They do have a heart, according to Dr. Moran, a biologist at the University of Hawaii. “But it’s a very wimpy heart.”
So the gut, as it churns, moves blood — actually, something called hemolymph — around the body. The hemolymph, like blood in mammals, carries oxygen.
What Dr. Moran and Steven J. Lane, of the University of Montana, and their colleagues have been investigating recently was how the sea spiders get that oxygen into their bodies in the first place.
The creatures don’t have lungs or gills, so the only other explanation seemed to be that the spiders were absorbing oxygen through their skin. But their skin is tough, a rigid exoskeleton that allows them to move around, and oxygen absorption through that would be unlikely.
So the researchers concentrated on numerous pores that dot the legs of sea spiders. It turns out they are deep enough to bring seawater through the exoskeleton, where the oxygen can be absorbed.