Remora

The remora's sucker is a modified first dorsal fin sitting flat on top of its head. It is an oval disc divided by a row of movable, blade-like ridges called lamellae, which are evolutionarily the spines of the fin laid over sideways. To attach, the remora presses the rim of the disc against the host to make a seal, then tilts the ridges upward, expanding the space under the disc and creating negative pressure — suction. Tiny spinules on the ridges grip the host's skin as well. The design is directional — pulling the remora backward (the way drag acts as the host swims) makes the ridges dig in and the grip tighten, while sliding it forward releases it easily, which is how a remora detaches itself and how you would remove one by hand.

Mostly not, and the relationship is usually described as commensalism (one benefits, the other is unaffected) shading into mutualism (both benefit). The remora gains transport, a bow-wave of protection from its large host, and food. The host gets some cleaning — remoras eat parasitic crustaceans off its skin, gills, and mouth — which is a genuine benefit. The downsides for the host are minor — added drag, the disc can leave a faint mark or slight abrasion, and a heavy load of remoras is a mild irritation, which is why manta rays and sharks sometimes breach or visit cleaning stations partly to dislodge them. Remoras that ride inside the mouths or gill chambers of mantas and large fish are more of a nuisance but still not seriously damaging.

They are often confused because both are small fish that travel with sharks, but they are not related and behave differently. A remora (family Echeneidae) physically attaches to the host with its head sucker and rides along. A pilot fish (Naucrates ductor) is a member of the jack family that swims freely alongside a shark, ray, or turtle, staying in the pressure wave around the host's body to save energy and darting out to grab scraps. Pilot fish never attach. The old sailors' belief that pilot fish "lead" sharks to prey is a myth; they simply shadow a moving host.

Yes. Divers and snorkellers in areas with lots of remoras, especially around large rays or whale sharks, are sometimes approached by a remora that attaches to a wetsuit, a leg, or the hull of a boat. It does not bite and cannot break the skin with the disc, so it is harmless, though the suction and the rasp of the ridges can feel startling and leave a temporary red mark. The remora is simply mistaking the diver for a passing host; it can be removed painlessly by sliding it forward toward its head, in the direction that disengages the ridges.