Over 60% of the Earth’s surface is covered by water over a mile deep. This vast habitat makes up 25% of all species on Earth. It is also home to some of Earth’s giants. The shift from small, shallow-water animals to ferocious, deep-dwelling monsters is a phenomenon known as deep-sea gigantism. This trend has been observed in various groups of marine animals – from crustaceans to cephalopods.

The Land of the Giants

As depth increases light intensity decreases exponentially until there is nothing but darkness. Sunlight rarely reaches beyond 200 m into the ocean; however, in the right conditions, it is possible for sunlight to reach depths of approximately 1000 m. Long wavelength light (e.g. red) is absorbed the fastest and therefore does not reach the ocean depths. This is why many deep-sea species have adapted red pigmentation, rendering them essentially invisible.

The three zones of light level in the ocean, determined by depth. NOAA.

Another characteristic of the deep-sea environment is the profound increase in pressure. For every 10 m increase in depth, the pressure increases by one atmosphere (atm). The Mariana Trench is 11 km deep, making it the deepest point in the ocean. At this depth, the pressure is over 1000 times that of the sea level atmosphere.

This substantial increase in pressure poses several risks for ocean-dwelling species, including nitrogen narcosis and blackouts. As a result, a huge variety of adaptations have evolved for coping with this extreme increase in pressure such as specially adapted enzymes and a reduced metabolic rate.

The location of the Mariana Trench, the deepest point in the ocean. DEEPSEA CHALLENGE.

Both latitude and depth play a role in determining the temperature of the ocean. Surface waters are more variable in temperature due to the differing amounts of solar radiation. Tropical surface waters can reach temperatures of 30°C, whereas the poles average at -2°C.

At low-mid latitudes, temperature generally decreases with depth. This is due to the decreasing intensity of sunlight as depth increases which results in consistent cold temperatures of around 0 – 3°C. These low temperatures have serious implications, and as such, deep-dwelling species possess a number of adaptations to overcome them.

So how exactly do giants overcome these extreme challenges?

Gigantism: Causes and Effects

Deep-sea gigantism is thought to occur for several reasons. Kleiber’s law is a biological principle that dictates how big an animal can grow. In 1932, Max Kleiber observed that an animal’s metabolic rate scales to ¾ the power of an animal’s mass. In short, the larger the animal is, the more energy efficient.

Due to the deep sea’s characteristically scarce food supply, this means that animals with larger bodies will be at an advantage. Despite being a controversial principle, it is widely acknowledged among scientists today and is a possible explanation for the giants of the deep sea.

Another ecological rule influencing body size is the island rule devised by Van Valen in 1973. This rule states that members of a species vary in size according to the resources available in the environment, with island dwelling animals evolving larger bodies.

The deep sea can be considered an insular environment, and researchers found that the low availability of food in the deep sea results in patterns following the island rule. Yet again, the characteristic lack of food in the deep sea may be an important factor resulting in the many deep-sea giants we see today.

The temperature change associated with depth is one of the most influential drivers of deep-sea gigantism in many species, especially crustaceans. Consistent with Bergmann’s rule (the tendency for species to grow to larger sizes in colder environments), researchers discovered that an increase in latitude and depth, and associated decrease in temperature, induced a slower metabolism. This in turn increased cell size, life span, and therefore an overall increase in body size.

This trend was especially apparent in deep-sea crustacea, in which continual growth is characteristic. The temperature explanation for deep-sea gigantism is considered more convincing than previously proposed hypotheses. This is because an animal’s metabolism is a function of their size and environmental temperature, and therefore the increased energy efficiency of deep-sea giants is a result of their gigantism rather than its cause.

We have explored some of the potential underlying causes of deep-sea gigantism. Now it is time to meet some of the largest known creatures on the planet.

Meet the Giants

The colossal squid (Mesonychoteuthis hamiltoni) is an elusive deep-sea predator and the largest of all known species of invertebrate, weighing in at half a tonne and reaching lengths of 13 metres. In line with Bergmann’s rule, this species is found at high latitudes in the southern hemisphere at depths of over 1000 metres. Most of our knowledge of colossal squids is derived from stomach samples of sperm whales. Only twelve in-tact specimens have been collected, none of which have been male and/or sexually mature. Because of this, not much is known about this giant’s behaviour and life history. This mysterious creature is classified as of ‘least concern’ by the IUCN Red List of Threatened Species, however, this is largely due to its uncharted nature.

A colossal squid specimen found in New Zealand waters. BBC News.

The seven-arm octopus (Haliphron atlanticus) is one of the largest known species of octopus (similar in size to the giant Pacific octopus), weighing 75 kilograms and reaching lengths of 3.5 metres. The seven-arm octopus is an elusive species and not much is known about its ecology due to the isolated nature of its habitat. However, a recent discovery by the Monterey Bay’s Aquarium’s deep diving robotic submersible catches a live individual on camera for only the third time in 27 years. The footage (shown below) shows a seven-arm octopus holding an egg-yolk jellyfish in its arms. The jellyfish appears to be partially consumed, with the stinging tentacles untouched. These are thought to be used as a defence or a tool to catch prey. The seven-arm octopus is also considered to be of ‘least concern’ by the IUCN Red List.

A seven-arm octopus consuming an egg-yolk jellyfish and possibly using the tentacles as a defence mechanism. MBARI.

Japanese spider crab (Macrocheira kaempferi) possesses the largest leg span (1.0 – 3.7 metres) of any arthropod and is found at depths of 50 to over 500 metres. Adults spawn in shallower waters (~50 metres) and individuals can live up to 100 years. However, this is rarely the case as these long-legged crustaceans are a delicacy in Japan and therefore the target of prosperous fisheries. Due to overfishing, new laws have been put in place to restrict harvesting during the mating season (January – April).

A Japanese spider crab to scale from the American Museum of Natural History. Popular Science Magazine, June 1920.

The giant isopod, Bathynomus giganteus, is another giant crustacean and a distant relative of the Japanese spider crab. Found on the deep-sea benthos, these pre-historic scavengers have confirmed lengths of 50 cm, roughly 50x larger than their terrestrial cousins. These giants are found all over the world, in the cold deep-water environments of the Atlantic, Pacific and Indian oceans. Unlike their decapod counterpart, the giant isopod is not a commercially fished species. However, they have been known to cause damage to fishing nets and have even been observed to take down sharks (see below). Scientists speculate that there may be undescribed species of giant deep-water isopods that have yet to be discovered.

The largest isopod, Bathynomus giganteus. NOAA.

The Land of Opportunity

Over 80% of the ocean remains unexplored and unmapped due to its intolerable nature. In recent years, there has been increased efforts to expand our knowledge of the deep-sea environment and its inhabitants through underwater observatories and deep-sea expeditions. However, the deep ocean still represents one of the only locations on Earth left mostly uncharted and presents immeasurable opportunities of discovery.

Deep-sea gigantism may be one of many phenomena observed in deep-sea biota. The existence of the colossal squid and numerous other giants begs the question: what else is out there?

Written by Lucas King