Big, Cold and Creepy
Researchers Study Gigantism Among Antarctic Sea Spiders
Scuba diver Art Woods slips through a hole in the Antarctic ice. He’s followed by Bret Tobalske. In an instant, the two UM professors leave a world of white ice and black volcanic rock for a gin-clear underwater sea, bursting with swaying, gaudy life. It’s also bracingly frigid at 28 degrees Fahrenheit.
Brittle stars, jelly fish, anemones, scallops, nudibranchs, ctenophores and octopuses brighten the shallow seas. Golds, yellows, oranges and reds infuse tropical bursts of color in the darkness. Weddell seals croon a haunting melody of chirps, twitters and space-age music. The ceiling of the ice billows in white waves.
Here, slender sea spiders reach gargantuan sizes. They wander the shallow ocean floor with the deliberate slow grace of sloths. In these fragile waters, some species grow to 20 inches across, big enough to clasp a human’s head. To study gigantism in sea spiders, the professors are willing to “slowly freeze to death,” as Woods describes their half-hour-long dives.
“Every time I jump in, I think this is the dive where I’m going to be warm,” says Woods, an exuberant researcher of arthropods, the evolution of body size and the role oxygen plays in marine systems. He’s studied sea spiders since 2007.
“Nothing prepares you for how insidious the cold is,” agrees the soft-spoken Tobalske, who’s known for his work in the evolution of bird flight.
Together, they’re tackling one of the great scientific questions: How and why do life forms evolve to become enormous, especially in the polar waters? Besides sea spiders, Antarctic giants include everything from worms to nudibranchs (sea slugs).
Antarctica’s bizarre sea life may stem from extended isolation. A circumpolar current around the continent acts like a down jacket in reverse – keeping the warmth out and the cold in. McMurdo Sound harbors an ecosystem rich in unexplored species and potential answers to life’s questions. For that reason, Woods, Tobalske and UM graduate student Steve Lane, along with two colleagues from the University of Hawaii, are willing to take multiple polar plunges with a laser focus on sea spiders.
This fall they will head back to the field, with a sense of time ticking on the planet’s clock. As the climate warms, the ocean’s acidity is rising. The polar ice melts earlier. The water is just shy of perfectly clear, barely perceptible changes witnessed by the scientists of the esteemed McMurdo Station, the Antarctic town dedicated to science and education.
A sea spider’s typical four pairs of legs seem impossibly long relative to a miniscule body. With deliberate steps, the sci-fi creature scavenges the sea floor for food, sucking up dead jellyfish and other delicacies through a slender proboscis.
“It was instant love,” says Woods of his first encounter with sea spiders in the Antarctic. He originally intended to study nudibranchs with colleague Amy Moran from University of Hawaii in the mid-2000s. Together, they added a side project on sea spiders that now is their main study.
Sea spiders are pycnogonids, part of an ancient lineage extending back 300 million to 400 million years, and in the same subphylum as horseshoe crabs. They’re not spiders and far different from crabs. There’s no respiratory system like gills or lungs. Instead, they rely on the diffusion of oxygen across their cuticles (like our skin). Their digestive and reproductive systems branch through their legs.
Giant sea spiders are rule breakers, appearing to defy a famous scientific premise, called the three-fourths scaling law, which states that as any animal gets bigger, its metabolic rate is lower relative to its size than that of a smaller animal. Their lack of a respiratory system is the likely explanation for the departure from the broad trend.
Not all sea spiders are giants. While most are still so little known that they lack names, scientists estimate there are 1,500 species, with 20 percent found in Antarctica. Both Woods and Tobalske also dive for fingernail-sized sea spiders in the San Juan Islands of Washington.
While theories proliferate about the Antarctic’s propensity for gigantism, Woods and Tobalske are testing a major idea called the oxygen hypothesis that’s tied to environmental factors – calm seas and cold waters. Oxygen is in high supply, because it dissolves easily in cold waters. At the same time, the metabolic rates of sea creatures are low, because of the cold.
“A lot of supply and little demand allows the evolution of giant forms, because it’s easy to get oxygen in through the surface of organisms,” says Woods. “We think gigantism is driven by the relative availability of oxygen, and to test that we’re doing oxygen physiology tests on sea spiders.”
While the oxygen-rich cold water predicts gigantism, calm waters also may be an essential quality. The long-legged sea spiders likely cannot withstand high currents or waves. Woods and Tobalske believe that the giants, then, would only evolve in calm places such as McMurdo Sound.
The waters may be calm, but conducting field tests is a colossal undertaking in itself. For Woods to position delicate electrodes in spiders requires absolute stillness. Likewise, when Tobalske lies on the shallow floor holding two cameras to film the biomechanics of slow-moving sea spiders, he forces himself to stay motionless. The cold is especially brutal when not moving. Raising a hand can bring in much-needed warmth by moving air around inside the dry suit from core to extremities.
“It’s very humbling how incredibly difficult and challenging it is to do research underwater,” Tobalske says. “The equivalent of tying your shoe takes magnitudes more effort. You’re very restricted. The gloves are huge. The water is so cold.”
Not all research takes place in rigorous conditions. In April, Tobalske traveled to Harvard University to conduct CT scans of cross sections of sea spiders of multiple sizes. The scans showed that the largest sea spiders have more pores in their body walls. Porousness serves well for gathering oxygen, but it compromises leg strength. As they suspected, the giant spiders can blow like leaves in the wind by the gentlest of currents.
Scientists only can study beneath the ice when it’s safe to dive. In summer, when the sea ice breaks up, a massive pulse of plankton enters the Antarctic like a “green, soupy pulse of energy,” according to Tobalske. How the spiders handle the influx and currents are still one of the great unknowns.
Tobalske and Woods first joined up when co-teaching a physiology class in 2010. They soon realized a shared interest in the evolution of gigantism, a subject that evokes dinosaurs roaming the planet.
“To understand the big pattern of evolution of body size, we’re applying fundamental principles about physiology and biomechanics,” says Woods.
Their studies take place beneath a looming planetary question: What will be the future of sea spiders and so many poorly understood creatures beneath the Antarctic ice? As the climate warms, an entire ecosystem teeters on the edge. So little is known about sea spiders that Woods and Tobalske are waiting to publish their first findings until the species are all identified.
“The rising temperatures are going to be hardest on the biggest individuals in the Antarctic seas,” says Woods. “Those will suffer disproportionately.”
Higher water temperatures mean less oxygen available. Without high oxygen levels, the big pores in sea spider legs cannot draw across the life-giving oxygen they need.
The more time Woods and Tobalske spend beneath the ice of the Antarctic, the more they fall under the spell of an ecosystem so strange that it turns all basic assumptions upside down, as if you open your freezer and find supersized goldfish thriving among the ice cubes.
Studying sea spiders in the controlled environment of tanks at McMurdo Station provides researchers a wealth of information, as long as they compare findings with the wild. When it comes to behavior, the biggest sea spiders react differently in the two environments. If disturbed in the wild while suspended in water, they pull their legs over their heads to take a comet aerodynamic shape, allowing them to plummet to the sea bottom and out of danger. In the lab, they remain spread out and floating when disturbed.
— By Marina Richie