Why some animals grow huge in the deep sea

This notebook has a squid in its header, so it seems fair to ask how squid get so big.

Deep-sea gigantism is the tendency of some deep-water animals to grow far larger than their shallow relatives. An amphipod is usually a small crustacean you could lose on a fingertip. Alicella gigantea reaches 34 cm and lives between about 3,900 and 8,900 metres. Sea spiders the size of a coin in shallow water can reach the size of a dinner plate at depth. The giant squid has a verified maximum of about 12 metres, and the colossal squid is heavier, with one 2007 specimen estimated at around 450 to 500 kg.

The strange part is that the deep sea looks like a terrible place to be big. It is dark and cold, and most of its food is whatever sinks from above.

The usual explanations

The first is temperature. Across many groups, animals in colder water tend to grow more slowly, live longer, and end up larger. Below about 600 metres the ocean stays close to 4°C. This cannot be the whole story, though, because size keeps changing with depth long after the temperature stops changing.

The second is food, and I find this one the most elegant. Metabolic rate usually grows more slowly than body mass, roughly with mass to the power of 0.75. This is Kleiber's law. An animal 16 times heavier needs only about 8 times the energy, so each kilogram costs half as much to run. A large body is also a bigger fuel tank, and it can travel further between meals. The giant isopod shows how extreme this can get: one kept at the Toba Aquarium in Japan went 5 years and 43 days without eating before it died in 2014. That is a captive animal with an unknown cause of death, so it proves little, but it is hard to forget.

The third is oxygen. Cold water holds more of it, and a slow metabolism needs less, so the limit on how big a body can get may loosen. A 1999 study linked oxygen to giant amphipods in polar waters, and a 2001 study found that the maximum size of deep-sea snails rose with dissolved oxygen.

The fourth is predators, which can push size in both directions. A giant squid has an eye of about 27 cm. One study estimated that this would let it see a sperm whale from more than 120 metres away in the dark, and suggested that escaping those whales may have helped drive both the huge eyes and the huge bodies. The authors were clear that this is a hypothesis, since nobody can easily watch these encounters.

The headline hides the average

What surprised me most is that gigantism is not the main trend. A global analysis of deep-sea seabed animals found that abundance and biomass fall exponentially with depth, and that average body size shrinks as well. Small organisms take over. One study in the North Atlantic found about 50 tiny meiofauna for each larger animal on the shelf, and more than 1,000 in the abyss.

So scarcity seems to push most of the community toward being small, and a few lineages toward being very large. Both can be sensible answers to the same problem. If there is little food, you can need very little, or you can be able to go a long way to find it.

We know less than the size charts suggest

These animals are also hard to measure. The first colossal squid filmed alive in its habitat was a 30 cm juvenile, recorded only in March 2025. When researchers reviewed size records for ocean giants in 2015, they brought the giant squid down from the often quoted 18 metres to about 12 verified metres. Old sightings, damaged carcasses and stretched tentacles all push the numbers up.

That is the part I would keep. A striking pattern arrived with four tidy explanations, and each one seems to hold for some animals and fail for others. Giants are also exactly what gets noticed, so the deep sea has the reputation of making things bigger, while most of it is making them smaller.