The form, structure and high-quality genome of a snailfish are reported in a paper published online this week in Nature Ecology & Evolution. This study reveals adaptations that enable the snailfish to survive in the deep, dark, high-pressure conditions of the Mariana Trench.

The deepest areas of the ocean lie between 6 and 11 km below the surface. Known as the hadal zone, it is among the most hostile areas on earth, due to high hydrostatic pressure, darkness, cold temperatures, low oxygen concentration and scarce food resources. Nevertheless, hundreds of species in the hadal zone have been identified, including liparid snailfishes. Snailfishes are top predators in the hadal food web and dominate the hadal fish fauna.

Wen Wang and colleagues caught several snailfishes from multiple locations in the Mariana Trench that were approximately 7 km deep. Unlike their relatives that live in tide pools, these hadal snailfishes demonstrate several adaptations to the deep sea, including transparent skin, big stomachs, thinner muscles, lightly ossified skeletons and incompletely closed skulls.

The authors found that osteocalcin, a gene that regulates tissue mineralization and skeletal development, is truncated in the hadal snailfish. This may contribute to the snailfish’s unusual skull and soft skeleton. Consistent with living in a dark environment, the hadal snailfish has lost several photoreceptor genes, resulting in poor vision in light. The authors also found multiple copies of genes that make cell membranes more fluid, which likely helps cells function at extreme pressures in the deepest ocean.

The authors conclude that the availability of this hadal snaifish genome may help illuminate species adaptation to extreme environments in the deep sea.