Image: NOAA/Monterey Bay Aquarium Research Institute

Sea spiders are some of the strangest animals to persist beneath the surface of the sea — and they have officially just gotten weirder. 

These marine arthropods are found in the Mediterranean, Caribbean, Arctic, and Antarctic oceans. They boast an average of 10-12 legs and can grow to astounding sizes of about three feet in width.

As opposed to normal veins and blood vessels, sea spiders have intestines that span the lengths of their limbs. While insects primarily utilize a substance called hemolymph in order to get oxygen to their cells, sea spiders do things a bit differently.

A recent study looking at gut peristalsis in sea spiders reveals they only use hemolymph around their gently pumping hearts — to provide oxygen to their head, abdomen, and the uppermost parts of their legs. 

Image: Sylke Rohrlach, Flickr

Upon closer observation, scientists determined that sea spiders actually use their guts to breathe. The squeezing motion that helps digest food is extra pronounced in sea spiders because they are actually using their guts to move oxygen throughout their extremities.

“So the question is, why aren’t these hearts doing the job they normally do in all the other organisms that we know about? It’s like they’ve offloaded the circulatory function onto the gut and the heart has this very restricted function right in the middle of the body,” the study’s lead author Art Woods stated to Live Science.

It was confirmed in 12 species of sea spiders that their digestive tracts are actually operated akin to circulatory systems, and researchers surmise that the answer to this oddity has to do with conserving energy.

The full study is published in the Journal of Current Biology.