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In zoology, deep-sea gigantism, or abyssal gigantism, is the tendency for species of deep-sea dwelling animals to be larger than their shallower-water relatives across a large taxonomic range.

Proposed explanations for this type of gigantism include necessary adaptation to colder temperature, food scarcity, reduced predation pressure and increased dissolved oxygen concentrations in the deep sea. The harsh conditions and inhospitality of the underwater environment in general, as well as the inaccessibility of the abyssal zone for most human-made underwater vehicles, have hindered the study of this topic.

In marine crustaceans, the trend of increasing size with depth has been observed in mysids, euphausiids, decapods, isopods, ostracods and amphipods. Non-arthropods in which deep-sea gigantism has been observed are cephalopods, cnidarians, and eels from the order Anguilliformes. 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. Deep-sea gigantism is not generally observed in the meiofauna (organisms that pass through a 1 mm (0.039 in) mesh), which actually exhibit the reverse trend of decreasing size with depth.

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. 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.

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 widely scattered resources.

Another possible influence is reduced predation pressure in deeper waters.

Dissolved oxygen levels are also thought to play a role in deep-sea gigantism. A 1999 study of benthic amphipod crustaceans found that maximum potential organism size directly correlates with the increased levels of dissolved oxygen levels in deeper waters. The solubility of dissolved oxygen in the oceans is known to be lower in oxygen-poor intermediary depths (ranging 200–1000 meters) until the increasing pressure, decreasing salinity levels, and colder temperatures of deeper water can contribute increasing solubility once more. The proposed theory behind this trend is that deep-sea gigantism could be an adaptive trait to combat asphyxiation in frigid, dense ocean waters. Larger organisms are able to intake more dissolved oxygen within the ocean, allowing for sufficient respiration. However, this increased absorption of oxygen runs the risk of toxicity poisoning where an organism can have oxygen levels that are so high that they become harmful and poisonous.

Warmer global temperatures may have an effect on the deep sea as much as the shallower surface waters of the ocean, as evidence suggests that deep-sea ecosystems can be sensitive to shifts within the climate. Following trends from the Paleocene-Eocene thermal maximum and related timescales, research suggests that current predictions of continuous greenhouse gas emissions and climate change will lead to higher ocean temperatures and a significant reduction in levels of dissolved oxygen in the deep sea. Should global warming lead to a warmer ocean state, thermohaline circulation would no longer be able to maintain an oxygen-rich deep sea, which would eventually lead to deep water becoming higher in both temperature and salinity. Based upon current theories regarding the existence of deep-sea gigantism, we would expect to see the phenomenon diminish in response to these changes in the environment, as it may become unfavorable or even impossible for these organisms to sustain a larger body form.

Quick Facts

• Proposed explanations for this type of gigantism include necessary adaptation to colder temperature, food scarcity, reduced predation pressure and increased dissolved oxygen concentrations in the deep sea.
• Dissolved oxygen levels are also thought to play a role in deep-sea gigantism.
• Deep-sea gigantism is not generally observed in the meiofauna (organisms that pass through a 1 mm (0.039 in) mesh), which actually exhibit the reverse trend of decreasing size with depth.
• The proposed theory behind this trend is that deep-sea gigantism could be an adaptive trait to combat asphyxiation in frigid, dense ocean waters.
• The harsh conditions and inhospitality of the underwater environment in general, as well as the inaccessibility of the abyssal zone for most human-made underwater vehicles, have hindered the study of this topic.

Source material: Wikipedia - "Deep-sea gigantism". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.