Nuts and Bolts: the cutest kind of shark.

Blotched catshark Scyliorhynus meadi

Everyone knows that sharks are terrifyingly good at smelling blood but, well, blood isn't everything. Not even if you're a shark.

A study of catsharks has discovered that some deep-water species glow. 

Why?

Well, some lizardfish and gobies may glow so they can recognise another of their own species. The medusa's tentacles glow so it can lure young rockfish to their doom - and the mantis shrimp uses its biofluorescence to make its signals easier to see.

But what of the catsharks? Well, it seems that while catsharks can see their own biofluorescence, their prey can't, so they're not using it to lure anything (if any creature is quite dumb enough to be lured towards a shark). Most other sharks can't see the biofuorescence, either, so it looks as if the catsharks are using their biofluorescence to talk to each other.

A female shark's glowing pattern, for instance, is different from the males', and that must avoid a lot of embarrassment. There are different patterns for each species, too, which must help stop things getting really weird.

The fact that some types of fish glow in the dark was discovered by accident when an eel photo-bombed Dr David Gruber when he was studying corals. Since then he has been working with fish expert John Sparks to try to work out what the glowing is saying.

Now Dr Gruber wants to investigate more marine animals. 'What I am really hoping is that it will draw us closer to these relatives. We emerged from the sea 200 million years ago...it has not been that long.'

I've never thought much about my relationship with sharks, but now Dr Gruber comes to mention it, we humans rely on a healthy glow to signal health and youth and beauty, too, don't we?

You know something? I feel even more nervous about going into the sea, now...

Word To Use Today: shark. This word comes arrived in English in the 1565 with Captain Hawkins' second expedition, but sadly no one knows where it came from. There's a chance, though, that it might be from the Mayan word for shark, xoc.