Deep sea gigantism is a tendency for animals to develop to an unusually large size when they live in the depths of the ocean. The Colossal Squid is a notable example of deep sea gigantism, and a number of other species with smaller counterparts in the shallow zones turn monstrously large in the deep sea. Given that much of the deepest part of the ocean remains unexplored, some scientists expect to find more giant creatures cruising the deep as they expand ocean exploration world wide.

Crustaceans are often prone to deep sea gigantism.

In addition to squid, the depths of the ocean also harbor huge isopods, giant crabs, massive tubeworms, and monstrously-sized fish. Crustaceans and mollusks seem to be especially prone to the condition. Some of the animals which live in the deeps are among the largest organisms in the world, and they have developed a number of adaptations which allow them to survive in the harsh environment at the bottom of the ocean.

The deeper into the ocean you go, the more intense the pressure gets. Animals that live in extremely deep water must have bodies which can withstand formidable pressure, and they also need to cope with cold, since the bottom of the ocean is extremely cold. Furthermore, the ocean's depths are pitch black, leading many deep sea animals to develop bioluminesce, and to have highly refined senses so that they can sense potential prey and predators.

The reason for deep sea gigantism is not really understood. Being large does confer some advantages, such as a greater ability to regulate body temperature, and a lack of need to be constantly on the move, but large animals also take longer to develop. Deep sea gigantism may reflect the slow pace of life near the ocean floor, with animals taking decades to mature. These giants can also live for a very long time, and they reach sexual maturity late, slowing the overall rate at which the species reproduces. Were humans to learn how to exploit the depths of the ocean, many deep sea giants could be in trouble, because they could probably not adapt quickly enough to cope.

Whether the giants or the normal-sized animals came first, deep sea gigantism represents some of the oldest of living creatures, since all life on Earth started out in the world's oceans. Some of these giant animals have ways of life markedly different from those of surface creatures. Giant tubeworms, for example, chemosynthesize for energy, using the energy from hydrothermal vents instead of the energy of the sun.