Slipper limpets growing on top of a mussel shell
Mussel farmers in the Menai Strait between Anglesey and Gwynedd have been issued with 'pest recognition' cards to keep out foreign shell fish.
The 'wanted' cards are part of a good practice code developed by the Countryside Council for Wales (CCW).
Unwanted invaders include American slipper limpets, which breed rampantly and can swamp native mussels.
Other species mussel farmers are being given help in spotting are the violet sea squirt and Chinese mitten crab.
In the past, foreign species have been introduced by mistake when juvenile shellfish are imported by the mussel industry.
Violet sea squirt and Carpet sea squirt (Japan) Can form sheet-like growths across the seabed smothering other life forms, competing for space and food
Solitary sea squirt (South America, Australia, South Africa) More sheet-like growths across the seabed
Slipper limpet (Canada and North America) Can rapidly cover seabed habitats.
American jack knife clam (North America) Highly invasive and forms dense colonies.
Chinese mitten crab (Russian Far East, South China) Very damaging to rivers and estuaries, the adults burrow into riverbanks, causing them to collapse, so increasing flood risk.
Veined rapa whelk (Japan and South China Sea) Major predator of commercial shellfish.
Wakame (Japan, China and Korea) Seaweed which replaces native types on rocky reefs.
Source: Countryside Council for Wales
Kate Smith, the CCW marine conservation officer, said a survey had suggested work to eradicate non-native species such as slipper limpets from the strait had been successful.
"This is hailed as one of the very few examples of a successful eradication of non-native species in the marine environment in the UK," she said.
"But prevention is better - and cheaper - than cure," she added.
Ms Smith said the code was very practical in its approach as it helped mussel farmers to spot the signs of invasive marine non-native species.
It would also help them to avoid accidentally bringing them into one of Europe's most cherished marine sites, she added.
The list features eight invasive non-native marine species for the mussel farmers to look out for, and also identifies red, amber and green zones where measures will apply.
Stephen Atkins from the North Western and North Wales Sea Fisheries Committee said he welcomed the introduction of the code.
"The committee is pleased to see the mussel industry's commitment to work within the standards it sets.
"Neither the Sea Fisheries Committee nor the Government presently have the legal powers to properly regulate the importation of mussel seed but this voluntary code will fill the gap until the measures in the proposed Marine Bill come into force," he said.