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With eight spindly legs, a minimalist body, and long proboscis, sea spiders look like something from the fever dream of an arachnophobic science fiction author. But sea spiders are very real, and they live all over the world. And in the dark, cold waters of the polar oceans, sea spiders grow big, huge—their legs spanning the width of your face.
Formally known as pycnogonids, sea spiders are a primitive group of marine arthropods. They’re not technically spiders. Most often, sea spiders are small, cryptic creatures.
Yet in the Arctic Ocean and Southern Ocean, sea spiders grow huge. But they’re not the only ones that do. The polar oceans are awash in creatures of unusual size, including copepods, echinoderms, and certain mollusks that grow larger than their more equatorial relatives—a phenomenon called “polar gigantism.”
Polar gigantism is a mystery. Though multiple hypotheses have been put forth to explain it, none has been proven—yet. One group of researchers thinks that Antarctic sea spiders hold the key to this ginormous riddle. But first they had to catch some sea spiders to put their theories to the test.
After drilling a hole in the thick sea ice, an insulated and dry-suited SCUBA diver drops through into the glass-clear water. The ice-filtered light, chirping Weddell seals, and seafloor sprinkled with ice crystals creates a magical atmosphere that is “really pleasant, except that you’re slowly freezing to death,” says Art Woods, one of the researchers on the project. The water temperature hovers around -1.5 to -1.8 °C, the freezing point of seawater.
Yet these chilling temperatures might be what holds the key to polar gigantism. Cold water can hold more dissolved oxygen than warm water, and the oxygen content of seawater near the coast of Antarctica is especially high. On top of that, colder temperatures slow the metabolisms of these cold-blooded creatures, and slower metabolisms consume less oxygen.
These factors combine to make sea spiders supersized because sea spiders rely on simple diffusion to get oxygen into and around their bodies. This system doesn’t work well in large-bodied organisms unless there is a lot of oxygen available, says Woods.
Perhaps the abundant oxygen and low demand allows for polar creatures’ mammoth sizes?
With their collected sea spiders, Woods and colleagues tested how changing the temperature and dissolved oxygen content of the water affects sea spiders’ physiology. So far their results seem to support the oxygen hypothesis for polar gigantism: larger sea spiders fare poorly in low oxygen water.
But cracking the puzzle of polar gigantism is about more than scratching an intellectual itch. If oxygen-rich cold water turns out to be the key to massive marine life, what happens as oceans warm and oxygen levels fall? Perhaps no one will notice the demise of supersized sea spiders, but seafood lovers will notice a world without jumbo lobsters and king crabs.