Hermit Crabs
Hermit crabs live inside empty snail shells in shallow water along beaches and in estuaries, small specimens on mudflats and large ones offshore. Some hermit crabs are entirely terrestrial, needing the water only to lay eggs. In the South Pacific, there are types that actually climb trees and very large ones that don't bother with a shell as adults.
The size of the crab determines what kind of shell, and upgrades are required as the crab grows. The Flat-Clawed Hermit Crab Pagurus arcuatus (right) is the largest in our area, and will often use Moon Snails and Whelks, but you will only find the big ones in deeper water. Small ones use Periwinkles and Oyster Drills.
Like good suburbanites, Hermit Crabs are always on the lookout for bigger and better shells, and when they find one they like, a quick switch is in order to see how it fits. They will also try to steal good shells from each other. This makes them very amusing to watch in an aquarium. Snail Fur is a type of hydroid that lives only on Hermit Crab shells, you could call it "crabgrass".
Hermit Crabs are not true crabs but are an intermediate form having a soft shrimp-like tail that is protected inside the snail shell. The four hind legs that appear to be missing are evolved into grippers that hold the crab inside the shell so strongly that you will tear the animal in half before you can extract it. The claws are usually shaped so that together they will exactly block off the opening when the crab retreats inside.
