Polar Gigantism

Where can you find sea spiders the size of dinner plates? Head to the frigid waters off Antarctica, and you'll likely run across one.

The size isn't normal for these arthropods. In other parts of the world, they're much smaller, a few centimeters or less. But in Antarctica, they experience what’s called polar gigantism, and it's exactly what it sounds like: "Marine invertebrates that occur at the poles in really cold water tend to have much larger body sizes than their relatives that live elsewhere," Art Woods, Ph.D., of the University of Montana, told weather.com.

It doesn't just happen to sea spiders. The phenomenon has been seen in marine sponges, worms, even the single-celled organisms studied by scientists like Sam Bowser, Ph.D., an investigator at the New York State Department of Health's Wadsworth Center. "Some of them are the size of your pinky nails," he told weather.com, about the organisms typically only visible under a microscope.

Antarctica has been a hot bed for polar gigantism but it's not the only place to see extra-large species. It occurs in the deep sea as well, aptly named deep-sea gigantism there, according to Amy Moran, Ph.D., of the University of Hawaii, who along with Woods, studies these species. "Different groups of organisms respond to weird environments in different ways," she told weather.com. "We’re trying to understand the animals in these environments."

In fact, why this happens remains an open-ended question, though there is one prevailing theory, and it has to do with oxygen. Pole-dwelling organisms have low metabolic rates because they are cold, living in what Woods describes as slow motion compared to, say, humans. They don't burn oxygen quickly. Yet there's an immense amount of oxygen available to them. "There's a lot of supply compared to how much demand there is for oxygen," he said. "We think that may allow them to evolve very large body sizes because they’re not starving for oxygen."