If you eat by shoving your entire writhing body into your meals, your dinner companions are probably going to leave. But your lack of table manners does open up a valuable opportunity that refined diners miss out on. You are literally surrounded by food. You’re slathered in carbohydates, fats, proteins and more. Rather than swallowing them, it would be very efficient if you could absorb these nutrients straight through your skin. And that’s exactly what a gruesome sea creature called the hagfish can do. Its entire body is like a simple gut.
Hagfish look like slimy eels, but they’re actually very different. They don’t have any jaws or a proper backbone (although they’re traditionally grouped within the back-boned ‘vertebrates’). If you handle one, it will cover your hands in slimy mucus while it literally ties itself in knots in its attempts to escape.
In the ocean, the hagfish’s habits are no less repulsive. They scavenge dead carcasses on the sea floor by burrowing inside and eating their way out. Inside their decaying meals, they swim amidst a nutrient-rich soup and they don’t let that opportunity go to waste. Chris Glover from the University of Canterbury has found that they can absorb these nutrients directly through their skin and gills.
Glover worked with the Pacific hagfish, stretching small samples of its skin over the rim of a glass tube. He submerged the tube in a beaker of seawater containing mildly radioactive sugar and amino acids, and some food colouring. The skin kept out the food colouring, but after a few hours, the water inside the tube was buzzing with radioactivity. Glover found the same thing in a similar experiment involving gill tissues.
Many other sea creatures, from worms to shellfish to starfish, can also absorb nutrients through their skin but the hagfish are the only vertebrates with the ability. They’re well suited to the task. A fish or turtle has an impermeable skin that buffers its innards against its salty environment, but the hagfish’s skin is more lenient. The animal actively shuttles ions and water across its skin so that the levels in its bodily fluids match those of the surrounding seawater. From this foundation, it’s not hard to imagine how it might have evolved to actively smuggle in nutrients.
In fact, Glover found that a hagfish’s skin can absorb nutrients faster than its intestines! This is important because these animals sit at the very base of the vertebrate family tree. It’s possible that our early ancestors had similarly absorbent skins. As they evolved to maintain their internal environment, and keep ions and water out, their skins lost the ability to take up nutrients. They may have had to develop a more complex and efficient gut to compensate.
Reference: Glover, Bucking & Wood. 2011. Adaptations to in situ feeding: novel nutrient acquisition pathways in an ancient vertebrate. http://dx.doi.org/10.1098/rspb.2010.2784