Why Do Gray Whales Have Barnacles? Whale Hitchhikers Explained

Why do gray whales have so many barnacles?

Look at any gray whale and you will see crusty white and gray patches on its head, back and tail. Most of these are whale barnacles, along with clusters of tiny crustaceans called whale lice and the round white scars left where old barnacles have dropped off.

Gray whales make excellent homes for barnacles for several reasons. They swim slowly, usually about 5–8 km/h (3–5 mph), so larvae have a chance to settle and stay attached. They spend long periods in shallow, calm waters such as the breeding lagoons of Baja California, where barnacle larvae can be common.

Whale barnacles are not just any barnacles that happen to land on a whale. They belong to a special family, the Coronulidae, that lives only on whales. The gray whale’s main barnacle, Cryptolepas rhachianecti, is found almost exclusively on gray whales and is rarely recorded on any other host.

How do whale barnacles attach and feed?

A barnacle starts life as a free-swimming larva, part of the plankton. It molts several times and then turns into a stage called a cyprid, which does not feed and must find a home quickly. Laboratory experiments suggest that whale barnacle cyprids are triggered to settle by chemical signals from the skin of suitable whales.

Once attached, the young barnacle builds a crown-shaped shell of calcium carbonate plates. Gray whale barnacles sink deeply into the whale’s skin, which grows up around the shell and anchors it firmly against the rush of water. Each barnacle grows up to about 3.8 cm (1.5 in) across.

The barnacle feeds by sweeping feathery legs called cirri through the passing water to catch plankton, much as a whale filters food on a far larger scale. As the whale swims, it carries the barnacle through fresh water full of food. Whale barnacles live for up to about a year and then fall off, which is why gray whales are covered in white ring-shaped scars. Barnacles also need neighbors to breed, so they settle in dense clusters.

What are whale lice?

Whale lice are not lice at all. They are small crustaceans in the family Cyamidae, related to the skeleton shrimp, and usually 5–25 mm (0.2–1 in) long, with flat bodies and hooked legs for gripping skin. Gray whales host several kinds, including the gray whale louse, Cyamus scammoni.

Unlike barnacles, whale lice have no free-swimming stage. They spend their whole lives on whales and move from one whale to another only by direct contact, such as between a mother and her calf or during mating. They gather in places sheltered from the flow of water, including around barnacles, in skin folds, around the blowholes and in wounds, where they eat flaking skin, damaged tissue and algae. The pale orange and white patches on gray whales are often dense clusters of whale lice.

Do barnacles hurt gray whales?

Scientists usually describe the relationship as commensal: the barnacle benefits and the whale is not obviously harmed. The barnacles take no food from the whale, and healthy gray whales carry them throughout their lives.

That does not mean barnacles are free. A heavily encrusted adult gray whale may carry several hundred pounds, well over 100 kg, of barnacles and lice, which adds weight and drag. Barnacles may also irritate the skin. Gray whales have been seen rubbing their bodies against pebbly seabeds, possibly to scrape off barnacles and lice, and many gray whales carry fewer barnacles on the right side of the head, the side most of them scrape along the seafloor when feeding.

Whale lice are usually classed as external parasites, because they feed on the whale’s skin. In most cases they cause only minor skin damage and no serious illness.

Barnacles on humpback whales

The humpback whale is another slow-swimming, coastal whale with plenty of passengers. Its main barnacle, Coronula diadema, is a larger, crown-shaped species up to about 5 cm (2 in) tall that clusters on the head, chin, long flippers and tail flukes.

Some scientists think barnacle-studded flippers and heads may work like knuckle-dusters. Humpbacks use their flippers and tails to strike at rivals during competition for females and at orcas during attacks, and a covering of hard barnacles could make those blows more damaging. This idea is plausible but has not been tested directly. You can read more about humpbacks confronting orcas in Do humpback whales protect other animals?

How barnacle scars help scientists

Barnacles are useful to whale researchers. Each gray whale’s pattern of skin color, scars, barnacle clusters and lice patches is unique, so scientists photograph the back and sides of whales and match them with catalogs to recognize individuals year after year. This photo-identification has tracked some female gray whales in the Mexican lagoons for decades.

Barnacle shells hold another record. As a barnacle grows, its shell takes up chemical traces, such as forms of oxygen, that reflect the temperature of the water around it. By analyzing the layers of a shell, scientists can reconstruct the waters the host whale passed through, and the same method has been applied to fossil whale barnacles to study the migrations of whales that lived long ago. To see how far modern gray whales carry their barnacles each year, read How far do gray whales migrate?