In the bathypelagic zone (1,000–4,000 metres deep) there is a total absence of sunlight. Bioluminescence (light produced by living creatures) is the only source of light.
Food is even scarcer than in the mesopelagic zone above. With less energy available, most of the fish are ‘sit and wait’ predators, or actively attract prey with bioluminescent lures. Bathypelagic organisms are mostly black, red or transparent, rendering them essentially invisible in the weak biological light. Bristlemouths and deep-sea angler fish are the commonest fish, typically less than 10 centimetres long. Their small size reduces metabolic demands. Compared with their mesopelagic relatives, bathypelagic fish tend to have lower metabolic rates, less developed gills and muscles, and small eyes – if they have them at all.
Made mostly of water, the fish are not able to be compressed by the great pressure at this depth. While the greatest ocean depth is nearly 11,000 metres, fish have been found only down to 8,400 metres, where the pressure is around 800 times that at the surface. This may be because essential enzyme functions are affected by such pressure.
Adapted to great depths, bizarre-looking fish from the bathypelagic zone look even less pretty when hauled to the surface, where the pressure is several hundred times lower. Their eyes bulge, and gases in their system expand, causing the fish to burst.
Because only 5% of the food produced up in the epipelagic zone reaches the deep sea, many deep-sea fish will eat anything that comes their way, exhibiting extraordinary adaptations to enable this. The swallower eel (Saccopharynx lavenbergi) and the gulper eel (Eurypharynx pelecanoides) have gigantic jaws, mouths lined with many teeth, and highly elastic stomachs that can accommodate prey larger than themselves.
Related to the gulper eel is the remarkable monognathid eel (Monognathus bertini), with a hooked fang made from fused frontal bones and linked to what may be venom glands. Glands in the snout and dorsal fin secrete substances that attract shrimps which the eel grasps, forces against the fang and injects with venom. This meal provides enough nutrients for the eel to mature, after which the lower jaw degenerates and the eel eventually dies.
Female deep-sea anglerfish typically have a bioluminescent lure on a dorsal spine or the chin. Prey are attracted to the lure and ingested with the inrush of water as the fish opens its huge mouth.
Finding a mate is also difficult in the deep ocean. Angler fish have developed an unusual solution. The female releases pheromones that the tiny male fish home in on. When they reach the female they bite her and hold on, never to let go. In some species the male’s jaw fuses with the female tissue, the two circulatory systems join up and the male degenerates until he is little more than a bag of testes subject entirely to the hormonal regime of his host. Being smaller, the male requires much less food, leaving more for the female’s reproduction requirements.
Other deep-sea fish are hermaphroditic (with male and female sex organs), ensuring that a chance encounter between any two fish will provide both eggs and sperm. However, most deep-sea invertebrates do not use such strategies. They may depend on bioluminescence to attract each other.