From the book "Straight to the South", part 2 "Shark archipelago", episode 12.
At night they set the lamp on the foredeck of our ship and we can see as trifle fish and plankton curl in its light attracting those who eat them. And here you are, we've been woken by a knock at the cabin door at 3am: "Shark!". Indeed, here it is, at arm's length from the boat.. Young, a little over three meters in length, but the real whale shark!
It realy covered with suckerfish, dorsal fin of which is transformed into the suction сup, consisting of thin plates, resembling the stripes on the shutters. When a fish swims independently, these plates laid flat. Attached to any surface, the suckerfish reduces the muscles in the suction cup, as a result of which the bands are rotated upwards, and a chamber with airless space (partial vacuum) will formed between them, which provides a secure joint. Suckerfish (lat. Echeneidae) can not independently adjust the depth of immersion, they do not have a swimming bubble. But they don't need it, they travel on seas and oceans on large turtles, sharks, whales and nautical courts with comfort and without special energy costs. They feed on scraps of host's food and enjoyed his patronage so they constantly fed and safe. Also they clean the host - they eat parasites, removing them from the skin, from gill slits and sometimes from the mouth of the shark. Here is such useful symbiosis :)