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A gruesome list of the 10 worst wildlife invaders has been revealed by the this week - the natural world's version of the '10 Most Wanted'. They've concentrated on the species threatening Britain's waterways and top of the list is the so-called 'killer shrimp', which has been discovered in three UK locations - two of them in Wales!

The tiny invader attacks native species like shrimp, young fish and insect larvae and has been known to destroy entire species to the point of extinction. It's been found in Cardiff Bay, at the in Port Talbot and at Grafham Water in Cambridgeshire.

We featured an item on the deadly shrimp on Country Focus earlier this year; I went to meet some of the Environment Agency's staff at their office in Cardiff Bay and they had an example of the shrimp in a test tube - it was tiny! Hard to believe something so small can cause so much havoc and has now been declared aquatic wildlife's 'public enemy number one'. I can remember the staff telling me that calling it a 'killer shrimp' was a bit misleading, because there's no risk to human health or water quality, but it's certainly one way to grab the headlines.

There's even a 'Killer Shrimp Wanted Poster' which has been issued by the very official sounding - to help people identify the rogue, and anyone who finds one is asked to email a photo to the for confirmation. People who use the waterways like sailors, rowers and anglers, are being asked to increase biosecurity measures - the mantra is 'Check, Clean, Dry' - to make sure all equipment is clean and dry before you leave the waterway, to prevent further spread.

But back to the 'Top Ten' and the worst wildlife invaders are costing around £1.7bn a year to tackle, according to the Environment Agency. That's a lot of money. When you read the list, many of the entries, like water primrose (straight in at number two) - don't sound too dangerous but then again, I'm not sure you'd want to get mixed up with topmouth gudgeon or giant hogweed. Here's the complete list:

1. Killer Shrimp
2. Water Primrose
3. Floating pennywort
4. American crayfish
5. Topmouth gudgeon
6. Giant hogweed
7. Japanese knotweed
8. Himalayan balsam
9. Mink
10. Parrot's feather

So is it safe to go back in the water?