The bathyal zone or bathypelagic – from Greek βαθύς (bathýs), deep – (also known as midnight zone) is the part of the pelagic zone that extends from a depth of 1000 to 4000 metres (3300 to 13000 feet) below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The average temperature hovers at about 4 °C (39 °F). Although larger by volume than the euphotic zone, the bathyal zone is less densely populated. Sunlight does not reach this zone, meaning there can be no primary production. It is known as the midnight zone because of this feature. Because of the lack of light, some species do not have eyes, however those possessing eyes in this zone include the viperfish and the frill shark. Many forms of nekton live in the bathyal zone, such as squid, large whales, and octopuses, but this zone is difficult for fish to live in. Sponges, brachiopods, sea stars, and echinoids are also common in the bathyal zone. The fish in this zone have become very energy efficient, since it is especially hard to find nutrients. Many have slow metabolic rates to conserve energy. The fish here have weak muscles, soft skin and slimy bodies. Animals in the bathyal zone are not threatened by predators that can see them, so they do not have powerful muscles. Animals need to have the right adaptations to survive in this lethal area. There are no plants because of the lack of sunlight necessary for photosynthesis. In the bathyal some of the world's largest whales feed. The adaptations of some of the fish that live there include small eyes and transparent skin.
Except where the ocean is exceptionally deep, the bathyal zone extends to the benthic zone on the ocean bed of that part of the continental slope that lies between 1000 and 4000 meters deep.
Enig, C. C. (1997). Bathyal zones on the Mediterranean continental slope: An attempt. Research on marine benthos: 9th Iberian Symposium on Studies of Marine Benthos. Madrid: MAPA, SGT. pp. 23–33. ISBN 8449102995.