As nouns the difference between crustacean and alga

is that crustacean is any arthropod of the subphylum Crustacea, including lobsters, crabs, shrimp, barnacles and woodlice while alga is any of many aquatic photosynthetic organisms, whose size ranges from a single cell to giant kelps and whose form is very diverse; some are eukaryotic and some prokaryotic; includes the seaweeds.