The brains behind Star Trek and Star Wars have some strange and bamboozling creatures to their credit, but if you take a good look at Earth, you'll find just as many startling and bizarre life-forms¹.

One such specimen took a London epicurean by surprise in 2005, when they took a look inside the mouth of the lovely, shiny red snapper they'd picked up at the fishmonger's. What they found – an ugly little beastie, related to the woodlouse – presumably knocked dinner right off the menu and sent them straight to the council's environmental health officer.

Inside the snapper's mouth, there lay nestled a nice, plump example of Cymothoa exigua  – the tongue-eating louse, for those non-fluent in Linnaean.

Parasitism is nothing unusual in the animal kingdom. Dog owners routinely treat their hairy chums to prevent worms and ticks, for example, and every school nurse must have more than a nodding acquaintance with head-lice. But the tongue-eating louse is a parasite with a difference. It's the only known instance where a parasite devours the organ of its host, and considerately goes on to become a replacement for the organ it destroyed.

The host fish respires, as most do, by taking water in through its mouth and expelling it through its gills, extracting dissolved oxygen from the water as it does so. It's at this point that our little friend finds the breach in the walls. C exigua enters along with the water. But rather than leaving the fish's body in the same way, it ends up clamping itself onto the tongue. There it stays, feeding on the blood it draws from the tongue, and growing plump and fat – it can grow almost as thick as a man's finger and up to about an inch (25mm) long.

With the host's tongue deprived of blood, the lack of oxygen and nutrients inevitably takes its toll. Something's got to give, and the tongue eventually shrivels away. Far from being the end of the story, that's where it starts to get interesting. With the fish's tongue destroyed, you might expect our little louse to make good its escape, to leave for pastures new and tongues afresh. Not a bit of it. This is a pest with a bit more staying power. With the withered tongue now useful to neither fish nor louse, C exigua remains clamped on to the muscles at the base of the tongue, continuing to draw blood. The fish seems to suffer no serious hardship as a result of its little lodger – in fact, the creature replicates the functions of the organ it destroyed. Effectively, it becomes a whole new tongue. The straightforward parasitic relationship becomes an odd kind of symbiosis, continuing 'till death us do part'.

C exigua is just one example of the vast variety of life that surrounds us. We're oblivious to most of it – perhaps that's no bad thing if we want to rest easy at night, unburdened by dreams of the beasts all around us. But let no one be over-fearful of London fishmongers; the tongue-eating louse is a native of much warmer, tropical climes. The one that washed up there was a more intrepid voyager than most. But if you do chance to stumble upon one, don't worry. Unless you're the tongue of a red snapper, the worst it will do is give you a nasty nip.