Deep-sea fishes have evolved complex survival strategies, including bioluminescent lures to attract prey and potential mates, gigantic or dwarf size depending on resource availability, and slow metabolic rates to conserve energy.
Deep-sea fish is a broad term that refers to species that inhabit the dark depths of the ocean, usually at depths of 600 metres or more.
The term deep-sea fish is not associated with a specific taxonomic group. It includes a wide variety of fish species that have evolved to thrive in the extreme conditions of the deep sea, exhibiting adaptations such as bioluminescence, enlarged eyes and jaws, as may be apparent from their appearance.
Many of them have oversized dagger-like teeth and expandable jaws capable of swallowing prey larger than themselves.
Their blubbery, slime-like bodies when seen on land are the result of pressure changes. Their bodies are generally flexible but not flabby, a trait that enables them to withstand the enormous pressures of the deep sea.
In some species of anglerfish, the male is extremely smaller than the female and spends its entire life attached to the female, gradually fusing with her body to become a permanent parasitic mate, providing sperm in exchange for nutrients. They have a terrific and direct solution to the difficult problem of finding a mate in the vast deep sea.
It was not until the late 19th and early 20th centuries, with the advent of technologically advanced submersibles and deep-sea trawling equipment, that scientists began to uncover the vast diversity of life in the deep sea.
Compared to the bright colours and streamlined bodies of coral reef fishes, deep-sea species are often dark, red or transparent. These colours effectively disappear in dim light, making them less visible to predators and prey.
Their bodies lack the underwater bladder common in shallow-water species and instead have high levels of unsaturated fatty acids in their tissues for buoyancy.
In terms of their conservation status, deep-sea fish may appear to be out of reach of humans, but they are not immune from human activity. Commercial deep-sea fishing, oil and gas extraction, deep-sea mining and climate change all pose certain threats to the survival of deep-sea fish.
The distribution and density of deep-sea fishes is still being studied, and it is difficult to identify countries with large populations of deep-sea fishes. However, it is likely that a significant number of deep-sea fishes are present in countries with extensive deep-sea environments within their maritime territories, such as the USA, Australia and Russia.
Breeding deep-sea fish in aquariums is a complex task due to the high pressure and low temperatures of their natural environment. One species of deep-sea fish, the orange roughy, holds the record for the longest-lived fish in aquariums, with one individual reported to have lived 20 years in captivity.
Due to the inaccessibility of their habitat, they do not interact with humans as much as shallow-water fish. However, like orange roughy and Patagonian toothfish, they are commercially caught for food and sold under the name chillian sea bass. Fishing methods for these fish, such as deep-sea trawling, have been criticised by some for their environmental impact, as they can cause significant damage to the delicate deep-sea ecosystem.
Several aquariums have successfully exhibited deep-sea species. For example, the Monterey Bay Aquarium in California has a ‘deep sea’ exhibit featuring species such as the Pacific Hagfish and California Headlight Fish.
Deep-sea fish are not generally known for their speed. However, the black swallower is one species with a particularly attractive diet. This fish is less than 25 cm long, but thanks to its highly elastic stomach, it can eat prey more than twice its length and more than ten times its mass. Thanks to this adaptation, the black swallower is able to make the most of the few feeding opportunities in the deep sea, where diets are scarce.
One of the things that characterises many deep-sea fishes is bioluminescence. This refers to the ability of an organism to produce and emit light. In the deep sea, where sunlight cannot reach, bioluminescence plays a variety of roles, such as attracting prey, communicating with potential mates and driving away predators. Bioluminescence is a testimony to the amazing adaptability of life.
Gigantism is a common feature of deep-sea fish. Known as deep-sea gigantism, this phenomenon means that they tend to be larger than shallow-water species. Scientists believe that this gigantism is an adaptation to the scarcity of food in the deep sea, and that the larger body size allows them to store large amounts of food when it is available. Incidentally, the giant orfish, which can grow to 11 metres in length, is the world’s longest bony fish.
It has evolved its own feeding mechanisms. Anglerfish, of course, have bioluminescent ‘lures’ which they use to attract unsuspecting prey. And the garouper eel has a huge mouth, which, to its surprise, can swallow prey larger than itself.
They are part of the deep-sea food web, consuming detritus, small fish and zooplankton, while being preyed upon by larger predators. Although much is still unknown, they may play an important role in marine ecosystems and, ultimately, in the global ecosystem.
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