Electroplankton is a music video game released for the Nintendo DS, in which you create music by touching things on the touch screen. In the US, it was released in limited number because it is not a mainstream game.
Along with not being considered a mainstream game, it's also commonly considered not to be a game at all, but rather a toy, as there is no real objective, and there is no way to actually lose (in fact, its probably even less of a game that Brain Age is, seeing as that game actually features an objective/time limit/and score).
The game, or rather "toy", is also oftentimes considered a demonstration of what the Nintendo DS hardware can do, as it takes advantage of most features, including the touch screen and microphone (with the latter one being featured more so than most other games that were released at the time).
There are ten different types of Plankton that you can touch that will produce different sounds and performances.
The Guinness World Records Gamer's Edition 2008 deemed Electroplankton the first game in which the player could compose their own music.
An Electroplankton themed stage (Hanenbow) appears in Super Smash Bros. Brawl and Super Smash Bros. Ultimate. If a character jumps on a leaf or touches the water, a sound will be produced. A character can also attack a leaf and the leaf's position will change as well as it's color and pitch.
There are separate mini games from the Electroplankton series. There are ten minigames released on the DSiWare at 200 Nintendo DSi points each. Five came out on November 9, 2009 and five more on November 23, 2009.