A species of Asian crab which can cause huge ecological damage has been found in the Dee estuary.
Chinese mitten crabs burrow into riverbanks, causing them to collapse, and pose a threat to native species.
The crabs - which are a delicacy in Asia - can also carry a parasitic lung fluke which can cause infection to humans via un-cooked meat.
Environment Agency Wales are monitoring the crabs and have asked anyone who spots one to call 01248 484076.
The crabs can grow to at least 80mm and live for five years.
They are well-established in London and also northwards to the River Tyne and on the south coast, westwards to the Teign.
The crabs are thought to have been brought to the UK in ballast tanks of commercial ships.
Chinese mitten crabs can cause extensive environmental damage
They may have entered the Dee after spreading from the Mersey estuary.
Huw Jones, of Environment Agency Wales said: "The appearance of this species is of concern to Environment Agency Wales and we will be working closely with partner organisations to monitor their spread."
Earlier this year, the Environment Agency warned anglers to report any sightings of the destructive Zebra mussel, which was spotted in a pool at Johnstown, near Wrexham.