Sea spiders are abundant in waters across the globe, and most are so small that you could hold one on the tip of your pinkie. But in the swirling waters around our planet’s icy poles, these spiders are giants. If you held the largest of these creatures, its gangly legs would just dangle off the palm of your hand.

Antarctic sea spiders got so big because some 30 million years ago, the Southern Ocean got cooler. This trait, known as polar gigantism, is thought to be essential to why they and many other cold-dwelling invertebrates of unusual size managed to survive.

Researchers wondered what allowed animals like these to reach such gigantic sizes. They also want to know what will happen as the waters they inhabit continue to get warmer, because it’s thought that extremely cold water marine animals can only tolerate a tiny range in temperature, making them particularly vulnerable to global warming.

In a study published Wednesday in Proceedings of the Royal Society B, a team of scientists challenged giant sea spiders collected in Antarctic waters to exercise to exhaustion in a kind of aquatic crossfit class.