Hobbies And Interests
Migrating Fish
The thermocline-twilight zone serves as home to a large variety of fishes. Some of these are able to migrate between the zones, swimming to the upper zones of the ocean to feed. The lanternfish is very common and can be found in such large numbers that they are visible on sonar. These fish have one of the common and notable adaptations of deep-sea organisms--bioluminescence, or the ability to glow. Bristlemouths are another type of migrating fish. Both of these fish have swim bladders that enable them to make this migration, which is difficult because of the drastic change in temperature and pressure. Some sharks, like the cookie cutter shark, are also able to migrate to and from this zone and have oily livers that serve as their swim bladders.
Non-Migrating Fish
Many fish in the thermocline-twilight zone have no swim bladder since the low oxygen content makes it difficult to obtain enough gas for one. Fish that do not migrate from the thermocline-twilight zone tend to be flabby rather than streamlined and do not move much. These fish often use bioluminescence to attract prey since light in the thermocline-twilight zone attracts attention. Because finding prey can be difficult, many of these fish are able to swallow prey larger than their mouths by unhinging their jaws and have very large stomachs so that they can eat whatever they find. Hatchetfish, dragonfish and anglerfish are three species of non-migrating fish; viperfish match the description of a non-migrating deep sea fish but have been found at higher depths, which suggests that they do migrate.
Jellyfish
Transparency is an adaptation that is extremely valuable. In the thermocline-twilight zone, transparent animals have an advantage in that they do not cast shadows and are thus nearly invisible to predators. Jellyfish are common. There are also many jelly-like animals that are not well understood as they disintegrate when hauled up in nets. This group includes animals known as larvaceans, which create a mucus-like material to trap marine snow for food. Siphonophores are jellyfish-like animals that are actually formed from a colony of related organisms; the Portugese man-of-war is a siphonophore.
Squid and Shrimp
Squid and shrimp are both very common. Squid, like giant squid and firefly squid, are able to adapt to deep water fairly easily because their bodies are largely composed of water, which means they are not impacted greatly by the intense pressure of deep water. Some species of squid and shrimp are able to eject a cloud of bioluminescent ink that blinds a potential predator. Bioluminescence can be defensive in other ways. For instance, both squid and shrimp sometimes have bioluminescent organs on their undersides, which means that a predator looking up at a bioluminescent organism has a difficult time seeing it since the light from the underside blends in with the light from the sun above.
Other Animals
The thermocline-twilight zone is home to a vast variety of animals. This includes many small invertebrates, such as worms and tubes. Krill are an important part of the ecosystem at this level and are eaten by many types of animals. Larvae from many animals and fish eggs are also commonly found at this level.