Explanations For Deep Sea Gigantism

At sea level, the atmospheric pressure is 1 atm, but we do not feel that the air is squeezing us since the fluids in our body resist this pressure with the same amount of force. However, with every 10.06 meters downwards the ocean, the pressure increases by 1 atm (1). Since the human ear was not evolved to be able to overcome rapid changes of pressure, as a result of increasing pressure, we start feeling a very uncomfortable push in our ears as we descend in the water. This is called a ‘squeeze’ and is caused when the eardrum is pushed inward because of the pressure difference between the middle ear and the external environment. If the pressure difference is not equalized, the squeeze can cause damage to our body (2).

If we are so easily affected by the increasing pressure towards the depths of the water, what’s up with the marine animals at the massive depths? Luckily, evolution has accounted for all of their needs. Some cetacean species have lungs that are able to compress completely in the presence of high pressure to force all of the gas out into the muscles and bloodstream where they are dissolved. Other adaptations include the ability to hold the breath for up to 2 hours, minimizing the heart rate to 4 beats per minute, or the ability of the body to shut down digestion, liver, and kidney functions during the dive (3). With the help of sonar tracking and attached time-depth recorders, zoologists have been able to discover that adaptations such as these have helped the sperm whale and bottlenose whale to be able to dive up to 1830 meters below sea level and to be able to stay at that depth up to an hour (4).

Since these adaptations allow animals to survive in the deep, dark sea, it could also be assumed that the intense pressures would stunt the growth of animals, causing them to have smaller forms than animals of the shallow waters. Edith Widder, CEO and senior scientist at Ocean Research and Conservation Association (ORCA), has seen with her bare eyes that this was actually not true (5). The video she was able to film with her team lasts less than 30 seconds and shows an eerie tentacle, followed by seven more, reaching towards an electronic jellyfish with hopes of having found its afternoon meal. When it realizes that it was not quite edible, it pushes away from the jellyfish, flowing away into the darkness. The owner of the eerie tentacles is the most famous of all deep-sea giants, the giant squid (see Figure 1). It lives up to its name with the largest ever discovered being 13 meters long, weighing nearly a ton, with an eye the size of a dinner plate (see Figure 2) (6). It is evidently larger than our fellow squids found in the coastal or oceanic waters, which have 1.6 cm body height at maximum.

Figure 1: Giant squid found in Scotland

Figure 2: The eye of a giant squid

This phenomenon is known as deep-sea gigantism and is explained by zoologists as the tendency of invertebrate species and other deep sea-dwelling animals to be larger than their relatives living in shallower waters (7). But how do these animals become weirdly giant, if there’s intense pressure underwater? Since marine animals are mostly made of water, which is only slightly compressible, pressure is not much of an issue. Along with this, the buoyant nature of water allows them to defy gravity; this is one explanation given to how deep-sea animals become giants. Two rules have been used to further explain this phenomenon; Kleiber’s rule and Bergmann’s rule. Kleiber’s rule, through the examination of the relationship between metabolic rate (R) and body mass (M) of an organism raised to the power 3/4, states that animals that are larger have a tendency to be more efficient (8). A whale naturally has a much greater body mass and size than a Flabellina (sea slug). Therefore it will also have a significantly greater metabolism. Larger animals have a smaller surface area to volume ratios, which helps with less energy being lost through heat. Since there is no light in ‘the midnight zone’ (or Bathypelagic Zone) of the ocean, no photosynthesis by plants can occur, thus food is scarce. This means that energy must be conserved, and the animals that have the strongest metabolism will survive. Bergmann’s rule rather states that an increase in the geographic latitude and depth will cause increased cell size, life span, and thus, an increase in the body size (9). This rule also explains how smaller animals such as sea spiders and sponges grow larger most prevalently in the poles when compared to their relatives from warmer habitats; the decreased temperature slows their metabolism down, causing them to live in ‘relative slow motion’ due to the availability of dissolved oxygen in colder waters. Another hypothesis is the lack of predators in the deep sea. The only predator that hunted the giant squid found was the sperm whale. Scientists have discovered undigested parts of the giant squid -such as its beak- in the stomachs of beach-stranded whales. Another evidence for the interaction between sperm whales and giant squids is the large sucker marks on the skin of sperm whales (see Figure 3) that are so large that can only be associated with the giant squid. The battle scars can only mean that the whale has survived the battle, meaning that it chose the giant squid as prey.

Figure 3: Sperm whale skin with sucker marks

Although not fully understood, deep-sea gigantism is still being explored. Several explanations as listed above have been used to explain the ‘how?’ part of the enlargement of these animals, but the ‘why?’ is still not discovered.

Works Cited

1. US Department of Commerce, National Oceanic and Atmospheric Administration. “How Does Pressure Change with Ocean Depth?” NOAA’s National Ocean Service, 1 June 2009, https://oceanservice.noaa.gov/facts/pressure.html.
2. “Ear Equalization and Ear Care for Scuba Divers.” PADI Blog — Scuba Diving and Freediving Tips, Dive Travel Insights & More, 26 Sept. 2018, https://blog.padi.com/ear-equalization-and-ear-care-for-scuba-divers/.
3. Omondi, Sharon. “How Do Deep-Sea Creatures Survive the High Water Pressure?” WorldAtlas, WorldAtlas, 10 Sept. 2019, https://www.worldatlas.com/articles/how-do-deep-sea-creatures-survive-the-high-water-pressure.html.
4. “How Do Deep-Diving Sea Creatures Withstand Huge Pressure Changes?” Scientific American, Scientific American, 21 Aug. 2006, https://www.scientificamerican.com/article/how-do-deep-diving-sea-cr/.
5. Langlois, Jill. “How Scientists Got That Giant Squid Video.” Animals, National Geographic, 3 May 2021, https://www.nationalgeographic.com/animals/article/giant-squid-us-waters-first-video.
6. “Largest Eye in the World, Giant Squid.” Smithsonian Ocean, 18 May 2018, https://ocean.si.edu/ocean-life/invertebrates/largest-eye-world-giant-squid.
7. Langlois, Jill. “How Scientists Got That Giant Squid Video.” Animals, National Geographic, 3 May 2021, https://www.nationalgeographic.com/animals/article/giant-squid-us-waters-first-video.
8. “Kleiber’s Law.” Oxford Reference, https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100039661.
9. Timofeev, S. F. “Bergmann’s Principle and Deep-Water Gigantism in Marine Crustaceans.” Biology Bulletin of the Russian Academy of Sciences, vol. 28, no. 6, Nov. 2001, pp. 646–650., https://doi.org/10.1023/a:1012336823275.