The frigid waters under the ice of Antarctica are home to giants. The cold oxygen-rich waters have given rise to some monsters of the marine world, not least the gangly-legged sea spiders. Little more than hollow tubes, it seems that these arthropods are even weirder than we thought, and pump oxygen around their bodies using their guts, rather than their hearts.
While there are over a thousand different species of sea spiders, the ones living in the polar regions grow to truly horrific sizes. Some of them measure up to 50 centimeters (20 inches) in diameter, with the vast majority of this accounted for by their spindly, wiry legs. But this poses problems, namely, how to move the oxygen they absorb through their exoskeleton to other parts of the body that might need it.
Smaller arthropods manage just fine with diffusion, but as organisms get bigger, this method becomes more and more inefficient. This has led in many cases, to the development of muscular organs, like the heart, which aids in moving oxygen and other gases around the body. But when researchers were looking at how blood flowed through the bodies of giant sea spiders, they noticed something curious. The heart seemed to only to be pumping weakly, while in contrast, the guts appeared to be showing incredible strong peristalsis, the wave-like movements of contraction.
“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,” explained H. Arthur Woods, who co-authored the study published in Current Biology. “In effect, sea spiders guts are 'space-filling' and ubiquitous in their bodies in the same way that our circulatory systems are space-filling and ubiquitous.”
A series of experiments involving 12 different species of polar sea spiders, in which they used dyes to track the movement of hemolymph (the spiders’ version of blood), and experimental manipulation of parts of the animals' guts, they were able to confirm what they thought.They found that the heart was not moving any fluid further than the spiders’ diminutive body, while the guts extending down each leg seemed to be doing the bulk of the work instead.
The study highlights the amazing variety and diversity that has evolved to solve the issues that many animals encounter. Whether or not the guts evolved primarily for digestion first, and were then co-opted for pumping, or the other way round, is not yet known, but it is hoped that perhaps the fossil record may be able to shed some light on this.