The Enigma of Deep Sea Gigantism

This article is based on the video titled “Why Do Deep Sea Creatures Evolve Into Giants?” by Real Science. The video delves into the fascinating world of deep-sea gigantism, a phenomenon where marine life forms in the deep sea grow significantly larger than their shallow-water counterparts. This article will further explore this intriguing subject, shedding light on the reasons behind this unique adaptation.


The Deep Sea and Its Inhabitants

The deep sea is a vast, dark, and nearly freezing cold environment, home to a variety of gigantic creatures. As we descend through the ocean layers, from the sunlit epipelagic zone to the pitch-black hadalpelagic zone, we encounter a range of organisms that have adapted to the extreme conditions. These include the giant Japanese spider crab, the big red jellyfish, the king of herrings oarfish, the giant squid, the Greenland shark, and the giant isopods. These creatures are all examples of deep-sea gigantism.

The Mystery of Deep Sea Gigantism

In the intensely cold and dark waters of the deep sea, the emergence of these leviathans raises intriguing questions. Is it a mere coincidence, or is it a feature of life in this inhospitable landscape? The scarcity of food below 400 meters, where sunlight tapers off and photosynthetic algae and plankton disappear, is a significant pressure for deep-sea animals. Most of them rely on marine snow, composed of dead plankton, fecal pellets, and bits of rotting corpses that fall to the sea floor. This scarcity of food and the high pressure of predation have led to the evolution of larger sizes, transitioning creatures from prey to top predators.

The Giants Squids

Among the most iconic deep-sea giants is the giant squid, an enigma of the deep. The largest individual ever found was 13 meters long, as long as a school bus, and weighed 275 kilograms. The colossal squid, sometimes called the Antarctic squid, is the largest invertebrate in the world. It’s shorter in length than the giant squid but can weigh between 500 and 700 kilograms. These squids have few predators due to their size and can comfortably prey on deep-sea fish and other squid species.

The Metabolic Efficiency of the Colossal Squid

The colossal squid, despite its intimidating size, is not the aggressive predator one might imagine. Instead, it exemplifies an important characteristic of deep-sea giants: an extremely slow metabolism. The metabolic rate of the colossal squid is so low that they only burn 45 calories per day and require a mere 0.03 kilograms of food per day. This slow pace of life is a survival strategy in the deep sea, where food is scarce and the environment is harsh.

The Greenland Shark

The Greenland shark, the largest fish in the Arctic Ocean and one of the largest sharks on earth, is another example of deep-sea gigantism. It lives at depths over two thousand meters where the water temperature is between negative two and seven degrees Celsius. This extremely cold environment has caused the shark to not just be huge but ancient. Greenland sharks are the longest living vertebrates in the world. Scientists estimate their average lifespan to be at least 272 years, with some individuals living well over 500 years. This astoundingly long life can be attributed to their exceedingly low metabolism, much like the colossal squid.

The Supergiant Amphipod

In the hadal trenches, six thousand to eleven thousand meters deep, we find the supergiant amphipod, Alicella gigantea, the largest amphipod ever discovered. Shallow water amphipods are usually around 5 millimeters to 20 millimeters in length, but these deep-water behemoths can grow up to 34 centimeters long. These amphipods are scavengers, eating any decomposing material they can find. Their large body size may help them store as much food and energy as possible when they can find it. Furthermore, their large body size allows them to travel greater distances in search of their next meal.

The Delicate Ecosystem of the Deep Sea

The deep sea is a delicate ecosystem. Many of these animals live on a knife edge of survival, and any change to their environment could mean the end for these giants. Overfishing, plastic pollution, changes in ocean chemistry due to climate change, and deep-sea mining are all threats to this incredible ecosystem. We are only beginning to understand the intrinsic connection between ocean ecosystems and the links between them and the terrestrial environments in which we live.

Bergmann’s Rule and Deep Sea Gigantism

Bergmann’s rule, which states that animals found in cold environments will be larger than those found in warm environments, has historically been applied to endotherms, or warm-blooded animals. However, it appears that this rule may also apply to ectotherms, or cold-blooded animals, such as the squids, crabs, and isopods that inhabit the deep sea. This is a controversial area of science, but it’s clear that certain species of ectotherm are indeed larger at colder temperatures.

The Role of Pressure and Temperature in Deep Sea Gigantism

The pressure and temperature of the deep sea may also play a role in gigantism. At such depths and cold temperatures, water would feel thicker than water at the surface. Larger body size may give these deep-sea invertebrates a respiratory advantage that helps them overcome the larger viscous forces in the water (Seibel, 2019).

The Unexpected Food Source of Deep Sea Giants

Recently, researchers discovered that some deep-sea creatures might be feeding on an entirely unexpected food source. A hadal amphipod, Hirondellea gigas, was found to have a unique cellulase enzyme that seemed to break down plant matter. This was surprising because no plant life can live in the hadal depths. However, large pieces of driftwood occasionally sink to the bottom of the deepest parts of the sea, and being able to convert this wood directly into energy is of high survival value for an animal that doesn’t have many options (Jamieson et al., 2019).

The video


The deep sea, a realm of perpetual darkness, intense pressure, and frigid temperatures, is home to an array of life forms that defy our conventional understanding of biology. These giants of the abyss, from the colossal squid to the Greenland shark, have evolved remarkable adaptations to survive in their harsh environment. Their slow metabolisms, unexpected diets, and sheer size are testament to the resilience and versatility of life on Earth.

This delicate ecosystem is under threat. Human activities such as overfishing, plastic pollution, changes in ocean chemistry due to climate change, and deep-sea mining pose significant risks to these unique environments and their inhabitants. The deep sea may seem like a distant, alien world, but it’s an integral part of our planet. Its health is intertwined with ours, and the survival of its inhabitants is a barometer for the overall health of our planet.

The deep sea is not just a place of scientific curiosity; it’s a world that holds key insights into life’s adaptability and resilience. The deep sea reminds us of the vastness of life’s possibilities and the importance of preserving our planet’s diverse ecosystems.


  1. Seibel, B. A. (2019). On the Depth and Scale of Metabolic Rate Variation: Scaling of Oxygen Consumption Rates and Enzymatic Activity in the Class Cephalopoda (Mollusca). Journal of Experimental Biology, 222(2), jeb191593. doi:10.1242/jeb.191593
  2. Jamieson, A. J., Fujii, T., Mayor, D. J., Solan, M., & Priede, I. G. (2010). Hadal trenches: the ecology of the deepest places on Earth. Trends in Ecology & Evolution, 25(3), 190-197. doi:10.1016/j.tree.2009.09.009

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