Amira Ismail
Standing beside an African Bush Elephant or a hippopotamus, it's impossible not to feel dwarfed by their massive presence. These are some of the Earth's largest animals, dominating the land with their sheer size. But what if I told you that in the ocean's mysterious depths, creatures far larger than any land giant silently thrive, hidden from view in the abyss? From Colossal Squids to Blue Whales, marine gigantism is a phenomenon that challenges our understanding of life on Earth. This phenomenon invites us to explore not only the reasons behind their impressive sizes but also the unique evolutionary traits that enable them to navigate the challenges of their underwater world and the added significance in the context of environmental change.
Understanding Marine Gigantism
Marine gigantism, particularly observed in deep-sea environments, is a captivating phenomenon where certain species grow to sizes significantly larger than their shallow-water counterparts. Various researchers have proposed theories to explain this occurrence, notably the oxygen-temperature hypothesis, which suggests that cold, oxygen-rich deep-sea waters facilitate the growth of larger organisms since in colder environments, metabolic demands decrease, allowing larger body sizes to thrive due to less oxygen limitation. However, research on Antarctic Sea spiders found no significant interaction between body size and dissolved oxygen levels affecting performance. When exposed to lower oxygen levels or higher temperatures, larger individuals did not show a greater decline in performance—measured by their ability to right themselves—compared to smaller ones. This data challenges the common hypothesis that larger organisms would be more adversely affected due to their higher metabolic demands and the notion that larger body sizes in polar environments are primarily due to high oxygen availability, suggesting alternative ecological or evolutionary processes may be involved.
Similar to the oxygen-temperature hypothesis, the Island Rule puts forth that isolated environments, like those found in the deep sea, create unique selection pressures that can lead to the evolution of larger sizes in some species while causing dwarfism in others. This theory suggests that when pressures from predators and competition with other species are reduced, smaller organisms are likely to evolve into larger ones. This change happens because intraspecific competition and sexual selection often prefer larger body sizes, as they can offer advantages like better access to resources and increased attractiveness to mates.
First articulated by the German biologist Carl Bergmann in the 19th century, Bergmann’s Principle further explores this idea, indicating that warm-blooded animals tend to be larger in colder climates, which may explain the prevalence of large species in polar regions. According to Bergmann, larger animals have less surface area compared to their volume than smaller animals do. This means that they lose heat more slowly to their environment. As a result, larger animals can maintain their body temperature better in colder climates. Ecological conditions also play a crucial role; deep-sea environments often exhibit low resource availability, prompting evolutionary adaptations that favor larger body sizes for increased metabolic efficiency and energy storage.
Marine gigantism in an era of climate change
Climate change profoundly impacts marine gigantism, presenting a myriad of challenges for large marine species and their ecosystems. As ocean temperatures rise, larger animals may initially benefit from their greater energy reserves and metabolic efficiencies, allowing them to adapt better to warmer conditions. However, this advantage is quickly overshadowed by detrimental effects such as ocean acidification and deoxygenation, which threaten the very habitats that support these giants. Disruption of food webs becomes a critical issue, as smaller prey species struggle to survive in increasingly hostile environments, leading to declines in the populations of larger predators that depend on them for sustenance. As species migrate to cooler waters, larger marine animals may face heightened competition for dwindling resources, further stressing their populations. Changes in reproductive patterns due to shifting water temperatures can disrupt breeding behaviors and success rates, leading to lower population growth and sustainability. Models indicate that decreasing oxygen levels in the ocean will affect where marine species can live and how much biomass they can support. This is largely due to the physiological changes these species will experience and the shrinking of their habitats. As a result, we can expect to see a decline in the body size of marine fish, as they struggle to adapt to these challenging conditions.
Why it matters
We cannot afford to ignore the problems caused by climate change in our oceans. We come to understand that the survival of larger marine species, such as the magnificent Blue Whale and the amazing gigantic squid, is entwined with our own as we observe their hardships.
Adopting sustainable fishing methods, cutting pollution, and halting climate change must be our top priorities. The livelihoods of many people who rely on healthy oceans for food and income are also being preserved by these initiatives, which go beyond simply conserving marine life.
Now is the moment to take action. Both our oceans and the communities that rely on them are in danger. We can contribute to ensuring that future generations will have the opportunity to live in a healthy thriving world.
Citations
[1 ] (2024). Proquest.com. https://www.proquest.com/scholarly-journals/giants-deep/docview/2887064709/se-2
[2] Woods, H. A., & Moran, A. L. (2020). Reconsidering the oxygen-temperature hypothesis of polar gigantism: successes, failures, and nuance. Integrative and Comparative Biology. https://doi.org/10.1093/icb/icaa088
[3] Woods, H. A., Moran, A. L., Arango, C. P., Mullen, L., & Shields, C. (2009). Oxygen hypothesis of polar gigantism not supported by performance of Antarctic pycnogonids in hypoxia. Proceedings of the Royal Society B: Biological Sciences, 276(1659), 1069-1075.
[4] Shishido, C. M., Woods, H. A., Lane, S. J., Toh, M. W. A., Tobalske, B. W., & Moran, A. L. (2019). Polar gigantism and the oxygen–temperature hypothesis: a test of upper thermal limits to body size in Antarctic pycnogonids. Proceedings of the Royal Society B, 286(1900), 20190124.
[5] Herczeg, G., Gonda, A., & Merilä, J. (2009). EVOLUTION OF GIGANTISM IN NINE-SPINED STICKLEBACKS. Evolution, 63(12), 3190–3200. https://doi.org/10.1111/j.1558-5646.2009.00781.x
[6] What is Bergmann’s Rule? - A Brief Account On The Deep-Sea Gigantism. (n.d.). BYJUS. https://byjus.com/biology/bergmanns-rule/
[7] Poloczanska, E. S., Burrows, M. T., Brown, C. J., García Molinos, J., Halpern, B. S., Hoegh-Guldberg, O., Kappel, C. V., Moore, P. J., Richardson, A. J., Schoeman, D. S., & Sydeman, W. J. (2016). Responses of Marine Organisms to Climate Change across Oceans. Frontiers in Marine Science, 3(1). https://doi.org/10.3389/fmars.2016.00062
[8] Guzman, A., Miller, O. L., & Gabor, C. R. (2023). Elevated water temperature initially affects reproduction and behavior but not cognitive performance or physiology in Gambusia affinis. General and Comparative Endocrinology, 340, 114307–114307. https://doi.org/10.1016/j.ygcen.2023.114307