What is a crestation?

'Crestation' is not a Word.

However, I believe you meant 'crustacean' as they both sound the same. A crustacean itself is any chiefly aquatic arthropod of the class Crustacea. Crustacea specifically means "having a crust or shell."

In simpler terms, it's an aquatic animal typically with a body covered in a hard shell or crust (a helpful way to remember this may be arthropods with crusts are called crustacean. Examples include lobsters, shrimps, crabs, barnacles, woodlice, etc.