At over 10 feet long, bobbit worms are the size of most snakes. They have poisonous bristles up and down their body that can cause significant nerve damage to anyone who dares touch them, and they eat by grabbing their victims with enormous strong mouths and sucking them down into the sand.
They are ambush hunters that wait in shallow waters with only about a tenth of their body poking out of the sand and their mouths stretched completely open. As soon as one of their antennae detects something in the water, they leap for it, even if it's significantly bigger than they are. Their bites, it should be noted, are so powerful that they'll sometimes accidentally just cut fish in half while trying to grab them.