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I have always liked looking at underwater ambush predators like wobbegongs, wolf fish, stargazers, and frogfish. There is something appealing about the way that a pile of pebbles will suddenly resolve into a wide toothy mouth with little pebble eyes. In freshwater environments from Nepal down through India, Myanmar, Bangladesh, and into Indonesia, the niche for highly camouflaged bottom predators is filled by Chaco Chaco, the frogmouth catfish.
The frogmouth catfish grows to a size of 8 inches (20 centimeters) and feeds on tiny shrimp and minnows. The animal is mostly head and can swallow creatures nearly as large as itself. Although the frogmouth catfish is already camouflaged, it prefers to bury itself in the mud or sand so that only its tiny black eyes protrude from the substrate.
The catfish are extremely committed to their camouflage and even when captured they will not move (although they growl menacingly when removed from the water). When kept in aquariums they rapidly raise the acidity of the tank which suggests that they either have potent gastric juices (for digesting large prey) or they might emit an unpleasant taste. Aquarium keepers sometimes keep the little predatory catfish in tanks which appear to be empty except for a blank spot of mud. The frogmouth catfish will only accept live food. Here is a video of one eating some shrimp!