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. This monster has spikes on its back as well as two glowing eyes hidden by its exoskeleton. As such, Crustys resemble another spiked monster, the Spikey. Crustys are large, have six legs and two enormous pincers.
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; the latter provided by their tough exoskeletons. A Crusty has a pink exoskeleton; however, since they can only be found underwater, their exoskeletons appear to be blue. In fight, a Crusty can use its massive pincers to inflict severe damage to an opponent. However, Crustys cannot use any magic, so they have a relatively low special attack.
Although Crustys have high physical defense, their magical defense is low. As such, Crustys have compensated for this weakness by developing the ability to shoot special light bubbles. These bubbles infect an attacker with the Mute status ailment. When Muted, an attacker will be unable to use magic, giving the Crusty the upper hand (or pincer, in this case). Crustys are related to the more powerful Star Crusters.
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