The twilight zone, also called the mesopelagic zone, is home to many species of squid and octopus, as well as some bioluminescent fish, including the hatchet fish. Some jellyfish-like animals called ctenophores and siphonophores live in the zone as well.
The twilight zone does not have any plant life because the darkness of the water prevents photosynthesis. Animals that live there must be strong enough to withstand colder temperatures and heavier water pressure than animals that live closer to the surface. Fish of the twilight zone, including the viperfish and dragonfish, tend to have heavier bodies because, unlike surface fish, they do not need to swim quickly. They often have large teeth and mouths because their prey may be bigger than they are.
Some animals travel in shallower waters at times. These include larger fish, such as swordfish, chain catsharks and cuttlefish. Several types of eels, including wolf eels and snipe eels, can also be found in the mesopelagic zone. Some herbivores may venture into the mesopelagic zone at times, but do not stay there due to lack of food. The animals that live there most of the time are carnivores or animals that survive on waste matter, such as fecal pellets and dead organisms, that sink into the twilight zone. These animals are called detritivores.