Pam, author of Tortoise Trails and possessor of bird feeders of which I am very very jealous, asked in relation to a recent post on turtles about 2 fish that look like sharks hitching on the turtles. Well these chaps are remora. There are a number of species around the world which range from cute, stripey juveniles living in whale sharks' ears to the big turtle sitting efforts in that post.
Above is a picture of an unattached remora (its blue because the flash didn't fire in the shallows) showing the "sucker" on top of its head. Those on top of a turtle are upside down. You can see the sucker is made up of various ridges which by manipulation can create an addhesive suction. You also often see unattached remora swimming alongside their host taking advantage of the aerodynamic (well hydrodynamic) draught (just like F1 or NASCAR drivers). They then fully detach and pick up scraps when the host feeds - you can see this behaviour in my turtle feeding frenzy post.
Here's a small remora hitching on a carribean reef shark's chin. As my dad always says if I was a shark I'd eat those free-loading little so and so's.