Like humans and other animals, sea spiders need oxygen to live. They need it to run their metabolisms, which supplies the power to do things like contract, break down food grow, secrete, and divide. One leading hypothesis about polar gigantism is that invertebrates that live in very cold water have evolved large body sizes because they their metabolisms are slowed down so much by the cold that they don’t need much oxygen. Our work is testing this idea. We’re measuring how fast large and small sea spiders use oxygen at different temperatures, and how readily they can get it from the water. Antarctica is a great place to work on this problem because of the constant cold and high oxygen of the ocean surrounding the frozen continent, and because sea spiders are incredibly abundant and diverse under Antarctic sea ice. Working on the physiology of animals in extreme conditions is a good way to understand the physiology of animals everywhere, because it highlights physiological challenges and the ways that animals adapt to meet them.
Things are bigger at the poles.
From the very first expeditions to the Antarctic, explorers observed that many types polar organisms had larger body sizes than related organisms from warmer parts of the world.
Scientists don’t know for sure why some polar organisms became giants, but they’ve got a few ideas.
One of those ideas is the oxygen hypothesis.
In polar oceans, the water is cold, which slows down the metabolic rates of the ectotherms that live there. In the Southern Ocean that surrounds Antarctica, these cold waters also contain high levels of oxygen. This means that there is a lot of oxygen available but not a lot of biological demand for it. As a consequence, it should be easy for animals to get oxygen to all of their tissues, even if the animals are big-bodied and their tissues are thick and poorly supplied with blood (or other oxygen-carrying fluids).
In warmer oceans, the relationship between oxygen demand and supply shifts: warm water raises the metabolic rates of animals living there, and it contains less oxygen. As a consequence, marine invertebrates in temperate and tropical oceans have a harder time getting enough oxygen. This oxygen problem may force these species to evolve smaller body sizes.
A more global statement of the idea: the oxygen hypothesis says that global patterns of body size in marine invertebrates (small in warm water, large in cold water) is due to global patterns in how temperature affects the supply of, and demand for, oxygen.
What we're doing to test the oxygen hypothesis.
For sea spiders, the oxygen hypothesis makes several straightforward predictions that we can test using well-established techniques.
Prediction 1: If we warm up individual sea spiders, their metabolic rates will rise and they will have lower levels of oxygen in their bodies. We think their aerobic performance, or how well they’ll be able to breathe, move around, and function, will decrease at temperatures that go too high.
Prediction 2: If we lower oxygen levels in sea water, large-bodied sea spiders will fare worse than small-bodied spiders in any one location, and all sea spiders from warm water will fare worse than those from cold water.
Prediction 3: In any one general location (e.g. west coast of North America or Antarctica), sea spiders living in particularly low-oxygen areas will be smaller or will have thinner cuticles in order to get more oxygen.
Prediction 4: Species that evolve particularly thick cuticles (for reasons like having to withstand forces in their environments from things like strong currents or fights with other sea spiders) will pay the price of getting oxygen less easily; they will have particularly low levels of oxygen in their bodies.
This last prediction gets at one of our aims in this work, which is to look for tradeoffs between cuticular toughness and cuticular permeability to oxygen. See the biomechanics section for more on this.
Why this work is important.
This work is important in two ways.
First, we’re testing a fundamental idea about what drives the evolution of different body sizes in marine environments all across the globe. Although we’re working on a single (but particularly interesting!) group of animals as a way of focusing our efforts, our results will evaluate a broad hypothesis that’s relevant potentially to all marine invertebrates.
Second, whether or not the oxygen hypothesis is right has something to say about what will happen to ectotherms during climate change. As global temperatures rise in the future, the temperatures of the world’s oceans will increase too. As ocean temperatures increase, the amount of dissolved oxygen in the water will decrease and the metabolic demand for oxygen will rise. This will put the metabolic squeeze on many marine invertebrates, and it may affect polar giants most of all. The more we know about how giant sea spiders have adapted to live in Antarctic waters, the better we can understand how they may be affected by warming oceans.