Sea Spiders Use Their Guts, Not Hearts, To Pump Oxygen

Sea spiders, which have inhabited Earth for over 500 million years, are fascinating creatures. The marine arthropods, which range in size from a millimeter long to as big as a dinner plate, have eight jointed legs that convene around a tiny body. Since their torsos are so small, sea spiders use their legs to conduct normal body functions such as digestion and reproduction. Now, it seems the creatures also have a unique breathing mechanism.

The latest discovery was made by a team led by University of Montana Associate Professor Arthur Woods. The researchers were curious to see how the spiders’ weak, tiny hearts managed to pump blood and oxygen from the central part of their bodies to the tips of their long, thin legs especially, in the larger Antarctic species.

To unravel the mystery, they injected fluorescent chemicals into the blood of 12 sea spider species from Antarctica and the US to see how far their small hearts were able to transport it. Not surprisingly, it was a very short distance. To make up for the shortcoming, the spiders have adapted by using their guts to pump blood. Woods says, “Unlike us, with our centrally located guts that are all confined to a single body cavity, the guts of sea spiders branch multiple times and sections of gut tube go down to the end of every leg.”

Just like in humans, the guts contract to move food along. However, since the spiders’ legs are not as flexible as the abdomen, they are unable to stretch or expand. Hence, when the spider pushes digestive fluids down its legs, the blood gets pushed up. Conversely, when the digestive fluids are pushed up, the blood flows back down. The scientists theorized that this action helps the oxygen, which is passively diffused into the animal’s legs, to be circulated throughout the body.

To test the thesis, the team lowered the oxygen levels in the surrounding seawater and observed that the sea spiders’ gut contractions instantly increased. Amy Moran, a marine ecologist at the University of Hawaii at Manoa, who helped with the study, says, “It’s like when you take a person up to altitude, and they breathe faster, and their heart rate goes up. The sea spiders are using their legs as gills and their guts as hearts.” The researchers, who published their findings in the journal Current Biology on July 10, 2017, believe that though this is the first time this kind of gut-based circulatory system has been observed, it may be more common than we realize.

Though classified as Chelicerates, a group that includes terrestrial spiders, horseshoe crabs, mites and ticks, sea spiders, or pycnogonids, are not considered “true” spiders. While some geneticists believe there is a distant relationship, Moran says, “They’re about as closely related to a terrestrial spider as a seahorse is to a horse.”

Resources: The Atlantic.com, National Geographic.com, the scientist.com