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The unique adaptations that animals of the deep-sea have evolved to cope with their harsh environment.

Body Color: This is often used by animals everywhere for camouflage and protection from predators. In the deep sea, animals' bodies are often transparent (such as many jellies and squids), black (such as blacksmelt fishexternal link), or even red (such as many shrimp and other squids). The absence of red light at these depths keeps them concealed from both predators and prey. Some mesopelagic fish such as hatchetfishexternal link have silvery sides that reflect the faint sunlight, making them hard to see.

Reproduction: Consider how hard it must be to find a mate in the vast dark depths. For most deep sea species, we do not know how they achieve this. Earlier we noted that unique light patterns may aid in this. Deep-sea anglerfish may use such light patterns as well as scents to find mates, but they also have another interesting reproductive adaptation. Males are tiny in comparison to females and attach themselves to their mate using hooked teeth, establishing a parasitic-like relationship for life. The blood vessels of the male merges with the female's so that he receives nourishment from her. In exchange, the female is provided with a very reliable sperm source, avoiding the problem of having to locate a new mate every breeding cycle.

Gigantism: Another possible adaptation that is not fully understood is called deep-sea gigantismexternal link. This is the tendency for certain types of animals to become truly enormous in size. A well-known example is the giant squid, but there are many others such as the colossal squid, the giant isopodexternal link, the king-of-herrings oarfish (which may be the source of sea-serpent legendsexternal link), and the recently captured giant amphipodexternal link from 7,000external link m in the Kermadec Trenchexternal link near New Zealand. While the giant tubeworms of hydrothermal vents grow well due to abundant energy supplies, the other gigantic animals live in food-poor habitats, and it is not known how they achieve such growth. It may simply be a result of the feature we examine next: long lives.

Long Lives: Many deep-sea organisms, including gigantic but also many smaller ones, have been found to live for decades or even centuries. Long-lived fishes include rattails or grenadiersexternal link and the orange roughy, which are of special concern as they are targets of deep-sea fisheriesexternal link. These species reproduce and grow to maturity very slowly, such that populations may take decades to recover (if at all) after being overfished. This has happened repeatedly to the orange roughy, a deep-sea fish easily found congregating around seamountsexternal link in the southern oceans. Once fisheries have wiped out one seamount population, they move on to another seamount.