UH Researchers Solve Mystery of Giant Sea Spiders

A giant sea spider, Colossendeis robusta, used in the thermal tolerance experiments done by Shishido and colleagues at McMurdo Station, Antarctica. Photo by Tim Dwyer, courtesy of ARCUS.

A large ocean arachnid of the polar oceans may be less vulnerable to ocean warming than researchers previously thought.

A team including two University of Hawaiʻi researchers ventured to the isolated throes of McMurdo Station in Antarctica to study why marine life living in the polar oceans and deep seas can grow so much bigger than other regions of the world.

University of Hawai‘i at Mānoa zoology PhD student Caitlin Shishido and UH researcher Amy Moran together with colleagues from the University of Montana studied giant sea spiders to test a prevailing theory that explains the phenomena of large marine animals. Called the “oxygen-temperature hypothesis,” the theory posits that animals living in extremely cold environments can grow larger due to slower metabolisms.

“The idea is, it’s a lot of work for animals to capture oxygen and bring it all the way to their cells,” said Shishido. “It’s a much bigger job for large animals than for small ones. If cold temperatures make you need less oxygen, you can grow to a larger size.”

Lead study author Caitlin Shishido, PhD candidate in Zoology at UH, at McMurdo Station, Antarctica in 2016. Photo by Amy
Moran.

The sea spiders—close relatives of land spiders that can breathe underwater through pores in their legs—were subjected to a series of exercises. The team attempted to exhaust the spiders by flipping them up-side down and counting the number of times they turned themselves right-side up at a range of temperatures: -1.8°C all the way up to 9°C. Contrary to expectations, the giant sea spiders showed just as much stamina as their smaller cousins on land.

“We were amazed that not only could the giant animals survive at much higher temperatures than they usually see, but they dealt with warm temperatures just like the smaller ones,” Shishido said. “That’s not supposed to happen; larger animals should exhaust their oxygen supply and run out of gas much sooner than small ones.”

This seemed especially odd for sea spiders who have no gills or lungs and rely on diffusion across the surface of their legs to get oxygen. So how do these creatures show such resilience in varying temperatures?

This was a mystery until Shishido and Aaron Toh, a UH undergraduate student, put the sea spiders under a microscope. They found that, as the spiders grow, their exoskeletons become more and more porous, allowing for greater oxygen intake.

“The exoskeletons of the really big ones look almost like Swiss cheese,” Shishido said.

These short-term experiments only provide a limited view of the effects of ocean warming on giant sea creatures, the researchers cautioned. The long-term effects have yet to be understood.

The study was published in the April 10 issue of Proceedings of the Royal Society of London.