biomedicalephemera:

Chelys fimbriata [now Chelus fimbriatus] - Mata mata

Some people say the Mata mata turtle looks like it’s smiling, because of its unusually-shaped mouth. The Indigenous South American nickname for the turtle, “matamata”, means “I kill”, according to Fritz Jurgen Obst. Whether that eponym meant that the turtle was good to kill and eat or the turtle killed a lot is unknown. The relatively large size and a meat quality similar to the Alligator Snapping Turtle makes the former meaning more probable.

In the wild, Chelus fimbriatus lives in stagnant waters, blackpools, and muddy streams around the Amazon rainforest. Its fringed neck and murky coloration, combined with algae that grows on its carapace, makes this turtle an excellent ambush hunter. When fish come near it, the mouth opens up, and the mata mata “vacuums” them in. This is in contrast to Alligator Snapping Turtles, which are similar ambush predators, but with a different strategy. The tongue of the snapping turtle acts as a lure, and unsuspecting fish swim right into its mouth.

Transactions of the Zoological Society of London, 1885.