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Bathypelagic fish

Below the mesopelagic zone it is pitch dark. This is the midnight (or bathypelagic zone), extending from 1,000 metres (3,281 ft) to the bottom deep-water benthic zone. If the water is exceptionally deep, the pelagic zone below 4,000 metres (13,123 ft) is sometimes called the lower midnight (or abyssopelagic zone). Temperatures in this zone range from 1 to 4 °C (34 to 39 °F) and it is completely aphotic.

Conditions are somewhat uniform throughout these zones; the darkness is complete, the pressure is crushing, and temperatures, nutrients and dissolved oxygen levels are all low.

Bathypelagic fish have special adaptations to cope with these conditions – they have slow metabolisms and unspecialized diets, being willing to eat anything that comes along. They prefer to sit and wait for food rather than waste energy searching for it. The behaviour of bathypelagic fish can be contrasted with the behaviour of mesopelagic fish. Mesopelagic fish are often highly mobile, whereas bathypelagic fish are almost all lie-in-wait predators, normally expending little energy in movement.

The dominant bathypelagic fishes are small bristlemouth and anglerfish; fangtooth, viperfish, daggertooth and barracudina are also common. These fishes are small, many about 10 centimetres (3.9 in) long, and not many longer than 25 centimetres (9.8 in). They spend most of their time waiting patiently in the water column for prey to appear or to be lured by their phosphors. What little energy is available in the bathypelagic zone filters from above in the form of detritus, faecal material, and the occasional invertebrate or mesopelagic fish. About 20 per cent of the food that has its origins in the epipelagic zone falls down to the mesopelagic zone, but only about 5 per cent filters down to the bathypelagic zone.

Bathypelagic fish are sedentary, adapted to outputting minimum energy in a habitat with very little food or available energy, not even sunlight, only bioluminescence. Their bodies are elongated with weak, watery muscles and skeletal structures. Since so much of the fish is water, they are not compressed by the great pressures at these depths. They often have extensible, hinged jaws with recurved teeth. They are slimy, without scales. The central nervous system is confined to the lateral line and olfactory systems, the eyes are small and may not function, and gills, kidneys and hearts, and swim bladders are small or missing. These are the same features found in fish larvae, which suggests that during their evolution, bathypelagic fish have acquired these features through neoteny. As with larvae, these features allow the fish to remain suspended in the water with little expenditure of energy.

Despite their ferocious appearance, these forms are mostly miniature fish with weak muscles, and are too small to represent any threat to humans. An exception to the generally small size of bathypelagic fish is the Yokozuna slickhead (Narcetes shonanmaruae), described in 2021, which is the largest known entirely bathypelagic bony fish at over 2.5 metres (8.2 ft) in length.

The swim bladders of deep-sea fish are either absent or scarcely operational, and bathypelagic fish do not normally undertake vertical migrations. Filling bladders at such great pressures incurs huge energy costs. Some deep-sea fishes have swim bladders which function while they are young and inhabit the upper epipelagic zone, but they wither or fill with fat when the fish move down to their adult habitat.

A couple of known exceptions are the cusk-eel (Holcomycteronus profundissimus), retrieved from 7160 meters deep, and the rough abyssal grenadier (Coryphaenoides yaquinae), found at 7259 meters depth. These species still have functional swim bladders, which allows them to maintain high bone density and strong jaws.

The most important sensory systems are usually the inner ear, which responds to sound, and the lateral line, which responds to changes in water pressure. The olfactory system can also be important for males who find females by smell. Bathypelagic fish are black, or sometimes red, with few photophores. When photophores are used, it is usually to entice prey or attract a mate. Because food is so scarce, bathypelagic predators are not selective in their feeding habits, but grab whatever comes close enough. They accomplish this by having a large mouth with sharp teeth for grabbing large prey and overlapping gill rakers which prevent small prey that have been swallowed from escaping.

It is not easy finding a mate in this zone. Some species depend on bioluminescence, where bioluminescent patterns are unique to specific species. Others are hermaphrodites, which doubles their chances of producing both eggs and sperm when an encounter occurs. The female anglerfish releases pheromones to attract tiny males. When a male finds her, he bites on to her and never lets go. When a male of the anglerfish species Haplophryne mollis bites into the skin of a female, he releases an enzyme that digests the skin of his mouth and her body, fusing the pair to the point where the two circulatory systems join up. The male then atrophies into nothing more than a pair of gonads. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.

Many forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges, brachiopods, sea stars, and echinoids, but this zone is difficult for fish to live in.