In one image, the sea spider assumes an elegant stance, two delicate legs poised above its body.
In another, the arthropod walks on angled legs near a starfish on an ocean floor teeming with other tiny sea critters.
The sea spiders grow to be just five or six millimeters in diameter in warmer waters, such as Puget Sound.
A decade ago, University of Montana researcher Art Woods dove into McMurdo Sound to study sea slugs, and he encountered dramatically large sea spiders, animals as big as dinner plates, beautiful as ballet dancers.
“We saw these sea spiders all over the place, and it was like, instant love,” said Woods, based at UM for 11 years.
This summer, a finding about sea spiders that Woods and his team published in Current Biology astounded the scientific community. The animal’s guts extend through its legs, and research showed the sea spider uses those guts to move oxygen through its body; its heart plays only a minor role and it has no lungs.
The sea spider is the only creature known to use its digestive muscles to circulate oxygen, according to the scientist.
National Geographic ran a story. So did The Atlantic. Gizmodo picked it up.
Last week, Woods and graduate student Steve Lane shared some of the other questions they have about the sea spider.
Their metabolic rates appear to be especially sensitive to temperature, Woods said. Will the warming oceans disproportionately affect the giant sea spiders even as the creatures shed new light on animal life?
Oxygen diffuses across the outer skin of the sea spider, the cuticle, and Lane wants to know how the pores change across body size.
“Sea spiders are bizarre. They don’t have any gills or lungs or anything,” he said.
Woods, associate professor in the Division of Biological Sciences, also talked about how the discovery demonstrates the possibilities evolution offers. The heart, after all, doesn’t have to be the motor that pushes gases through the body.
“It shows the diversity of possible solutions that evolution sometimes arrives at for common problems,” Woods said.
Woods and Lane wanted to study sea spiders because they’re “poster kids” for a phenomenon called “polar gigantism” among marine invertebrates that live in the Arctic and Antarctic waters.
Creatures at the poles tend to have larger body sizes compared to their relatives that live in other places, and sea spiders in particular appeared prominent in Antarctica.
“They’re big and visible, and they’re out there crawling around, and they seem to be an important part of the food chain,” Woods said.
By comparison, Woods and Lane dove off Friday Harbor on San Juan Island in Washington, and the sea spiders are so small, it took Lane days to find them even with his sharp eyesight.
In October and November 2015 and 2016, the team traveled to McMurdo Station in Antarctica, a research station the National Science Foundation notes is the hub of the U.S. Antarctic Program.
The National Science Foundation grant collaborators included researchers from UM — Woods, Lane, and associate professor Bret Tobalske, studying the biomechanics of sea spiders — and associate professor Amy Moran and graduate student Caitlin Shishido from the University of Hawaii.
There, the team dove into 28 degree water to collect sea spiders and test some hypotheses about why polar gigantism exists. All animals that are aerobic need oxygen, and in general, it’s hard for larger organisms to get enough oxygen, Woods said.
Sea spiders take in oxygen through their cuticle, or outer layer, so if they’re really big, the supply can be a problem, he said. That’s because it’s too little surface area compared to the volume inside burning up oxygen.
But two things happen in cold water, Woods said. First, metabolic rates are low, and metabolic processes go slowly in the cold, so there’s not much demand for oxygen. At the same time, cold water holds a lot of oxygen.
“In polar waters, you have a lot of oxygen available, and the organisms aren’t using it very fast,” Woods said. “We think that tilts the balance.”
In other words, he said, the conditions appear ripe for the sea spider to grow large.
It’s hard to know their age, but Woods has a romantic idea the ones some 10 inches across must be 50 years old, maybe even centenarians.
The dives the crew took were all day affairs.
Just gearing up took 20 or 30 minutes, Lane said. The researchers would put on long underwear and polar fleece onesies under dry suits and a couple of pairs of gloves, sometimes with hand warmers.
They’d load gear into bags at the station and place air tanks and other equipment into a vehicle called a PistenBulley, with tracks that crawl over the snow and ice.
That time of year, the ice is about six feet to eight feet thick, and the team would drive out from the station with a giant drill some 3.5 feet in diameter. At the site, the auger would bore a hole through the ice.
They’d pull “a glorified fishing hut” over it for warmth, Lane said.
Then, the divers would plunge into the water. They’d carry flashlights, although light penetrates the ice that time of year.
If the dry suits didn’t leak because of user error, their core bodies would stay warm, but their hands would start freezing after 20 minutes or so. They limited most of the dives to 30 or 35 minutes; most were at a depth of around 70 to 100 feet, with the limit at 120 feet.
“The animals were found all over,” Lane said. “Ten feet deep to presumably much farther down below.”
Lane and Tobalske did the longest dive, a little more than an hour in relatively shallow water. Two dives would take all day, including time to gear up and warm up for a couple of hours between dives.
“It’s exhausting just to be that cold,” Woods said.
Back at the lab, the team kept sea spiders in a seawater table that pumped fresh cold water directly from McMurdo Sound, and Woods spent hours looking at the way fluids moved through their bodies.
A lot of arthropods, creatures such as insects, spiders and shrimp, have complex guts that spill beyond their core and fill up the nooks and crannies of their extremities. The guts of sea spiders, for instance, extend through their legs.
As Woods watched their movements, he made an odd observation.
“It struck me that there was so much more peristalsis (wavelike movement of the intestines) than there should be just for digestion alone,” he said.
He thought it made sense for the movement to have another function, and he wondered if the muscles were pushing not just food, but gases.
So Woods tested that idea. If a human isn’t getting enough oxygen, the person starts breathing faster, and the same is true for sea spiders.
In the lab, he manipulated oxygen levels in the water to find out how the sea spiders responded.
“Do they hyperventilate with their guts? And they did.”
The heart provides blood flow only to a central core, not to the entire animal.
One knee jerk reaction some scientists have to animals’ dispersed guts is to assume they need complex guts for digesting food, Woods said. His research shows that evolution might have different reasons for developing complex guts, to move gases instead.
“To me, what it says about evolution is it’s tinkering with different systems,” Woods said.
The sea spider offers more mysteries to consider in the future.
Evidence from Shishido’s work suggests the metabolic rates of sea spiders are especially sensitive to temperature fluctuations, Woods said.
“As the polar oceans warm, how is that going to affect the marine invertebrates that have lived there? And is it going to affect the biggest ones disproportionately?” he said.
He would like to conduct more rigorous testing to explore that question.
Lane wants to know more about the animal’s exoskeleton. Why, for example, does the sea spider appear to groom itself? And how does the cuticle do two different jobs that appear to be in conflict? How is it porous enough to allow in oxygen but also thick enough to support the sea spider’s body?
On the whole, Woods wants the finding that sea spiders use digestive systems to move oxygen to open people’s minds to wider possibilities in nature.
“One thing I hope is that this work will spark people to think more broadly about some of these respiratory problems that organisms face,” Woods said.