From the Super Mario Wiki
Crabbers are a type of Crab found in beach galaxies of Super Mario Galaxy and Super Mario Galaxy 2. They are shown to have a big left pincer and a small right one. Mario has to defeat the creature by Spinning at its rear. This is a little tricky, as Crabbers always face Mario. So, Mario first has to spin in its face, or Jump on it. The Crabber will defend itself with its big pincer. When the Crabber defends its face, it doesn't follow Mario, so he can Spin at its exposed backside to defeat it.
Crabbers come in two colors: red and cyan. Red Crabbers are less skittish than blue ones, so they are easier to defeat, as they come closer to hurt Mario, and also leave Star Bits behind when defeated. Cyan Crabbers are harder to defeat, They fear when Mario or Luigi is near them and run away, and leave a 1-Up Mushroom behind when defeated.
Crabbers are found in galaxies with a beach, such as Deep Dark Galaxy. They walk sideways like normal crabs, and try to defend their rear by facing Mario when close.
Crabbers reappear once again, in some beach galaxies such as Starshine Beach Galaxy. Yoshi also appears in this galaxy, and can stuff Crabbers in his mouth. Yoshi cannot convert them into Star Bits, but can spit out their big pincer in the same way as a Spiny Egg or Bullet Bill. The pincer acts as a boomerang, as it flies out for some distance, then returns.
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