, shrimps, krill, barnacles), Cnidaria (e.g. coral, anemones, box jellies, jellyfish) and fishes.
The most common hadal vertebrate species are liparid snailfishes (small tadpole-shaped cold-water fishes with pelvic fins forming a sucker; related to lumpfish and sea snail), which live at the widest depth range of any marine fish family (habitats ranging from intertidal to depths exceeding 8,100 m). Recent studies have shown that snailfish are top predators in the hadal food chain. However, very little is known about the genetic basis and evolutionary history of snailfishes’ adaptation to deep-sea life. Authors [see attached article] sequenced the genome of a snailfish species, Pseudoliparis swirei, found at a depth of 7,415 m. Unlike closely related shallow sea species, P. swirei has transparent, unpigmented skin and scales, thin and incompletely ossified bones, an inflated stomach, and a non-closed skull.
Phylogenetic analyses show that P. swirei diverged from a close relative living near the sea surface about 20 million years ago and has robust genetic diversity. Genomic analyses reveal that: [a] the bone Gla protein (bglap) gene has a frameshift mutation that appears to cause early termination of cartilage calcification; (2) cell membrane fluidity and transport protein activity in P. swirei appear to have been enhanced by changes in protein sequences and gene expansion (i.e. more genes; increased genome size) and (3) the stability of its proteins appear to have been increased by critical mutations in genes encoding the trimethylamine N-oxide-synthesizing enzyme and hsp90 chaperone protein. These data provide insights into morphological, physiological, and molecular evolution of the hadal vertebrates.
Nature Ecol May 2o19; https://doi.org/10.1038/s41559-019-0864-8