In zoology, deep-sea gigantism, also known as abyssal gigantism, is the tendency for species of invertebrates and other deep-sea dwelling animals to be larger than their shallower-water relatives. Proposed explanations for this type of gigantism include colder temperature, food scarcity, and reduced predation pressure in the deep sea. The inaccessibility of abyssal habitats has hindered the study of this topic.
In marine crustaceans, the trend of increasing size with depth has been observed in mysids, euphausiids, decapods, isopods, and amphipods. Non-arthropods in which deep-sea gigantism has been observed are cephalopods, cnidarians, and eels from the order Anguilliformes.
|“||Other [animals] attain under them gigantic proportions. It is especially certain crustacea which exhibit this latter peculiarity, but not all crustacea, for the crayfish like forms in the deep sea are of ordinary size. I have already referred to a gigantic Pycnogonid [sea spider] dredged by us. Mr. Agassiz dredged a gigantic Isopod eleven inches in length. We also dredged a gigantic Ostracod. For over 125 years, scientists have contemplated the extreme size of Bathynomus giganteus. – Henry Nottidge Moseley, 1880||”|
Examples of deep-sea gigantism include the big red jellyfish, the giant isopod, giant ostracod, the giant sea spider, the giant amphipod, the Japanese spider crab, the giant oarfish, 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 12 m), Onykia robusta, Taningia danae, Galiteuthis phyllura, Kondakovia longimana, and the bigfin squid.
In crustaceans, it has been proposed that the explanation for the increase in size with depth is similar to that for the increase in size with latitude (Bergmann's rule): both trends involve increasing size with decreasing temperature. 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 (the latter also being associated with delayed sexual maturity), 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 parameter.
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.
Food scarcity at depths greater than 400 m is also thought to be a factor, since larger body size can improve ability to forage for scattered resources. In organisms with planktonic eggs or larvae, another possible advantage is that larger offspring, with greater initial stored food reserves, can drift for greater distances. As an example of adaptations to this situation, giant isopods gorge on food when available, distending their bodies to the point of compromising ability to locomote; they can also survive 5 years without food in captivity.
According to Kleiber's rule, the larger an animal gets, the more efficient its metabolism becomes; i.e., an animal's metabolic rate scales to roughly the ¾ power of its mass. Under conditions of limited food supply, this may provide additional benefit to large size.
Reduced predation pressure
An additional possible influence is reduced predation pressure in deeper waters. A study of brachiopods found that predation was nearly an order of magnitude less frequent at the greatest depths than in shallow waters.
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.
A Stygiomedusa jellyfish, which can grow up to 10 m (33 ft) in length.
A deepwater stingray, which can reach up to 2.7 meters by 1.5 meters in size.
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