From the Super Mario Wiki
Crusty is a powerful crab-like crustacean that lives underwater, typically in the waters in and around Jonathan Jones' Sunken Ship during the events of Super Mario RPG: Legend of the Seven Stars. Crustys resemble another spiked monster, the Spikey, but the two are unrelated. A Crusty actually has a pink exoskeleton; however, since they can only be found underwater, they appear blue and their real coloration is never seen.
Crustys are very powerful monsters but are also very rare; they can only be found aiding other types of underwater monsters in battle. These monstrous crustaceans have high physical attack and high physical defense. A Crusty uses its massive pincers to inflict severe damage to an opponent 2/3 of the time, or fires a light bubble 1/3 of the time that does no damage, but with the effect of inflicting Silence. Crustys do not have any magic.
Although Crustys have high physical defense, their magical defense is low, and they are weak to Fire and Thunder. As such, Mario and Mallow can dispose of them fairly easily, especially once the latter has learned Shocker. Crustys are related to the more powerful Star Crusters.
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