Remora

Remora sometimes called a suckerfish or sharksucker, is any of a family (Echeneidae) of ray-finned fish in the order Carangiformes. They are small fish that live on and around sharks. They eat stray bits of food left by the shark and tiny shrimp-like parasites that live on the shark's skin. They have sucker-like disks on their heads with which they attach to the shark. 
Both the shark and the remora benefit from the pairing, but the remore benefits much more; this is commensalism. Remora are also known as sucker fish or shark sucker.