Zebra Mussels slow up boat movement

These little freshwater devils, the size of my fingernail, can grow up to 2" long.  They attach themselves to the bottom of boats, water outlets, buoys even other varieties of freshwater mussels and starving them.  The ones in Bristol Harbour came in with ships from Russia carrying wood from inland waterways - in the bilges probably.  They can spawn over a million eggs a year!  Adults can even survive out of water for about 7 days but when they are scraped off  the bottom of boats here they are usually pushed back into the Harbour because the smell of dead mussels is unbelievably awful.  

https://en.wikipedia.org/wiki/Zebra_mussel  will give you the low down on this pest for in the Great Lakes in America that is certainly what they have become