Do you see that pink wiggly thing in the turtle's mouth? It looks just like a worm, doesn't it? Well, it's actually part of the turtle! The alligator snapping turtle has a vermiform (worm-shaped) appendage on the end of his tongue, which means he never has to run to the bait shop or sporting goods store when he wants to catch something to eat. All he has to do is find a comfy spot on the river bottom, open his mouth, and sit as still as possible. The fake worm coupled with the cave-like appearance of the turtle's head is enough to draw curious and hungry fish. Then, as you might imagine, the fish becomes the meal.
This is an example of aggressive mimicry (aka Pekhamian mimicry, named after George and Elizabeth Pekham). Basically, this means a predator appears to be something else -- something harmless -- so that the prey doesn't recognize it as a threat. This way, the predator doesn't have to do any actual hunting. He just waits for the prey to come along. A lure isn't required for an animal to be considered an aggressive mimic - the ability to closely approach prey is enough - but the lure definitely makes things easier for the turtle.
Note: A human going fishing is a different situation. For something to be considered an aggressive mimic, the mimicry cannot be intentional. In other words, the turtle was born to fish. He naturally has all the equipment he needs. A human has to go out and buy rods, reels and bait.