A Yellow Kneed Sea Spider — Calipallenid pycnogonid. Image credits: Sylke Rohrlach.

Contrary to their name and appearance, spiders and sea spiders are not really related. Sea spiders are not even arachnids, though they have much in common with the group. They have long legs and a small body size, inhabiting many different parts of the world, from New Zealand, and the Pacific coast of the United States, to the Mediterranean Sea and the Caribbean Sea. They can be found as deep as 7,000 meters (23,000 ft), but can also thrive at well camouflaged beneath the rocks and algae in estuarine habitats. They’re a diverse and interesting group, and — as this study reveals — they sometimes breathe through their legs.

Stretching my legs

“The fundamental constraint shaping animal systems for internal gas transport is the slow pace of diffusion,” the study reads. The problem is that these sea spiders, despite having a heart, don’t have any of the classic breathing structures (read: gills or lungs). Their hearts are also weak, unable to pump blood properly from their long legs to their small bodies. So instead, they have to rely on a different mechanism.

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They seem to use their thin, porous external skeleton. Namely, their legs.

“They do all their business in their legs,” explains Amy Moran, a marine ecologist at the University of Hawaii at Manoa. “Their gonads are in the legs, and the females store eggs in their legs.”

They’re basically using their unusual structure to compensate for their physiological disadvantages.

“They don’t have any specialized structures for gas exchange,” adds Moran, co-author of a new study on sea spiders. “We have lungs. Fish have gills. Sea spiders don’t have anything except a large surface area.”

She and her colleagues collected live specimens of 12 sea spider species. They dyed and traced their blood, finding wave-like movements of the gut that move not only food around, but also oxygen. This allows them to not only gather the oxygen they need but also move it around their bodies. Sebastian Kvist, an invertebrate ecologist at the Royal Ontario Museum in Toronto who was not involved in this study told National Geographic that other marine animals might do the same thing, but we just haven’t discovered it yet.

The origin of this process is uncertain. It may be an adaptation derived from earlier evolution of branching gut structures, something known from Cambrian fossils of arthropods. To better understand the evolution of this trait, researchers now want to examine gas transport in extant arthropods with complex guts.
Journal Reference: H. Arthur Woods, Steven J. Lane, Caitlin Shishido, Bret W. Tobalske, Claudia P. Arango, Amy L. Moran — Respiratory gut peristalsis by sea spiders. DOI: http://dx.doi.org/10.1016/j.cub.2017.05.062