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Introduction
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Deep Sea Gigantism Explained

There is a pattern in the deep sea of organisms growing far larger than many creatures from the shallows. The reasons behind it remain a mystery, but the phenomenon has been given a name.

Deep Sea Gigantism - the tendency of deep sea creatures to grow far larger than their shallow water relatives. There are a number of theories that try to explain this phenomenon. Read on for more.

Deep Sea Gigantism Explained
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Kleiber's Rule

Larger Animals are More Efficient

Kleiber's Rule states that 'animals that are larger will tend to be more efficient.' This can be demonstrated by comparing a whale to a small fish. The whale, with a mass hundreds of times larger than that of the fish, will have a greater metabolism. The smaller surface area to volume ratio gives the whale the advantage of conserving greater energy, with less energy lost to the surroundings through heat. In the deep, food is scarce, with most of the nutrients falling as marine snow - a trickle of organic debris from shallower waters. Thus, food is scarce, so there is an incentive to conserve this energy and to grow larger.

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Furthermore, larger animals have the ability to ingest larger prey. They will be more likely to survive environmental extremities or attacks from predators, and are able to cover larger distances in search of food or a mate. Reproductive success is also therefore aided by an organism being larger.

Abyssal Gigantism

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To understand this phenomenon, we first must understand why larger organisms are more efficient, and thus why deep sea creatures like the giant isopod tend to grow to these sizes.

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Sleeper Shark

Somniosidae

Sleeper sharks dwell at depths of 6,600 feet (2,000m). At 23 feet long (7m), they are among the largest shark species on Earth. And yet, they are nearly blind, they cruise along at a steady 1.7 miles per hour, and scavenge on rotting corpses at whale-fall sites.

Bergman's Rule

Sea Animals Grow Larger in Cold Climates

Bergman's rule states that 'sea animals tend to increase in body size with a decrease in temperature.' This certainly matches observations in colder regions like the arctic deep, where some of the largest ocean creatures like Greenland sharks, and giant sea spiders are found. The low temperatures lead to their cells growing larger. By slowing their metabolic rate, the cold also means creatures in such climates have longer lifespans.

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Slowed metabolisms cause polar creatures to live in 'relative slow motion', and the rich supply of oxygen found in cold water allows them to grow to gigantic sizes.

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Sea Spiders

Pycnogonida

These arthropods are commonly found scuttling along much of the ocean floor. They are abundant, with 1,300 known species, and are usually rather small at 0.04 inches (1mm). But in the depths of the Antarctic, they can be 3 feet long (1m). this is because the cold water carries more oxygen, so more of it diffuses into the sea spider's body, allowing it to grow larger.
 

The Island Rule

deep sea isolation

In 2006, biologist Craig R McClain investigated the gradient from the shallows to the deep sea. His findings seemed to mirror the framework of terrestrial islands, where these isolated areas of land develop indigenous biodiversity. More specifically, he found that the deep sea seemed to mirror the 'Island Rule', an idea that small-bodied life on islands grows much larger in its isolation than life on major land-masses, due to limited resources, predation and competition.

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According to McClain, 'the deep sea is functionally similar to an island'. The deep also has limited resources, few predators, and is isolated from the shallows. It follows that the diversification of life in the deep mirrors that on islands - the isolated communities develop larger body sizes in response to the conditions stated above.