Eunice Aphroditois, The Bobbit Worm

Eunice aphroditois, the Bobbit worm, is an aquatic predatory polychaete worm dwelling at the ocean floor.
This organism buries its long body into an ocean bed composed of gravel, mud or corals, where it waits patiently for outside stimulus to reach one of its five antennae.
Armed with sharp teeth, it is known to attack with such speeds that its prey is sometimes sliced in half. Although the worm hunts for food, it is omnivorous. It is also covered in bristles that are capable of a sting that results in permanent numbness in humans.
Little is known about the sexual habits and life span of this worm, but researchers hypothesize that sexual reproduction occurs at an early stage, maybe even when the worm is about 100 millimetres (3.9 in) in length; this is very early, considering that these worms can grow to sizes of nearly 3 metres (9.8 ft) in some cases (although most observations point to a much lower average length of 1 metre (3 ft 3 in)) and an average of 25 millimetres (0.98 in) in diameter. A long lifespan may very well explain the size of these creatures.