The Deepest Fish Currently Known from the Ocean
- 演化之聲
- Mar 13
- 3 min read
Updated: Apr 10
Most people know that the deepest trench on Earth is the Mariana Trench. It is also the place where the deepest-living fish currently known has been discovered: Pseudoliparis swirei, commonly referred to as the Mariana snailfish. This species was first discovered in 2014 and was formally described and named in 2017. It belongs to the family Liparidae within the order Scorpaeniformes. Most individuals have been recorded at depths between 6,898 and 7,966 meters. Observations below roughly 8,200 meters are extremely rare, suggesting that this depth may approach the physiological limit that teleost fishes can tolerate.
During expeditions conducted in 2014 and 2017, researchers collected specimens using free-falling baited traps. Each trap was equipped with acoustic release systems, cameras, and flotation devices, and carried bait consisting of mackerel enclosed in a mesh bag. After deployment, the apparatus descended to the target depth and remained on the seafloor. The bait quickly attracted swarms of small crustaceans, which in turn drew groups of Pseudoliparis swirei. This setup allowed researchers to observe the fishes' behavior and morphology in their natural environment.
Video recordings and captured specimens indicate that the diet of this species mainly consists of small crustaceans, especially deep-sea amphipods that gather around bait on the trench floor. A total of 37 individuals were recovered during these expeditions. These specimens were brought to the surface for detailed anatomical examination and genetic sequencing, allowing researchers to clarify the species' taxonomic placement and evolutionary relationships.
Adult individuals of Pseudoliparis swirei typically reach a length of about 22 centimeters. Their bodies are pale pinkish-white and semi-transparent, allowing internal organs to be visible through the thin skin. Compared with closely related species, this fish has a relatively small head and a distinctive lower lobe on the pectoral fin. Males possess a forward-pointing genital papilla, which makes sex identification relatively straightforward.
The reproductive biology of this species also reveals unusual characteristics. Female individuals produce two clearly different size classes of eggs. In one examined female, the larger eggs numbered 23 and measured more than 5 millimeters in diameter, with the largest reaching approximately 9.4 millimeters. The smaller eggs were far more numerous, reaching as many as 851 in the same individual, but each was less than half the diameter of the larger eggs. The reason for this pronounced size difference remains unclear, though it may reflect different developmental stages of the eggs or a reproductive strategy adapted to the extreme conditions of the deep ocean.
Another intriguing observation concerns the distribution of individuals across depth zones. Smaller, presumably younger individuals tend to occur at greater depths, whereas larger adults are more commonly found in slightly shallower regions of the trench. This pattern may relate to differences in developmental stages, ecological requirements, or the distribution of available food resources.
The discovery of Pseudoliparis swirei provides important insight into how vertebrates adapt to one of the most extreme environments on Earth. At depths approaching eight kilometers, organisms must withstand crushing pressure, near-freezing temperatures, and extremely limited food supply. Yet within these harsh conditions, members of the Liparidae have evolved into one of the most successful fish groups inhabiting the hadal zone, forming a distinctive community of fishes uniquely adapted to life in the deepest parts of the ocean.
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Author: Shui-Ye You
Reference:
Gerringer ME et al. (2017). Pseudoliparis swirei sp. nov.: A newly-discovered hadal snailfish (Scorpaeniformes: Liparidae) from the Mariana Trench. Zootaxa.
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