Deep-sea fish
Characteristics

The fish of the deep-sea have evolved various adaptations to survive in this region. Since many of these fish live in regions where there is no natural illumination, they cannot rely solely on their eyesight for locating prey and mates and avoiding predators; deep-sea fish have evolved appropriately to the extreme sub-photic region in which they live. Many of these organisms are blind and rely on their other senses, such as sensitivities to changes in local pressure and smell, to catch their food and avoid being caught. Those that aren't blind have large and sensitive eyes that can use bioluminescent light. These eyes can be as much as 100 times more sensitive to light than human eyes. Rhodopsin (Rh1) is a protein found in the eye’s rod cells that helps animals see in dim light. While most vertebrates usually have one Rh1 opsin gene, some deep-sea fish have several Rh1 genes, and one species, the silver spinyfin (Diretmus argenteus), has 38. This proliferation of Rh1 genes may help deep-sea fish to see in the depths of the ocean. Also, to avoid predation, many species are dark to blend in with their environment.

Many deep-sea fish are bioluminescent, with extremely large eyes adapted to the dark. Bioluminescent organisms are capable of producing light biologically through the agitation of molecules of luciferin, which then produce light. This process must be done in the presence of oxygen. These organisms are common in the mesopelagic region and below (200 metres (656 ft) and below). More than 50% of deep-sea fish, as well as some species of shrimp and squid, are capable of bioluminescence. About 80% of these organisms have photophores – light producing glandular cells that contain luminous bacteria bordered by dark colourings. Some of these photophores contain lenses, much like those in the eyes of humans, which can intensify or lessen the emanation of light. The ability to produce light only requires 1% of the organism's energy and has many purposes: It is used to search for food and attract prey, like the anglerfish; claim territory through patrol; communicate and find a mate, and distract or temporarily blind predators to escape. Also, in the mesopelagic where some light still penetrates, some organisms camouflage themselves from predators below them by illuminating their bellies to match the colour and intensity of light from above so that no shadow is cast. This tactic is known as counter-illumination.

The lifecycle of deep-sea fish can be exclusively deep-water, although some species are born in shallower water and sink upon maturation. Regardless of the depth where eggs and larvae reside, they are typically pelagic. This planktonic — drifting — lifestyle requires neutral buoyancy. In order to maintain this, the eggs and larvae often contain oil droplets in their plasma. When these organisms are in their fully matured state they need other adaptations to maintain their positions in the water column. In general, water's density causes upthrust — the aspect of buoyancy that makes organisms float. To counteract this, the density of an organism must be greater than that of the surrounding water. Most animal tissues are denser than water, so they must find an equilibrium to make them float. Many organisms develop swim bladders (gas cavities) to stay afloat, but because of the high pressure of their environment, deep-sea fishes usually do not have this organ. Instead they exhibit structures similar to hydrofoils in order to provide hydrodynamic lift. It has also been found that the deeper a fish lives, the more jelly-like its flesh and the more minimal its bone structure. They reduce their tissue density through high fat content, reduction of skeletal weight — accomplished through reductions of size, thickness and mineral content — and water accumulation makes them slower and less agile than surface fish.

Due to the poor level of photosynthetic light reaching deep-sea environments, most fish need to rely on organic matter sinking from higher levels, or, in rare cases, hydrothermal vents for nutrients. This makes the deep-sea much poorer in productivity than shallower regions. Also, animals in the pelagic environment are sparse and food doesn't come along frequently. Because of this, organisms need adaptations that allow them to survive. Some have long feelers to help them locate prey or attract mates in the pitch black of the deep ocean. The deep-sea angler fish in particular has a long fishing-rod-like adaptation protruding from its face, on the end of which is a bioluminescent piece of skin that wriggles like a worm to lure its prey. Some must consume other fish that are the same size or larger than them and they need adaptations to help digest them efficiently. Great sharp teeth, hinged jaws, disproportionately large mouths, and expandable bodies are a few of the characteristics that deep-sea fishes have for this purpose. The gulper eel is one example of an organism that displays these characteristics.

Fish in the different pelagic and deep-water benthic zones are physically structured, and behave in ways, that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic anglerfishes, and the deep-water benthic rattails.

Ray finned species, with spiny fins, are rare among deep-sea fishes, which suggests that deep-sea fish are ancient and so well adapted to their environment that invasions by more modern fishes have been unsuccessful. The few ray fins that do exist are mainly in the Beryciformes and Lampriformes, which are also ancient forms. Most deep-sea pelagic fishes belong to their own orders, suggesting a long evolution in deep-sea environments. In contrast, deep-water benthic species, are in orders that include many related shallow-water fishes.
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