(CN) – Researchers have discovered sea spiders rely on waves of gut contractions to circulate fluids and breathe, rather than with a beating heart as most animals do.
These spindly arthropods – invertebrate animals that have an exoskeleton – have unique guts that spread throughout their bodies and facilitate respiration, according to a study published Monday in the journal Current Biology.
“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,” said H. Arthur Woods of the University of Montana, Missoula, lead author of the new report. “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.”
The team began studying sea spiders in Antarctica as part of a mission to explore a phenomenon known as “polar gigantism.” Polar sea spiders tend to have larger bodies than their relatives in more temperate climates, which has led scientists to speculate about how they manage basic life processes.
After spending “a lot of time just watching blood and gut flows in sea spiders,” Woods realized that their hearts were beating weakly. While the spiders’ hearts were not able to move blood beyond the central portion of their bodies, he also noticed that their guts showed strong, organized waves of contractions.
“My ‘a-ha!’ moment was to consider that maybe all that sloshing of blood and guts was not about digestion but instead about moving respiratory gases around,” Woods said.
The team tested this theory on 12 sea spider species, using video microscopy of tracers in the animals’ hemolymph – the equivalent of blood for invertebrates – and guts, as well as experimental manipulation of gut contraction.
The scientists found that after taking in oxygen through their cuticles, the spiders use gut peristalsis to move nutrients throughout the body. Though the human gut uses peristalsis – waves of involuntary muscle constriction and relaxation – to mix and transport gut content, the peristaltic waves observed in the spiders are much stronger than would be needed for digestion.
Woods believes the discovery of new fossils might help to clarify the evolutionary origins of this odd respiratory system, which could also shed light on similarly complex guts in other modern arthropods.