The symbiotic relationship between a barnacle and a whale is known as commensalism, which describes an inter-species interaction that benefits at least one participant. In the case of whales and barnacles, the barnacles are the ones that derive true benefit from the relationship. The whales, on the other hand, are largely unaffected by their non-harmful parasites.
Whales do not gain a significant benefit from the presence of barnacles on their bodies. However, their presence is not especially damaging or harmful to the whales either. Because of the size and strength of whales, they are able to navigate the ocean waters with little impediment from the added weight and resistance. In some cases, whales can even use a heavy layer of barnacles as a plate of armor against attacks from other whales.
Barnacles, on the other hand, gain a tremendous advantage from their partnership with whales. By attaching themselves to the belly, back and sides of whales, they enable themselves to hitch a free ride through nutrient-rich water environments. This allows barnacles to consume micro-organisms that float freely in the water, which would otherwise be inaccessible to the barnacles.
Barnacles begin their lives as free-floating larvae, and they transform into fully developed barnacles only after they have implanted onto a host (the skin of a whale). This symbiotic relationship has helped barnacles continue to survive and thrive as one of the Earth's oldest living animals.