Rays 'throw up their guts'

The whole cleaning process lasts about four seconds

"Throwing your guts up" may not be the most elegant phrase to describe the action of vomiting, but in some fish species it is certainly an accurate one.

UK researchers have shown that a thornback ray will literally turn its stomach inside out to eject an unwanted meal. For a very short time, even the stomach itself will be pushed out through the mouth to get a good cleaning.

The ray thrashes its head from side to side to make sure everything is washed way.

Many animals vomit. It is a very important protection mechanism designed to rid the body of dangerous materials that are either toxic or indigestible.

And although the type of vomiting reflex seen in the thornback may sound dramatic, it is probably more common than was thought.

How to invert a tube

"We don't really know the mechanism involved but it obviously involves some muscular movements," Dr David Sims, from Aberdeen University's Department of Zoology, told BBC News Online.

"The problem is how to invert a tube. When you take your socks off they always go inside out and that is what the ray has got to do. So, muscle contractions of the gut and throat area are probably responsible."

Dr Sims and colleagues watched and filmed several trawler-caught thornbacks (Raja clavata) swimming in their laboratory.

The rays were given an injection of veratine hydrochloride to make them sick. This usually occurred within 10 minutes of the emetic being administered. Vomiting was frequent, with one animal "chucking its guts up" nine times in four discrete bursts.

Poisons and parasites

The whole procedure is very short with the stomach ejected, cleaned and swallowed again in about four seconds.

"This behaviour has been seen in rays and sharks and is therefore more widespread than is currently realised," Dr Sims said. "It is a way for these creatures that eat prey whole to get rid of indigestible materials. It would work for poisons and parasites as well."

Certain frogs and toads can also do it, except they actually use a hand to clean the stomach when it is hanging outside the mouth.

By studying different creatures that display similar behaviour, scientists might gain fresh insight into the evolution of the autonomic nervous system.

This controls the many organs and muscles within the body cavity that function in an involuntary, reflex manner.

The research is published in the journal Nature.