Sea Spiders Pump Blood With Their Guts, Not Their Hearts

sea spider, ammothea
A preserved specimen of the bottom-dwelling sea spider genus Ammothea. Mark Conlon/Getty Images

The heart is a pretty important organ. That's true not only for us humans, but also for the seemingly endless array of other creatures and critters that roam the planet. The human heart pumps blood to the rest of our bodies through a network of arteries, veins and capillaries. It's also an essential part of one of the more important functions of the human body: breathing. When we breathe in air, blood pumped to the lungs by the heart picks up oxygen to transport that vital element to the rest of our bodies. When we breathe out, we expel harmful carbon dioxide — caused by a buildup of internal waste — that has been brought up to the lungs via blood pumping from the heart.

sea spider
Biologist Art Woods holds an Antarctic sea spider specimen in his University of Montana laboratory.
University of Montana

Well, at least most of us do. See, there are these creatures called sea spiders. These marine arthropods of the class Pycnogonida, which visually resemble land-based spiders with longer legs (or the terrifying facehuggers from the "Alien" films), but are more closely related to crustaceans, don't need their hearts to breathe.


Researchers from the University of Montana, Missoula recently examined 12 different species of sea spider, discovering that the deepwater animals use their guts as the pump that sends blood and oxygen throughout their bodies. The scientists recently published their findings in a study in the journal Current Biology.

The circulation process is made easier by the fact that these creatures' guts extend into each of their legs with tubes aiding the flow of precious cargo. A series of ongoing muscle movements ensures that the blood and oxygen keeps circulating.

"My 'aha!' moment was to consider that maybe all that sloshing of blood and guts was not about digestion but instead about moving respiratory gases around," said biologist and lead author H. Arthur Woods, in a release announcing the finding.

If you're wondering why anyone should care about what one creepy-looking creature well below the surface of the seas goes through to maintain proper circulation, the researchers say they are trying to determine whether sea spiders evolved over time from heart-based to gut-based blood flow, shedding greater light on the process of evolution and the variety of life here on Earth.

sea spider
A sea spider photographed on the seafloor.
Timothy R. Dwyer (PolarTREC 2016)/Courtesy of ARCUS