They are overfed, oversexed and over here.

More than 7,000 giant American bullfrogs have been found living in a pond in East Sussex.

The streetwise amphibians can grow to twice the size of normal frogs and elsewhere they have been known to eat fish, birds and even baby alligators.

Following the discovery at an undisclosed location, English Nature has put out wanted posters and a tape recording of the bullfrog's distinctive call.

Amphibian expert Jim Foster said: "We don't know how they got there. It seems someone may have got some tadpoles from a garden centre and released them.

"The owner just found them in his lake. He wasn't very happy at all because it meant quite a disturbance to his own wildlife.

"They could cause quite a lot of damage if they spread. We are very keen to trace them.

"I'm not sure what attracted them to East Sussex to breed."

English Nature is currently removing the bullfrog colony, mostly made up of tadpoles and froglets, from the pond.

Hundreds are thought to remain.

Bullfrogs, which take up to three years to reach maturity, grow to 8in in length and can cover a distance of almost two miles across land.

Mr Foster said: "They are not very fussy about what they eat.

"They mainly consume invertebrates but some times they enjoy fish, small birds and there are reports of them devouring baby alligators in the American swamps."

The male bullfrog has distinctly large ears and emits its own version of the "yo" greeting used in American rap.

Mr Foster said: "I suppose it's a way of getting the females round to its pad.

"It's extremely loud. When you get a lot of them together you can hear them hundreds of metres away.

"They are a worry. We would like to hear from people who think they have seen one."

The bullfrog, previously imported as a pet, was banned from Britain in 1997 because of concerns over its potential impact on wildlife.

The Sussex bullfrogs have been put to sleep.

An English Nature spokesman said: "It was all done humanely. Sadly, the numbers were so great we had no other option."

English Nature has compiled a tape recording of the bullfrog's call for worried pond-owners.

If you think you have found one or want a copy of the tape, call 01733 455101.