Butis butis, the crazy fish, duckbill sleeper, or upside-down sleeper, Ikan Ubi is a species of sleeper goby that are native to brackish and freshwater coastal habitats of the Indian Ocean and the western Pacific Ocean from the African coast to the islands of Fiji. They prefer well-vegetated waters and can frequently be found in mangrove swamps. They are small, drably-colored fish, reaching a maximum length of only 15 cm (5.9 in). They are predatory and are known for their behavior of swimming vertically – or even upside down – while hunting.

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