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In zoology, deep-sea gigantism, also known as abyssal gigantism, is the tendency for species of invertebrates and other deep-sea dwelling animals to display a larger size than their shallower-water relatives. Examples of this phenomenon include the giant isopod, the giant amphipod, the Japanese spider crab, the king of herrings (an oarfish of up to 12 m), the deepwater stingray, the seven-arm octopus, and a number of squid species: the colossal squid (up to 14 m in length), the giant squid (up to 13 m), Onykia robusta, Taningia danae, Galiteuthis phyllura, Kondakovia longimana, and bigfin squids. Some other very large fish found in the deep ocean, such as the Greenland shark and the Pacific sleeper shark, would not normally be considered examples because they sometimes visit the surface and are not larger than comparable species that spend more time in shallower water, such as the great white shark.
It is not known whether deep-sea gigantism comes about as a result of adaptation for scarcer food resources (therefore delaying sexual maturity and resulting in greater size), greater pressure, or for other reasons.
In the case of marine crustaceans, it has been proposed that the increase in size with depth occurs for the same reasons as the increase in size with latitude (Bergmann's rule): both trends involve increasing size with decreasing temperature. The trend with depth has been observed in mysids, euphausiids, decapods, isopods, and amphipods. The trend with latitude has been observed in some of the same groups, both in comparisons of related species as well as within widely distributed species. Decreasing temperature is thought to result in increased cell size and increased life span, both of which lead to an increase in maximum body size (continued growth throughout life is characteristic of crustaceans). In Arctic and Antarctic seas where there is a reduced vertical temperature gradient, there is also a reduced trend towards increased body size with depth, arguing against hydrostatic pressure being an important factor.
Temperature does not appear to have a similar role in influencing the size of giant tube worms. Riftia pachyptila, which lives in hydrothermal vent communities at ambient temperatures of 2–30 °C, reaches lengths of 2.7 m, comparable to those of Lamellibrachia luymesi, which lives in cold seeps. The former, however, has rapid growth rates and short life spans of about 2 years, while the latter is slow growing and may live over 250 years.
A giant isopod (Bathynomus giganteus) may reach up to 0.76 metres (2.5 ft) in length.
A Japanese spider crab whose outstretched legs measured 12 ft (3.7 m) across.
- Foster's rule
- Insular dwarfism
- Island gigantism
- Largest organisms
- Cephalopod size
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