From the Super Mario Wiki
Spike Eels are enemies found in Super Mario 3D Land. They are purple colored eels with purple spikes on their back and three big yellow spikes on their head, that slightly resemble hair. They have a big mouth, a white lip and a single sharp tooth.
Spike Eels appear only in underwater tropical levels, notably in World 7-1, World 3-2 and Special World 2-2. They stay in holes and swim in Mario's way, not unlike Unagis and Gringills. They take 15 hits to defeat with the Fire Flower or Boomerang Flower, but can be defeated in a single hit with invincibility or by transforming into Statue Mario and crushing them. They can also be attacked from the front or behind. Their tails are also spiked.
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