The
bathyal zone or
bathypelagic – from Greek (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
primary production, if any, is almost nonexistent. 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 known 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.