On a summer day in Southeast Alaska, a pod of humpback whales breaks the surface to the delight of a boatload of whale watchers. A crust of barnacles covers the whales' chins, and the edges of their big pectoral fins. These whales have hitch-hikers on board, a species of barnacle that only lives on humpback whales.
Whale barnacles are different than shoreline barnacles and are unique to the species of whale they piggyback on. One species lives only on humpbacks, and another only lives on gray whales. They colonize the skin of these filter-feeding whales in huge numbers - a humpback whale can host almost 1,000 pounds of barnacles. That's a lot of baggage, but relative to the humpback's 80,000-pound body, it's about as much extra weight as clothing on a person.
Barnacles begin their lives as free swimming planktonic larvae that settle onto a substrate and develop into the sturdy, immobile barnacles were familiar with. But how does a planktonic barnacle larvae hook up with a swimming whale? During the breeding season, when the whales mill around together in warm, shallow waters, the planktonic larvae picks up a chemical signal when the whale is close by. It swims over and hangs on. Once onboard, the larvae crawls to the head or fins, where the flow of water is consistent. Once they've settled on a location, the barnacle morphs into the immobile adult form.