Why are the South American matamata and alligator snapper turtle unusual feeders?

The South American matamata is a weird looking fish that lives at the bottom of rivers in South America. Its body is covered with protuberances and algae, and it looks like a pile of debris at the bottom of the river. Smaller fish are fooled by its appearance, and as they come to nibble at the protuberances, the matamata opens its huge mouth wide, and sucks in a flood of water along with the fish!

 The alligator snapper turtle too, lies in murky water, looking like a pile of mud. It lies with its jaws wide open, and there is a structure inside its mouth that looks like the two ends of an earthworm. Fish are fooled into thinking that this is a tasty morsel of food. As they approach the turtle, the turtle snaps its jaws shut, and captures its prey!