Remora and sharks have a commensalism relationship which, according to the Encyclopedia Britannica, is a relationship between two species in which one benefits from the other without either being harmed. Commonly called a suckerfish, the remora is a pelagic marine fish that attaches to the shark and feeds on it. Unlike parasitic species, the remora does not penetrate the flesh of the shark or otherwise cause it damage.
According to the Animal Diversity Web, an online publication by the University of Michigan, the remora requires the continual travel of the shark and cannot survive in still waters. There are several species of remora, and some have even been found living in the mouths of sharks. Though it is unknown if the sharks consider the remora as undesirable, no remora remains have ever been discovered in the digestive tract of a shark.
Ancient Greek and Roman civilizations believed the remora had magical powers, with the Romans attributing the death of Emperor Caligula to the fish holding back his ship and allowing it to be overtaken by the enemy. Today, remora are used by fishermen around the world as bait. They attach a line to a living species, and, once it has attached to a larger fish or turtle, the fishermen slowly pull in their catch.