“The hadal zone”—a term that refers to the feared Greek underworld—is used to describe the deepest portion of the ocean, including trenches, that extends from 20,000–36,000 feet (6,000-11,000 meters) deep. At these depths, sunlight is long gone, temperature is a frigid 34–36 °F (1–2°C), and hydrostatic pressures can reach 1,100 times that of atmospheric pressure.

One might expect to find only the most menacing creatures down there, or perhaps nothing at all, but we see neither case to be true: meet the little pink hadal snailfish. Hadal snailfishes surpass all other vertebrate capabilities of inhabiting hadal depths and earn the title “deepest living fishes”.

A hadal snailfish (c.f. Notoliparis antonbrunni) from the Atacama Trench, also known as the Peru-Chile Trench. Photo by Alan Jamieson, Newcastle University, featured in Gerringer’s paper.

Not only is the snailfish family (Liparidae) noteworthy for their abilities to inhabit the most extreme depths, but the entire family also spans the largest vertical range of any known vertebrate (0–27,000 feet/8,200 meters). Multiple species have been found throughout 9 of the 27 subduction hadal trenches, but all are commonly referred to as the hadal snailfishes. This speciation raises interesting questions about evolution into the trenches.

How do these snailfishes establish success at these depths despite the extreme conditions of the deep sea? In a recent IOB paper titled, “On the Success of the Hadal Snailfishes”, Mackenzie Gerringer, PhD outlines the evolutionary drivers of key adaptations that allow for vertebrate life in the hadal trenches in review of current literature. In the context of examining nearby families inhabiting the abyssal zone (the area above the hadal zone from 13,000—20,000 feet/4,000—6,000 meters) reasons for the success of hadal snailfishes become evident.

Among deep-sea adaptations are mechanisms of tolerating extreme hydrostatic pressure, feeding, and lifecycles.

High pressure can result in detrimental impacts to protein structure and therefore enzyme function, as well as membrane fluidity if not addressed effectively. A molecule called trimethylamine oxide (TMAO) stabilizes proteins under pressure and is found in increasing cellular concentrations with increasing depth of fishes. However, there is a limit to how much of this molecule can be present within cells, which proposes a depth limit for fishes at 27,00 feet/8,200 meters. Luckily for the hadal snailfishes, TMAO is present in sufficient amounts to allow for enzymatic function up to this depth.

Adaptations regarding feeding mechanics paired with food availability contribute just as well to the hadal snailfishes’ success. Snailfishes across the ocean feed on small crustaceans, which become increasingly available in the deep ocean trenches as amphipod populations flourish. It helps that snailfishes also possess large pharyngeal jaws, which are evident on micro-CT scans of fishes and allow the predators to more easily crush their prey. Sources of prey for other vertebrate families are scarcely found this deep.

Micro-CT scan shows the bones of a hadal snailfish (Pseudoliparis swirei) from the Mariana Trench. Pharyngeal jaws can be seen in the middle of the skull and appear as round structures covered in tooth-like projections. These are referred to as a second pair of jaws and help in the crushing of prey. Scale bar= 1cm. Photo by Abbey Dias at the Karel F. Liem Bioimaging Center, Friday Harbor Laboratories, University of Washington.

Another defining characteristic of the deep ocean is the ever-changing environment. Since hadal trenches are formed due to subduction zones, the environment is highly unstable because of the shifting tectonic plates. It is therefore advantageous that snailfishes have short lifecycles and continuously spawn in order to continue their legacy of success in the deep.

Although the deep sea remains greatly understudied, it represents the largest portion of the Earth’s habitable biosphere. It is increasingly important to recognize the organisms that find their homes here and understand their roles as vital contributors to the global ocean ecosystem.

Hadal snailfishes (Pseudoliparis swirei) in the Mariana Trench gathering around a deep-sea lander to feed on giant deep-sea amphipods (snailfishes’ crustaceous prey). Photo by Alan Jamieson, Newcastle University, featured in Gerringer’s paper.

Abbey Dias is an undergraduate researcher interested in deep-sea biology and climate change in extreme environments. Her research is conducted at Friday Harbor Laboratories and she currently attends Whitman College in Walla Walla, WA. To find out more about deep-sea science and her scuba diving adventures, follow @oceanabbeyy on twitter and LinkedIn.