Pumping guts not heart keep sea spiders alive

Researchers have discovered a scientific oddity – a sea creature that uses its gut, not its heart, to pump blood around its body.

Sea spiders are found in the Mediterranean and Caribbean, as well as the Arctic and Antarctic Oceans. The scientists discovered this peculiarity when researching why some animals in polar regions have larger bodies than their temperate or tropical counterparts.

During the study, scientists observed that sea spiders’ hearts pumped blood only to their central section. But this was coupled with vigorous gut peristalsis – however, sea spider guts are very different to ours. Dr H. Arthur Woods, from the University of Montana and first author of the study, said: “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.”

Sea spiders take in oxygen directly through their cuticles, but their peristaltic waves are more vigorous than would be needed for digestion. 12 species of these marine arthropods were studied after tracers were injected into the animals’ haemolymph – a fluid similar to vertebrates' blood. Observing the tracer move around the body and experimental manipulation of the guts’ ability to contract enabled the researchers to prove this gut peristalsis theory.

The scientists are currently unsure of whether the sea spiders’ guts arose initially for digestive purposes and its circulatory benefits came later or vice versa. The scientists are hoping that potential future fossil finds may help answer their evolutionary beginnings as well as understanding gas transport in other arthropods with similar digestive systems. The research was published in Current Biology.