Underwater 'Spider' Breathes Through Its Legs

Strange creatures with eight spindly legs that crawl along the ocean floor, sea spiders may look like arachnids, but they're actually not.

There are about 1,000 sea spider species worldwide, ranging from an inch long to as big as a dinner plate. They're mostly all legs, with a tiny body and pointy proboscis that sucks juice out of unwitting prey.

The alien-looking sea spider is actually a distant relative of land-dwelling arachnids.

Photograph by Timothy R. Dwyer (PolarTREC 2016), Courtesy of ARCUS

“They do all their business in their legs,” explains Amy Moran, a marine ecologist at the University of Hawaii at Manoa. “Their gonads are in the legs, and the females store eggs in their legs.”

Sea spiders obtain oxygen by diffusing water over their thin, porous external skeleton. “They don’t have any specialized structures for gas exchange,” says Moran, co-author of a new study on sea spiders. “We have lungs. Fish have gills. Sea spiders don’t have anything except a large surface area.”

Like most arthropods, sea spiders have hearts. But they're weak, unable to move both oxygen and blood from the tips of the sea spiders' pencil-like legs to the central part of their bodies. (See National Geographic pictures of odd-looking sea animals.)

So scientists wondered—do they breathe through their legs?

The new research suggests they do. Their guts, which extend to the bottom of their legs, regularly contract to move oxygen around their bodies, according to a study published July 10 in the journal Current Biology.

A Gut As a Heart

The scientists collected live specimens of 12 sea spider species from Antarctica and the U.S. West Coast. After dyeing and tracing some of the sea spiders' blood, they found the wave-like movements of the gut pump not just food, but also oxygen. (See animals that live under the ice in Antarctica.)

“It’s super cool. They can exchange gases much more efficiently this way,” says Sebastian Kvist, an invertebrate ecologist at the Royal Ontario Museum in Toronto who was not involved in this study.

“I’m curious to know what other marine animals do this that we don’t know about.”

“Our own guts contract in waves to move food, and people assumed it was just for digestion when they saw this movement in sea spiders,” Moran explains.

This new research shows it goes beyond that: “These sea spiders are using the gut as a heart.”

Because sea spiders have been around for 500 million years, the research may also help scientists learn more about how circulatory systems evolved in various animals.

Follow Shaena Montanari on Twitter.