Miami - Sea spiders do not move blood and oxygen through their bodies with the help of vigorously beating hearts, like most creatures, but with guts that act like pumps, researchers said on Monday.
These guts extend through their entire long, gangly bodies, researchers said 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 lead author H Arthur Woods of the University of Montana, Missoula.
Woods became fascinated with giant sea spiders while stationed in Antarctica, where he said he found himself spending "a lot of time just watching blood and gut flows in sea spiders".
He noticed their hearts were beating only weakly, and moved blood only in the central portion of their bodies.
But their guts showed strong and organised waves of contractions.
The process is called peristalsis, and it happens in humans too, with waves of involuntary constriction and relaxation of muscles.
Its purpose in people is to aid digestion, mix up the contents of the gut and move them through the intestines.
Peristaltic waves in sea spiders are far stronger than would be needed for digestion, because they must also get enough oxygen through the body.
"The findings highlight the vast evolutionary diversity of solutions to problems that all animals encounter," said the report.
Future fossil discoveries might help scientists better understand the origins of this odd survival strategy.