Crabs and Lobsters

Crabs and Lobsters are both large, hard-shelled crustaceans with big powerful snapping claws that are unable to speak and live on the beach and in water. They are hostile to woodlanders and vermin alike; mindless beasts much like swans. None have ever been seen slayed in the Redwall series.

Mossflower
In Mossflower, a blue-black Crab attacks Martin and his friends, and is cleverly "defeated" by Gonff, who puts a staff of wood in between its pincers and then dances with it!

Mariel of Redwall
In Mariel of Redwall, a monstrous Lobster resides in a pool by Bobbo's house, where Mariel] needs to retrieve a precious metal swallow. She and Dandin go down to get it, but then they are assaulted by said Lobster. When the two mice distracted the brute, Boobo's pet, Firl swam down and snagged the bird, and they all escaped safely.

Lord Brocktree
A sizable group of Spider Crabs resided beneath the tunnels of Salamandastron, and ate the stoat Fraul.

Books
Crabs/Lobsters appear in Mossflower, Mariel of Redwall and Lord Brocktree.