Deep-sea gigantism

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A live 10 m (33 ft) colossal squid, the largest cephalopod, hauled in next to a fishing boat in the Ross Sea off Antarctica.

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 shallow-water counterparts. 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 shallow water, e.g., the great white shark.

[edit] Explanations

It is not known whether this effect 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.

One example for which scientists do have an explanation is the giant tube worm. These creatures live off hydrothermal vents, which supply them with enormous amounts of energy.

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.^[1] The trend with depth has been observed in mysids, euphausiids, decapods, isopods, and amphipods.^[1] 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.^[1] 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).^[1] 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.^[1]

[edit] Gallery

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.
Examination of a 9 m (30 ft) giant squid, the second largest cephalopod, that washed ashore in Norway.
A robust clubhook squid, the third largest squid, whose mantle reaches 2 m (6.6 ft) in length, caught off Alaska.
Navy SEALs display a 7 m (23 ft) king of herrings oarfish, caught off California.