a sea spider on the sea floor
Timothy R. Dwyer (PolarTREC 2016), Courtesy of ARCUS

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Meet the only animal known to pump blood with its guts

High school biology classes teach us that the circulatory system pumps blood and the digestive system pumps food. But sea spiders (pictured) apparently skipped that lesson: They pump blood using their guts. Researchers discovered the remarkable physiological strategy after injecting dye into sea spiders—common inhabitants of the world's oceans named for their resemblance to land-based spiders—and watching the flow of blood. They noticed that the animals’ hearts were beating weakly. But the digestive system—which is unusually extensive in sea spiders, running down each leg—was contracting in waves, moving food in the gut as well as blood in the surrounding hemocoel cavity, the spider equivalent of veins and arteries. Additional experiments using 12 sea spider species from Antarctica and the United States confirmed that gut contractions propel the flow of blood and oxygen in this group of marine-dwelling arthropods, the team reports today in Current Biology. It’s not clear exactly why the digestive system does the job, but it could save energy having gut contractions move food and blood. Sea spiders also have an unusual need to move oxygen from their extremities to the core of their bodies because they lack gills, and most oxygen is taken up by diffusion through the surface of their long legs. It’s the first time a blood-pumping system like this has been found in nature, but the authors suspect that other animals may do something similar.