Mother nature’s giants in the deep sea: Exploring the rationale behind their enormousness                                  

(By Sulagna Maity on 17th January 2024)

In the picture below, you can see the majestic African bush elephants, also known as the African Savannah elephants. They are the largest terrestrial (land) animals, towering over us at a height of 10-13 feet and weighing between 3600-6450 kg. The largest known elephant weighed an astounding 10,886 kg, which is approximately 175 times the weight of an average human! And yet, as impressive as these creatures are, they are not the largest animal on our planet. This honour goes to the Antarctic blue whale (pictured above), which can weigh around 180,000 kg and reach up to 98 feet in height.

Besides blue whales, other marine animals including deep-sea stingrays or giant stingrays (up to 8.9 feet in length), tiger sharks (up to 10-14 feet in length), seven-arm octopuses (length around 11 feet) and big red jellyfish (length between 2.6 to 4.9 feet) also attain large sizes as compared to terrestrial animals. But why? One might question how these sea creatures are enormous despite the oceanic environment’s challenges such as limited resources, cold temperatures, and ocean currents (which are directional movements of seawater by gravity and wind). Could it be their adaptation strategy to survive in the deeper, harsh parts of the ocean?

Deep-sea gigantism is an interesting phenomenon where species dwelling in the deeper regions of the ocean grow much larger than the shallow-water species. This astonishing adaptation has intrigued scientists for years, leaving them curious about the size disparity. The exact reason behind this phenomenon is still unclear but numerous theories have been proposed. One of them is Bergmann’s Rule. This ecological rule states that the species living in colder environments tend to have larger body sizes whereas small-sized species tend to primarily live in warmer regions. Here is how the rule works: larger animals have a smaller surface area to volume ratio so they are capable of conserving heat more effectively than smaller animals.

Another possible explanation is Kleiber’s Law. According to this law, an animal’s metabolic rate scales roughly the ¾ power of its body mass which means larger animals have a low metabolic rate relative to their body size, causing them to utilise less energy per gram of tissue to perform basic functions. For example, the Greenland shark (Somniosus microcephalus) is one of the largest (about 24 feet long), long-lived (approximately 250 years) sharks having a low metabolic rate and requiring very little energy to sustain themselves in a natural environment. Whether this is the same case for giant squids or whales is not explored yet.

However, the above two theories cannot be the sole reasons for deep-sea gigantism. Factors like food scarcity, hydrostatic or water pressure and even growth hormone level of certain marine vertebrates can contribute to deep-sea gigantism, regardless of water temperature.

Then comes the role of dissolved oxygen present in the water. Deeper regions of the sea have colder water than the surface or shallow regions. High temperatures decrease the solubility of oxygen in water, meaning colder water retains more dissolved oxygen than warmer water. A study by Chapelle and Peck found that the maximum size of sea organisms is directly proportional to the increased level of dissolved oxygen in the deep sea. In 2001, McClain and Rex studied the effect of dissolved oxygen on deep-sea gastropods (a group of animals including snails, slugs, and sea snails). By using deep-sea turrids (a type of sea snail), they found that the maximum size of the turrids increased with higher oxygen concentration. However, the overall concentrations of dissolved oxygen in the ocean have been reduced due to ocean warming. This could impact the metabolic processes of marine animals.

Now, let’s investigate this deep-sea gigantism from the molecular perspective. Body weight is also associated with many genes or a combination of genes. In 2023, researcher Felipe Andre Silva and his team at the University of Cidade in Brazil focused on the genetic factors of Cetaceans (sea mammals like dolphins, whales, and porpoises). They examined 19 species of Cetaceans and found that the molecular evolution of four genes (GHSR, IGFBP7, PLAG1 and NCAPG) is responsible for the increased or colossal size of the sea mammals. Each of these genes had previously been identified as being involved in cell growth and increased body size in domesticated cattle. NCAPG gene was even found to be linked to growth in humans. However, it is still uncertain whether the same genes are responsible for the massive size of non-mammalian marine animals such as sharks, stingrays and squids.

Deep sea gigantism remains an interesting phenomenon. With this new genetic information, we’re one step closer to understanding the cause of the enormousness of the sea creatures. But there’s still so much we are unaware of, and the inhospitable environment of the deep sea makes it difficult for scientists to conduct in-depth research on marine animals. However, this does not dampen the enthusiasm of the scientists who are on a quest to unravel the mysteries behind the magnificent enormity of marine animals. With cutting-edge technology and continued research, the day is fast approaching when we will finally unlock the secrets of the ocean’s greatest wonders.

Feature photo credit: Ben Phillips