Why Do Deep-Sea Creatures Grow So Large? Exploring Temperature, Oxygen, Food, and Fragmented Habitats

Before discussing deep-sea creatures, the video first clarifies what "giant" means. Even among land elephants, sizes range from the small Bornean elephant to the imposing African elephant. Whales in the ocean also vary in size by species, from small species to the blue whale, which exceeds 30 m. The blue whale is far larger than the African elephant, yet calling the African elephant "giant" doesn't feel wrong. In other words, the video explains, what matters is how large an animal is within its own group.

Viewed from this perspective, in the deep sea a relative of the pill bug, which on land measures only a few centimeters, grows to nearly 50 cm. This is the giant isopod. Amphipods, too, can exceed 30 cm in the deep sea. Most squid in shallow waters are a few dozen centimeters long, but the deep sea harbors squid exceeding 10 m in body length, and the giant squid, the prime example, is one of the largest invertebrates on Earth. Many cases have thus been confirmed in which deep-sea creatures grow larger than their relatives of the same lineage on land or in shallow seas, and this phenomenon is called "deep-sea gigantism."

The first factor the video cites is temperature. Water temperatures in the deep sea below 1,000 m stay at 2–4°C year-round, with almost no variation. In such a cold environment, organisms' metabolism drops considerably and their energy consumption falls. At the same time, growth slows, but lifespans tend to lengthen. By growing slowly and living long, the body can more easily grow large over time, according to the video.

The tendency for bodies to grow larger in cold environments has long been known among land animals as well. Known as Bergmann's rule, it is an adaptive hypothesis holding that reducing the ratio of surface area to volume makes maintaining body temperature more efficient. The sun bear of the tropics and the polar bear of cold regions are cited as representative examples.

So does this rule also apply to the deep sea? According to the video, in the early 2000s it was argued that Bergmann's rule and deep-sea gigantism are not separate phenomena, but can both be explained by the same mechanism tied to falling temperatures. Based on surveys focused mainly on crustaceans, a hypothesis was proposed that the main driver of deep-sea gigantism is temperature, not water pressure. The idea is that low temperatures increase cell size and extend lifespan, and as a result, bodies grow larger as well.

The video cites oxygen as another element of the deep-sea environment. In general, cold water can dissolve more oxygen than warm water. Furthermore, if metabolism slows at low temperatures, organisms need less oxygen. For this reason, some deep-sea environments are thought to provide conditions that make it easier to sustain a large body.

In fact, the relationship between oxygen levels in the water and body size has been studied in deep-sea snails, and it has reportedly been suggested that oxygen supply may set the upper limit on body size. However, there are also oxygen-poor areas in the deep sea, so gigantism cannot be explained by oxygen alone. The video nonetheless positions it as certainly one of the important factors.

Up to this point, the discussion covers physiological effects of the cold environment; the video next shifts its focus to the environment itself. A major feature of the deep sea is its scarcity of food. Photosynthesis does not occur in the deep sea, and many animals depend on organic matter sinking from upper layers. This "marine snow" of carcasses, excrement, and the like slowly drifts down and accumulates, but its quantity is limited and food is not always available.

What gives an advantage in such an environment is the strategy of eating a lot at once and enduring a long time. Large-bodied organisms can store more energy, and if they can conserve energy, they can withstand long periods of starvation. The giant isopod is a classic example: it is known to be able to live for years without eating by storing food in its huge stomach to stockpile nutrients while drastically lowering its metabolism. A giant isopod kept at Toba Aquarium, known as "No. 1," went without food for more than five years until its death was confirmed in 2014. The video states that gigantism in the deep sea is also the result of evolution for surviving periods when food cannot be obtained.

The video further points out that deep-sea habitats are scattered. The deep sea is vast, but places where organisms tend to gather are limited to areas such as around sunken organic matter, so habitats exist as isolated points. The seafloor, which seems to stretch on endlessly, is in fact a world fragmented like islands, the video says.

In such an environment, the need to grow quickly and reproduce early becomes relatively weaker, leaving room to keep growing over a long period. On the other hand, places where resources are concentrated also see fierce competition. For example, many organisms gather where a whale carcass has sunk, producing temporarily intense competition. The video explains that competition in the deep sea does not occur uniformly across the entire ocean, but consists of local battles taking place independently in each location.

Low temperatures, oxygen, scarce food, and fragmented habitats. The combination of these factors can explain deep-sea gigantism to some extent. However, the video adds the caveat that not all deep-sea creatures grow large, and many remain small.

As an example, it introduces a 2005 study. Researchers placed baited cameras in the deep sea and examined the deep-sea fish that gathered. Observations were made over a wide range of depths, from several hundred meters to several thousand meters, and the body sizes of fish with different ecological strategies were compared. The results showed that scavenger-type fish, which feed on carcasses, tended to grow larger as depth increased. Meanwhile, fish not dependent on such resources showed the opposite tendency, becoming smaller with greater depth. Based on these results, the researchers stated that when considering the deep sea, it is necessary to take into account not only physiological factors but also differences in food distribution. In light of this, the video concludes that deep-sea gigantism is not a simple law but a phenomenon produced by differences in each organism's survival strategy.

Finally, the video states that deep-sea exploration has only just begun and that most of the deep sea remains unknown. It then introduces a recent example in which a Japanese research team used baited cameras to discover a large fish, the yokozuna slickhead, in the deep sea at depths exceeding 2,000 m. This fish, over 2 m in total length, belongs to the slickhead family, whose members are typically around 40 cm, and it was found to be one of the largest bony fish.

In this study, the researchers first estimated the fish's habitat through environmental DNA analysis and then narrowed down observation sites, successfully confirming individuals efficiently. The video describes this as a discovery showing that deep-sea research, which has traditionally relied heavily on chance, is entering a stage of targeted discovery, and says that this method could further accelerate the discovery of unknown deep-sea creatures. The video closes by saying that the giant creatures found so far are only the prologue, and that still-unknown giant creatures must be lurking in the dark depths of the sea.