Urchin

The Urchin is a slow, blue sea urchin that moves between tunnels in the underwater levels of Super Mario World. They are invincible, and Yoshi can't eat them, but he can step on them. Players must time their movements carefully as to avoid an Urchin's spikes.

Several Urchins make a small appearance in the Super Mario World episode "Mama Luigi". In the episode, Luigi and Yoshi, while being chased through an underwater tunnel by a Rip Van Fish, almost swim directly into several Urchins. Luckily, Luigi and Yoshi manage to avoid bumping into these Urchins.

In Nintendo Power's Super Mario Adventures comic, several Urchins are seen to act as guards in the moat surrounding the Koopalings tower.

The Urchins in Super Mario World: Super Mario Advance 2 can get killed by Luigi riding on Yoshi and swallowing a Blurp and spitting it at the urchin.

Though they were seemingly going to make an unexpected return in Yoshi Touch & Go, Urchins were not seen in the final game. They were, however, featured in the Balloon Trip tech demo that can be thought of as a Yoshi Touch & Go beta, where they would (perhaps strangely) float in the skies and take one of Baby Mario's balloons away if he was to touch one of them; they could also never be trapped in bubbles, making them some of the more challenging enemies featured in the demo. By the final game's release, these Urchins were redesigned as the plant-like Briers.

Urchins also make an appearance in Super Princess Peach. They appeared in the Subrella sections of the game. They would move back and forth, and could not be defeated.

Urchins, or at least creatures that look identical to Urchins, appear in Super Mario Galaxy in the Beach Bowl Galaxy. They, however, unlike Urchins do not swim around in the water and stay on the sea bottom. Urchins also appear on land, in the Freezeflame Galaxy. They will act exactly the same as they do underwater. There are also red Urchins, to go along with the fire area of the stage, they will act exactly the same as the normal Urchins, and can be killed if the player tricks them into entering the lava or if they are hit by a fireball shot by Fire Mario or Luigi.

Trivia

 * The Sammer Guy "Urchin Lung" from Super Paper Mario may be a reference to an Urchin.
 * Hacking into Super Mario RPG: Legend of the Seven Stars reveals that game was initially planned to feature what would appear to be a tiny Urchin, likely as some kind of item.