In the mysterious depths of the world’s coldest oceans, a remarkable phenomenon known as gigantism unfolds. Here, invertebrate sea creatures such as squids, sea spiders, worms, and various other species defy their smaller counterparts in other parts of the world by growing to astonishing sizes. Gigantism, as it’s termed, poses an intriguing question: What compels these deep-sea inhabitants to become giants?
One striking example of this phenomenon is the colossal squid (Mesonychoteuthis hamiltoni) found in subantarctic waters, which can grow to be about 14 times longer than the arrow squid (Nototodarus sloanii) commonly found around New Zealand. Then there’s the deep Pacific waters, home to a sea sponge the size of a minivan. But what drives such colossal growth in creatures dwelling in the frigid and remote ocean depths? It appears that survival in these harsh environments may be the primary motivator, with unique conditions in these cold waters facilitating their remarkable size.
In the deepest ocean regions, resources are profoundly limited, akin to the constraints seen in island ecosystems. A study published in the Journal of Biogeography in 2006 reveals that much of the food available in the deep sea originates in shallower waters, with only a fraction trickling down to the ocean’s depths. When food is scarce, being larger offers a significant advantage, as explained by Alicia Bitondo, a senior aquarist at the Monterey Bay Aquarium in California, who specializes in deep-sea species.
Larger animals have the ability to move faster and cover more ground in search of sustenance or to find a mate. They possess more efficient metabolisms and excel at food storage. Therefore, when a substantial food source like a large carcass descends into deeper waters, larger predators can consume more and store that energy for a longer period.
Cold temperatures in the deep sea can also contribute to gigantism by significantly slowing down the metabolisms of these creatures. Many species in this ecosystem experience slow growth and maturation rates, exemplified by the Greenland shark (Somniosus microcephalus). These sharks can reach lengths of up to 24 feet (7.3 meters) and weigh up to 1.5 tons (1.4 metric tons), but their growth is stretched across a lifespan spanning centuries.
For instance, Greenland sharks grow approximately 0.4 inches (1 centimeter) per year and do not reach sexual maturity until around 150 years old. Bitondo attributes their extended lifespan and immense size, in part, to the absence of predators in the deep sea.
Before humans encountered these deep-sea giants, they were discovered near the South Pole, particularly near Antarctica, where gigantism happens closer to the surface. Giant sea slugs, sponges, worms, sea spiders, and even colossal single-celled organisms inhabit these shallower waters, some of which are within scuba diving range at depths as shallow as 30 feet (9.1 meters). Art Woods, an ecophysiologist and professor at the University of Montana in Missoula, suggests that something about Antarctica might enable giant species to live closer to the surface. He speculates that gigantism in Antarctica could be linked to the abundant oxygen supply in the frigid waters surrounding the continent.
In polar waters, oxygen concentrations are high, according to the U.S. Geological Survey (USGS). However, animals in these environments have remarkably slow oxygen consumption rates due to cold water temperatures, which lower their metabolic rates. This high oxygen supply surpasses the animals’ oxygen demands, potentially freeing them from growth constraints. The environment seemingly “allows them to develop larger body size and tissue size without suffering oxygen deprivation,” Woods explains. While a rich oxygen supply may not directly drive sea creatures to become large, it certainly enables such growth.
Nevertheless, even for these polar giants, there appears to be a size limit. A study published in the journal Proceedings of the Royal Society B in 2017, led by Wood, examined giant Arctic sea spiders that can reach sizes of up to 12 inches (30.5 centimeters) long, roughly the size of a dinner plate. The research revealed that larger sea spiders had lower oxygen levels in their bodies. Aerobic metabolism relies on an adequate oxygen supply, and if it drops too low, tissues can suffer from oxygen deprivation.
The deep and frigid oceans continue to astound us with their tales of gigantism, unveiling the remarkable ways in which life adapts and thrives in these extreme environments. As scientists delve deeper into these mysteries, we gain a greater appreciation for the awe-inspiring diversity of life on our planet.
