Giant oar fish are dangerous

Deep sea gigantism - Deep-sea gigantism

Investigation of a 9 m giant squid, the second largest cephalopod, that washed ashore in Norway.

In zoology it is Deep sea gigantism the tendency for species of invertebrates and other deep-sea animals to be larger in a large taxonomic area than their relatives in shallower water. Suggested explanations for this type of gigantism include colder temperatures, food shortages, reduced predatory pressures, and increased levels of dissolved oxygen in the deep sea. The inaccessibility of abyssal habitats has hampered research on this topic.

Taxonomic area

In marine crustaceans, the trend of an increase in size with depth was 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 of the Anguilliformes.

Other [animals] reach gigantic proportions among them. It is particularly certain crustaceans that have this latter characteristic, but not all crustaceans, as the cancerous forms in the deep sea are of common size. I have already referred to a gigantic pycnogonid [spider crab] that we were dredging. Mr. Agassiz has dredged a gigantic isopod that is eight inches long. We also dredged a huge ostracod. For over 125 years, scientists have considered the extreme size of Bathynomus giganteus . - Henry Nottidge Moseley, 1880

Examples of deep-sea gigantism are the great red jellyfish, the giant isopod, the giant ostracode, the giant spider crab, the giant amphipod, the Japanese spider crab, the giant oarfish, the deepwater stingray, the seven-armed squid, and a number of species of squid: 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.

Deep-sea gigantism is generally not observed in meiofauna (organisms that traverse a 1mm web), which actually have the opposite trend of decreasing size with depth.


Lower temperature

In crustaceans, it has been suggested that the explanation for increase in size with depth is similar to that for increase in size with latitude (Bergmann's rule): both trends involve an increase in size with decreasing temperature. The trend with latitude has been observed in some of the same groups, comparing both related species and widely distributed species. It is believed that a falling temperature leads to an increase in cell size and a prolonged lifespan (the latter is also linked to delayed sexual maturity), both of which lead to an increase in maximum body size (continuous growth throughout life is characteristic of crustaceans ). In Arctic and Antarctic seas, where there is a decreased vertical temperature gradient, there is also a decreased trend towards increasing body size with depth, with hydrostatic pressure being an important parameter.

Temperature does not appear to play a similar role in influencing the size of giant tube worms. Riftia pachyptila who lives in hydrothermal venting communities at ambient temperatures of 2–30 ° C, reaches a length of 2.7 m, comparable to that of Lamellibrachia luymesi who lives in cold seeps. However, the former has rapid growth rates and short lifetimes of around 2 years, while the latter grows slowly and can live over 250 years.

Food shortages

Food scarcity is also believed to be a factor at depths greater than 400 m, as larger body size can improve the ability to forage for widely dispersed resources. In organisms with planktonic eggs or larvae, another possible advantage is that larger offspring with larger initially stored food reserves can drift over greater distances. As an example of adaptations to this situation, giant isopods feed on food when available and stretch their bodies to the point that their ability to move around is impaired. They can also survive in captivity for 5 years without food.

According to Kleiber's rule, the larger the animal, the more efficient its metabolism. This means that the metabolic rate of an animal is roughly scaled to the ¾ force of its mass. In conditions of limited food supply, this can be of great benefit for large sizes.

Reduced piracy

Another possible influence is the reduced predatory pressure in deeper waters. A study of brachiopods found that predators were almost an order of magnitude less common in the greatest depths than in shallow waters.

Increased dissolved oxygen

It is also believed that dissolved oxygen levels play a role in deep-sea gigantism. A 1999 study of amphipod benthic crustaceans found that the maximum potential size of the organism correlates directly with increased levels of dissolved oxygen in deeper waters. It is known that the solubility of dissolved oxygen in the oceans increases with increasing pressure, decreasing salinity and decreasing temperature with depth.

The proposed theory behind this trend is that deep-sea gigantism could be an adaptive trait for combating asphyxiation in marine waters. Larger organisms can take up more dissolved oxygen in the ocean, which allows adequate breathing. However, this increased oxygen uptake carries the risk of toxicity poisoning if an organism can have oxygen levels that are so high that they become harmful and toxic.


  • A giant isopod ( Bathynomus giganteus ) can reach a length of up to 0.76 m.

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