Sea Spiders Move Oxygen with Pumping Guts, Marine Biologists Say | Biology

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.

A sea spider on the sea floor. Image credit: Timothy R. Dwyer / PolarTREC 2016 / ARCUS.

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…

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