Remora

The remora is a slender fish, family Echeneidae, best known for the flattened, ridged disc on top of its head, a modified dorsal fin that acts as a powerful suction pad. It uses this disc to attach itself to sharks, rays, sea turtles, and other large marine animals, hitching a ride and feeding on scraps left by its host or on parasites picked from the host's skin. The relationship is generally considered harmless to the host, making it a well known example of commensalism in the ocean.