Polar News

Giant Sea Spiders, Frigid Divers, and the Oxygen Hypothesis

Would you dive into a frigid ocean to get close to a giant sea spider? Team Pycno, five researchers led by Amy Moran at the University of Hawaii at Manoa spent from October to early December 2015 stationed at McMurdo Station, Antarctica to examine how cold-blooded animals in the polar oceans live and function in such a frigid environment, and trying to understand their unique adaptations and biology. In particular, the team studied Antarctic sea spiders (pycnogonids).


Sea spiders aren't true spiders. While they live all over the world, the really big ones (Colossendeis megalonyx) live in the Southern Ocean around Antarctica. In fact, according to Art Woods, an associate professor of biology at the University of Montana, when Team Pycno searched off the coast of Washington state in June 2015, the largest sea spider they found was about the size of a dime. The giant sea spider, on the other hand, is the stuff of nightmares. It uses a proboscis to suck the life out of its prey and can have a leg span of 25 centimeters. The odd animals don’t even have proper bodies, let alone a respiratory system. They absorb oxygen directly from the surrounding ocean through the outermost layer of their exoskeletons. And yet, scientists believe that oxygen may be the key to why these creatures are so much larger than their counterparts in warmer climes.

Could it be the oxygen?
It’s not just the sea spiders that grow to a tremendous size in the Southern Ocean. Sponges, sea stars, worms and other invertebrates also exhibit polar gigantism. And while not everything grows giant in these conditions, something causes some species to grow much larger than they do anywhere else. The team is testing the hypothesis 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. To test this theory, they measured 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. What’s more, the water there is crystal clear, allowing divers to see for hundreds of feet.

The cold water of the Southern Ocean slows down the metabolic rates of the ectotherms that live there and it also contain high levels of oxygen. In other words, there is a lot of oxygen available but not a lot of biological demand for it. As a consequence, the researchers say that 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).

However, team lead Amy Moran adds that oxygen is not necessarily the only reason for polar gigantism. A lack of predators could allow the animals to grow larger than they do environments where size makes them more appealing to other fauna looking for a meal. Another potential contributor is the lack of strong currents in the Southern Ocean. It’s possible that the spiders, for example, have a difficult time clinging to the sea bed in a stronger current, which means they would have a more difficult time controlling their own movements.

The oxygen hypothesis leads to several predictions that were tested during Team Pycno’s 2015 expedition.

Prediction 1: If the researchers warm up individual sea spiders, their metabolic rates will rise and they will have lower levels of oxygen in their bodies. The team thinks 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 the researchers 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 the team’s primary goals, which is to look for tradeoffs between cuticular toughness and cuticular permeability to oxygen. To test these predictions, the team set up a number of experiments stimulating conditions ranging from ocean currents to different oxygen and temperature conditions to observing how well sea spiders groom algae and bacteria off themselves.

Speaking to ECO Magazine about the four prediction, team leader Amy Moran of the University of Hawaii at Manoa said it was still too early to give any definitive answers, but added, “From the data we've analyzed so far, it appears the metabolism of Antarctic sea spiders is quite sensitive to temperature, but we don't (yet) see any signs that the large-bodied ones are more affected by high temps or low O2 than the small ones. We have some indication that large pycnos have more porous cuticles that may let oxygen diffuse in more easily.”

The team’s second expedition to McMurdo will take place October-December 2016. They will also travel to the Friday Harbor Labs in Washington State to work on warmer-water pycnos in the summer of 2016.

Team Pycno says their work tests a fundamental idea about what drives the evolution of different body sizes in marine environments all across the globe. Whether or not the oxygen hypothesis is right has implications for what will happen to ectotherms during climate change. As global temperatures rise in the future, the temperatures of the world’s oceans will also increase; and as temperatures increase, the amount of dissolved oxygen in the water will decrease and the metabolic demand for oxygen will rise. This metabolic squeeze may affect polar giants most of all.

Speaking to The Antarctic Sun, Moran said, “These Antarctic giants, because they’ve adapted for millions of years to constantly cold, constantly high oxygen conditions, in theory they’re fragile metabolic flowers that can’t take much temperature change.”

One thing is for sure. Much of what the researchers observe will amount to new entries into the scientific literature. This is vital, because the more we know about how giant sea spiders have adapted to live in Antarctic waters, the better we can understand how they (and other species) may be affected by warming oceans.

For more information, including a video blog, dive gallery, and links, click here.

The Antarctic Sun article cited is located here.

This research is funded by the Office of Polar Programs at the National Science Foundation.

ECO Magazine is a marine science publication committed to bringing scientists and professionals the latest ground-breaking research, industry news, and job opportunities from around the world.


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