This fish hitchhikes across the ocean using a suction disc on its head

A remora's first dorsal fin has evolved into a flat, ridged sucker that clamps onto sharks, whales, turtles and even boats, letting the fish travel for free and snack on the host's leftover scraps and parasites. Sliding backward tightens the suction; swimming forward releases it, so the remora can detach in an instant when a better ride comes along.

— Wikipedia contributors, Remora — Family Echeneidae