But don't worry, they are of the marine variety.

Angler Roger Tipple was digging for lugworms when he came across a heavily egg-laden Chinese mitten crab, which is now under guard in solitary confinement at the Hunstanton Sea Life Sanctuary.

An invader from the Yellow Sea, the crabs have been a major concern for UK environmentalists since numbers began to multiply in the River Thames in the early 1990s.

Having since colonised the Humber, Medway, Tyne and Wharf, they were discovered lurking in The Ouse in 2005.

Sea Life marine expert Mr Ben Stone said: "They are a problem because they burrow into river banks causing landslides and erosion."

Mr Stone had just completed a two-day course studying marine aliens at the neighbouring Yarmouth Sea Life Centre when the crab was found and Mr Tipple's alarming discovery raises further concerns about the mitten crab's ability to spread.

Mr Stone added: "The Chinese mitten crab, so called because they have furry front claws, was one of the first things we learned about.

"They live in freshwater but can migrate downstream to estuaries in the spring to breed. Clearly they are moving along the coast too and the larvae from their eggs could end up in more British rivers."

Staff at the sanctuary have notified the Environment Agency and are keeping the crab in an isolated display tank to avoid any hatched larvae entering the main water circulation system and potentially going back out to sea.