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Gut Instinct

Anatomy of a pycnogonid: A: head; B: thorax; C: abdomen 1: proboscis; 2: chelifores; 3: palps; 4: ovigers; 5: egg sacs; 6a–6d: four pairs of legsWhen an animal is all legs and almost no body, questions abound - and one question in particular springs to mind - where do they keep all their essential biological parts? Most mammals have plenty of internal space for the heart, lungs, and gastrointestinal organs. Not so for sea spiders! Some species are as large as a dinner plate (i.e., Antarctic sea spiders), with legs-for-days, but others have a tiny “torso” that is basically an attachment site for legs. So, how do they manage to reproduce or circulate blood, not to mention, process their food?

Sea spiders manage these important functions with their long legs. Interestingly, females grow ovaries on their legs and release them through pores. During mating, the male climbs over the female to fertilize her eggs and then carries them until they hatch. Similar to seahorses, male sea spiders carry their offspring until birth –and while the male seahorse swims away soon after - male sea spider cares for the young after they are born!

Not only are sea spiders’ reproductive organs located in their legs, but so are their guts. The distance between the mouth and anus is so small that the intestines reach down each leg so that food can be adequately processed. The guts contract to move food along just like our intestines do.

Sea spiders do not have any respiratory organs, instead, they receive oxygen through passive diffusion across their 4-6 pairs of legs. But because these pycnogonids can become so large, scientists wondered how they get the amount of oxygen they need to survive. Larger animals need to get plenty of oxygen into their bloodstreams and need to be able to pump that blood around their bodies. Passive diffusion might be fine in a small body, but Atlantic sea spiders tip the scale for size. Drs. Amy Moran and Arthur Woods, of the University of Hawaii at Manoa, recently discovered that sea spider hearts are not large enough to efficiently pump blood throughout their whole body. Instead, they circulate blood using their guts!

Each leg contains blood vessels as well as intestines, so as food is processed, blood is also circulated through the legs and into the body. However, the legs of a sea spider are not expandable, so, when digestive fluid is pushed in one direction, it forces blood to flow in the opposite direction. After oxygen diffuses in the legs, it travels to the creature’s body cavity through contractions of the guts. Once the blood reaches the body cavity, the heart is then able to circulate it around the body and head.

Sea spiders are able to live without having a specialized system for pumping blood because they effectively use their legs as gills and their guts as hearts – a gut check for all of us – we are not the only complicated creatures on the planet!  





Woods, H. Arthur, Lane, Steven J., Shishido, Caitlin, Tobalske, Bret, Arango, Claudia P., Moran, Amy L. 2017. Respiratory gut peristalsis by sea spiders. Current Biology, Volume 27, Issue 13, R638 - R639

External anatomy of Nymphon sea spider. After G. O. Sars (1895).


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