From the Super Mario Wiki
Urchins are spiked aquatic creatures that have appeared as enemies throughout the Mario series. As their name and appearance suggest, they are based on real-life Sea Urchins. They are slow, yet though enemies that Mario must avoid usually because they cannot be defeated by normal means, like a simple Stomp. Although they live mostly underwater, Urchins can also live on land.
 Super Mario series
 Super Mario World
In Super Mario World, Urchins move between tunnels in Forest of Illusion 2. Yoshi can't eat them, but he can bounce on them. They can be defeated by Grab Blocks. They however can't be defeated by using a fireball. Players must time their movements carefully as to avoid an Urchin's spikes.
 Super Mario Galaxy
Urchins appear in Super Mario Galaxy in the Beach Bowl Galaxy and the Deep Dark Galaxy. However, they do not swim around in the water; instead, they will stay on the sea bottom, and when they see Mario, they start chasing him by rolling towards him. Urchins also appear on land in the Freezeflame Galaxy; they act exactly the same as they do underwater. There are also red Urchins to go along with the fiery 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 Fire Luigi. The underwater versions can be destroyed with a shell. They release six Star Bits when defeated.
 New Super Mario Bros. Wii
Urchins appear in New Super Mario Bros. Wii along with a bigger version of them called Mega Urchins. Urchins start floating up and down like their Super Mario World incarnation, but when frozen with an Ice Flower or Penguin Suit they will float briefly on the water before breaking free and getting knocked out underwater permanently. In this state, the Urchin can block underwater Warp Pipes that spray out jet-streams, which can be used to the player's advantage or disadvantage, depending on the situation. They could also be defeated with a fireball, a Koopa Shell or a Starman. Mega Urchins can only be defeated with a Starman. In the game, when there is a "bah" in the background music, Urchins will breifly open thier mouths, releasing bubbles, as if they were saying "bah" underwater.
 Super Mario Galaxy 2
Urchins reappear in Super Mario Galaxy 2, behaving the same way as in the predecessor. They appear in the Starshine Beach Galaxy and in the Slimy Spring Galaxy. Red Urchins also appear in this game in the Battle Belt Galaxy.
 New Super Mario Bros. 2
Urchins also appear in New Super Mario Bros. 2, where they behave the same way they did in New Super Mario Bros. Wii, and have the same appearance. Mini Urchins also appear, acting exactly like their bigger versions, only standing still.
 New Super Mario Bros. U
Urchins also make an appearance in the Wii U game New Super Mario Bros. U. In one of the Sparkling Waters levels, Urchins are placed over water geysers used to rise them to the platform surface. In the level Urchin Shoals, Urchins and their big relatives are the main features.
 Super Mario World TV series
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.
 Nintendo Adventure Books
 Super Mario Adventures
 Yoshi Touch & Go
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. In the demo, they float in the skies and take one of Baby Mario's balloons away if he touches one of them. They can 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.
 Super Princess Peach
Urchins also make an appearance in Super Princess Peach. They appear in the Subrella sections of the game. They move back and forth, and cannot be defeated.
 Mario Party 9
In Mario Party 9, Urchins appear in the minigame Pier Pressure. The ten fishing rods either have Cheep-Cheeps or Urchins at the end of them; if the player ends up choosing one with an Urchin on it, he or she is eliminated.
Blooper also tosses Urchins at the players in its boss minigame, Blooper Barrage. They target a specific player, and their course can be redirected by hitting it with a cannonball. Getting hit by an Urchin results in that player losing one point; hitting it right before it is about to hit the player sends in into the sea below. Additionally, in the minigame Bowser's Block Battle, if Blooper appears on the stage, it attacks by throwing Urchins at the players.
 Super Mario Galaxy Trading Card Description
Urchins are spiky blue fish that Mario must face in an underwater challenge in the Beach Bowl Galaxy and a land mission in the Freezeflame Galaxy. They roll along the seafloor or ground trying to poke Mario with their spikes. There are also red Urchins in lava areas that Mario can hit with a fireball or trick into falling into the lava.
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