Sometimes symbionts benefit at the expense of the host, this is called as parasitism. The giant tapeworms that derive food and shelter by the guts of whales are considered parasites because they may weaken their whale hosts.

Marine parasites are very common. Few marine species escape having at least one kind of parasite.Some barnacles live on the skin of whales. They do not affect their host at all, most such barnacles obtain shelter, food, and some other benefit without affecting the whales one way or the other.