In zoology

Zoology

Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...

, deep-sea gigantism, also known as abyssal gigantism, is the tendency for species of crustaceans, 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

Giant isopod

A giant isopod is any of approximately nine species of large isopods in the genus Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic...

, the Japanese spider crab

Japanese spider crab

The , Macrocheira kaempferi, is a species of marine crab that lives in the waters around Japan. It has the largest leg span of any arthropod, reaching up to and weighing up to . It is the subject of small-scale fishery.-Description:...

, the king of herrings

King of herrings

The king of herrings or giant oarfish, an oarfish of the family Regalecidae, is the world's longest bony fish. Rarely sighted, it is found in all the world's oceans at depths of between 300 and 1000 meters...

 (an oarfish

Oarfish

Oarfish are large, greatly elongated, pelagic Lampriform fishes comprising the small family Regalecidae. Found in all temperate to tropical oceans yet rarely seen, the oarfish family contains four species in two genera...

 of up to 12 m), the Seven-arm Octopus

Seven-arm Octopus

The Seven-arm Octopus is the largest known species of octopus based on scientific records, with a total estimated length of 4 m and weight of 75 kg...

, and a number of squid species: the Colossal Squid

Colossal Squid

The colossal squid , sometimes called the Antarctic or giant cranch squid, is believed to be the largest squid species in terms of mass. It is the only known member of the genus Mesonychoteuthis...

 (up to 14 m in length), the giant squid

Giant squid

The giant squid is a deep-ocean dwelling squid in the family Architeuthidae, represented by as many as eight species...

 (up to 13 m), the Robust Clubhook Squid

Robust Clubhook Squid

The Robust Clubhook Squid , often cited by the older name Moroteuthis robusta, is a species of squid in the family Onychoteuthidae. It is the third largest species of squid after the Colossal Squid and giant squid; and largest of the genus Onykia, reaching a mantle length of . The tentacular clubs...

, the Dana Octopus Squid

Dana Octopus Squid

The Dana Octopus Squid is a species of squid in the family Octopoteuthidae. It is one of the largest known squid species, reaching a mantle length of and total length of...

, Galiteuthis phyllura

Galiteuthis phyllura

Galiteuthis phyllura is a species of glass squid, possibly the largest in the genus.In 1984, the Russian stern-trawler Novoulianovsk brought up the remains of a gigantic specimen of G. phyllura from a depth of 1000–1300 m in the Sea of Okhotsk...

, Kondakovia longimana

Kondakovia longimana

Kondakovia longimana, also known as the Giant Warty Squid or Longarm Octopus Squid, is a large species of hooked squid. It attains a mantle length of at least 85 cm and probably over 1.15 m. The largest complete specimen of this species, measuring 2.3 m in total length, was found in Antarctica in...

, and the bigfin squid

Bigfin squid

The bigfin squids are a group of rarely seen cephalopods with a distinctive morphology. They are placed in the genus Magnapinna and family Magnapinnidae...

s.

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.