Giants of the deep

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Tales of sea monsters have been shared among humans for as long as we have had contact with the sea. Sea serpents, gigantic whales, and massive squids and octopuses have been described many times over the years, dating as far back as the works of Aristotle in the 4th century. There may, however, be some truth to these tales. After all, the ocean has a tendency to cultivate creatures much larger than those residing on land. What is it that allows the sea to produce such giants?

The African elephant is the largest animal species on land, with the largest individuals growing up to 4m tall, but it is dwarfed by the largest marine animal, the blue whale, which reaches 30m in length. The vast size of the blue whale is an example of how large animals can grow when unconstrained by the effects of gravity. The buoyancy provided by the ocean means that sea creatures can grow as large as is necessary to support their existence, and such whales need to grow large enough to prevent them from losing too much body heat to the surrounding water. They are, like elephants, warm-blooded mammals after all. However, the blue whale is not the only marine animal to grow to vast sizes - there are many deepsea creatures that similarly dwarf their terrestrial or shallow-water counterparts, a concept known as deep-sea gigantism.

The giant isopod (a massive crustacean related to woodlice, shrimp, crabs, and so on) is just one example of a deep-sea dweller that grows much larger than its terrestrial relatives. Living on the ocean floor at depths of over 500m, its body length can grow up to 50cm, which is impressive considering that terrestrial and shallow-water isopods rarely exceed 3-5cm in length. Similarly, the Japanese spider crab occupies the same habitat, and its carapace, the main part of its body, can reach 40cm, while its claws span up to 3.7m. In comparison, the European spider crab, a related shallow-water species, only reaches a carapace length of up to 20cm and claw span of 50cm.

The phenomenon of gigantism does not just apply to crustaceans. The giant squid, a creature that has fascinated humans for many years and may be the source of tales of monsters like the kraken, resides at depths of around 1,000m and grows to lengths of 13m. Comparably, the deeper-dwelling colossal squid grows as long as 10m, but is much greater in mass, reaching half a tonne. It is also believed to have the largest eyes of any animal on Earth, having a diameter of 30-40cm. Meanwhile, most other species of squid living in the photic zone (the top 200m of the ocean) only grow to about 60cm in size.

It is clear that species living at great depths tend to grow larger, but the factors driving this phenomenon are less well understood. Deep-sea gigantism is difficult to study, given the inaccessible nature of the abyss. However, it may be explained by two theories. The first is Bergmann's rule which posits that animals are more likely to grow to larger sizes in colder regions than those living in warmer environments. This is because cold habitats are believed to produce animals with larger cells and, thus, longer lifespans. Given that the depths of the ocean are as cold as 0-3°C, it seems plausible that Bergmann's rule would apply here. The other is Kleiber's law, which states that larger animals are more metabolically efficient. That is, a larger animal uses less energy to respire per unit body mass than a smaller one. It is possible that the larger body size of deep-sea dwellers allows them to conserve more energy between meals in a habitat where food is scarce and is, therefore, more beneficial.

As technology progresses and expeditions to the deep become more viable, our understanding of this phenomenon will no doubt improve. Until then, however, deep-sea gigantism will remain another of the ocean's mysteries.

Rachel Herbert-Goddard is a freelance writer based in the UK.

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Many deep-sea creatures similarly dwarf their terrestrial or shallow-water counterparts, a concept known as deep-sea gigantism.