Lanternfish rule a realm where sunlight never reaches
Below the sunlit epipelagic zone lies a vast dark ocean where lanternfish dominate among deep-sea fish. Anglerfish, viperfish, and bristlemouths share bathypelagic and abyssopelagic depths. Bioluminescence, pressure tolerance, and fragile gas spaces shape how these animals live and die.
Sunlight drives photosynthesis only in the top 200 metres or so, which leaves about nine-tenths of the ocean's volume dark. Down there the water seldom tops 3 °C and can drop to −1.8 °C, oxygen runs short, and pressure ranges from 20 to 1,000 atmospheres, climbing by one atmosphere every ten metres. Twilight persists between 200 and 1,000 metres, where an oxygen-minimum layer sits somewhere around 700 to 1,000 metres; below a kilometre the bathypelagic and abyssopelagic zones get no light at all. Lanternfish are by far the most numerous residents, alongside flashlight fish, bristlemouths, viperfish, anglerfish and cookiecutter sharks.
Italy's Palombini Shale, laid down about 130 million years ago on the abyssal floor of the vanished Piemont-Liguria Ocean, preserves swimming and feeding traces left by neoteleost fish. Before that find, no deep-sea bony fish older than about 50 million years was known. Late Cretaceous rocks in Canada and Angola hold frilled sharks, catsharks and other hexanchid relatives that still favour deep water, while Eocene beds at Monte Solane in Italy and in Iran's Pabdeh Formation are rich in stomiiform fish. Some researchers link the colonisation of the abyss to extra nutrients reaching the sea after flowering plants spread on land.
Food arrives mostly as marine snow, a steady fall of dead plankton, faeces, sand and dust whose flakes can grow several centimetres wide and take weeks to sink, although most of it is eaten well before reaching the bottom. Lanternfish, hatchetfish and lightfish crowd around seamounts and continental slopes, where their prey also gathers. Because their bodies carry the same pressure as the surrounding water they are not crushed, but their cell membranes stiffen, so they pack in more unsaturated fatty acids to stay supple.
More than half of deep-sea fish make their own light, and about 80 percent of those use photophores, glands filled with glowing bacteria, some fitted with lenses to brighten or dim the output, at a cost of just 1 percent of the animal's energy. Some species are blind and hunt by smell and pressure changes, while others have eyes up to 100 times more sensitive than ours. Most die in captivity, and their gas-filled spaces can burst when they are hauled to the surface.
Source: Deep-sea fish
