Scientists have long believed that “polar gigantism” was a result of a combination of cold-driven low metabolic rates and high oxygen availability in the polar oceans, This hypothesis is known as the “oxygen–temperature hypothesis.”
If this hypothesis is true, then it would make sense that polar giants would be more susceptible to warmer temperatures. The University of Hawai’i at Manoa zoology Ph.D. student Caitlin Shishido, along with UH researcher Amy Moran and her colleagues at the University of Montana, went to Antarctica to test the theory.
The scientists decided to study Antarctica’s giant sea spiders because it’s so cold, along with the high oxygen availability due to upwelling of deep waters around the continent. The study – published in the April 10 issue of Proceedings of the Royal Society of London – was carried out at McMurdo Station, Antarctica, and at UH.
“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.”
All about sea spiders
Sea spiders are also called Pantopoda or pycnogonids. They are marine arthropods in the class Pycnogonida and the order Pantopoda. There are over 1,300 species of sea spiders known today and they are found in oceans all over the planet.
Their leg spans vary from 1.0 millimeters (0.04 inches) to over 70 centimeters (2.3 feet). Most sea spiders tend toward the smaller leg span range and are found in shallow waters. But they can grow to be quite large in Antarctic and deep waters.
Just to ease people’s minds, sea spiders are not true spiders or even arachnids, although their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans.
There has been some dispute about this classification because genetic evidence places sea spiders closer to being the sister group to all other living arthropods
Testing the theory
To test the “Oxygen-Temperature Hypothesis,” the researchers collected sea spiders in a range of sizes near McMurdo Station. The performance of each spider in the experiment was then tested at four water temperatures ranging from the ambient level of -1.8 degrees Celsius up to 9C.
The testing consisted of flipping the spiders on their backs and counting the number of times they were able to right themselves at a range of temperatures. What blew the researcher’s minds was that the giant spiders kept up with the small ones at every temperature level.
“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 should be especially true for sea spiders because they are “skin breathers.” Sea spiders have no lungs or gills – instead, they get their oxygen from a process called diffusion, absorbing oxygen across the surfaces of their legs. How they do this was a mystery.
When Shishido and Aaron Toh, a UH undergraduate student, used microscopes to look closely at their legs, they discovered the legs of sea spiders are covered with pores, and the researchers found that as the sea spiders grow, their exoskeletons become more and more porous, with bigger holes.
“The exoskeletons of the really big ones look almost like Swiss cheese,” said Shishido.
In discussing the conclusions of their research, the scientists write: “Future ocean warming will undoubtedly have profound effects on Antarctic marine organisms and ecosystems. However, even among these polar giants, whose large body size is thought to confer a particular vulnerability to climate change, predicting ‘winners’ or ‘losers’ requires a more nuanced, whole-organism approach that integrates across many levels of a species’ ecology, life history and physiology.
