Remoras also known as suckerfish

Remoras are the fish you see clinging to sharks, whales or turtles. They do this with their dorsal fins that have been evolved into a sucker-like organ allowing them to attach themselves to flat services. They'll spawn on sea turtles, and on whale sharks (see post: whale sharks), and rarely by themselves. They will detach themselves from sea turtles that goes on land. If a player is wearing a chest plate the remoras would have no problem attaching themselves to the back of the player. The only ways to get a remora off is by going on land, taking off the chest plate, or have another player or mob damage the remora. If a remora is fed rotten meat, it becomes friendly. A friendly remora will only detach itself if the chest plate is taken off. It'll even stick with you if you fall into lava. Status effects will last twice as long to the remoras than other mobs. So, if you throw an invisibility II splash potion on them, the effect will last 12 min instead of 6 min. That might seem useless, but the remoras share their status effects, because they are so kind <3 If the remora on your back has water breathing II status effect, you'll also have that water breathing II status effect. One problem though the remoras can't breath on land. So, if you surface with a friendly one your back it'll take damage after some time. 

They can also attach themselves to the bottom of boats, and swim sideways and upside down if they want to :)