Marine arthropods called sea spiders use gut peristalsis to move hemolymph and oxygen throughout most of their bodies, according to a team of researchers led by Dr. H. Arthur Woods from the University of Montana.

Sea spiders, or pycnogonids, are small, primarily bottom-dwelling marine arthropods that superficially resemble the true, terrestrial spiders.

There are over 1,300 known species, ranging in size from 1 mm to over 90 cm.

They are found in the Mediterranean and Caribbean Seas as well as in the Arctic and Antarctic Oceans.

Relative to other arthropods, sea spiders have long legs in contrast to a small body size. The number of walking legs is usually 8, but species with 10 and 12 legs exist.

Unlike true spiders, sea spiders do not possess a specialized respiratory system — oxygen is absorbed by the legs and is transported via hemolymph (fluid, analogous to the blood in vertebrates) to the rest of the body.

“The sea spiders have an unusual gut in the first place,” Dr. Woods 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.”

“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.”

So, how do sea spiders use that branching system to move fluids? The answer is gut peristalsis.

In fact, the human gut also uses peristalsis — waves of involuntary constriction and relaxation of muscles — to mix gut contents and move them along. Sea spiders show peristaltic waves that are much more vigorous than would be needed for digestion.

Dr. Woods and co-authors made that discovery after an Antarctic mission to explore a phenomenon known as ‘polar gigantism.’

Biologists had long observed that polar species, including giant sea spiders, have larger bodies than their more temperate or tropical relatives.

That trend raises a lot of intriguing questions about how the polar species manage basic life processes, including how to get enough oxygen into their bodies.

“One of the things that make sea spiders a great organism for study is that they are really skinny and, using a microscope, you can see easily into their bodies,” Dr. Woods said.

“In my first season at McMurdo Antarctic Station, I found myself spending a lot of time just watching blood and gut flows in sea spiders.”

“I soon realized that the sea spiders’ hearts were beating only weakly. Their hearts weren’t moving any blood beyond the spiders’ central portion. In contrast, their guts showed very strong and organized waves of peristaltic contractions.”

“My ‘aha!’ moment was to consider that maybe all that sloshing of blood and guts was not about digestion but instead about moving respiratory gases around,” he said.

A series of experiments and observations in 12 species sea spiders, involving video microscopy of tracers in the animals’ hemolymph and guts, together with experimental manipulation of the guts’ ability to contract, allowed the team to test out and confirm that hypothesis.

“The findings highlight the vast evolutionary diversity of solutions to problems that all animals encounter,” the authors said.

“It’s not clear whether the sea spiders’ space-filling guts first arose for purely digestive functions and the respiratory benefits came later or vice versa.”

“Future fossil finds might help to weigh in on the evolutionary origins of the animals’ unusual respiratory strategy,” Dr. Woods said.

“It would also be worthwhile to explore gas transport in other living arthropods with similarly complex guts.”

The findings were published in the July 10 issue of the journal Current Biology.

_____

H. Arthur Woods et al. 2017. Respiratory gut peristalsis by sea spiders. Current Biology 27 (13): R638-R639; doi: 10.1016/j.cub.2017.05.062