Barnacles & Whales

Whale barnacles are specialized crustaceans in the order Sessilia that have adapted to live exclusively on the bodies of cetaceans. The most common and well-studied species is Coronula diadema, which is found predominantly on humpback whales. Other notable species include Coronula reginae, Cryptolepas rhachianecti (found mainly on gray whales), and Xenobalanus globicipitis, a stalked barnacle that attaches to the fins and flukes of dolphins and smaller whales. Unlike free-living barnacles that cement themselves to rocks and hard surfaces, whale barnacles have evolved specialized shell morphologies that allow them to embed into whale skin. Coronula diadema, for instance, has a broad, cup-shaped shell with radiating ribs that anchor into the epidermis of the whale. The barnacle does not bore into the skin but rather encourages the whale's skin to grow around its shell base, creating a secure biological anchor. Whale barnacles are filter feeders. They extend feathery appendages called cirri from their shells to capture plankton, small crustaceans, and organic particles from the water as the whale swims. This constant movement through nutrient-rich waters gives whale barnacles a significant feeding advantage over their stationary rock-dwelling cousins, which must wait for tides and currents to bring food to them. The life cycle of whale barnacles is closely tied to their hosts. Free-swimming barnacle larvae, called cyprids, must locate and settle on a whale to survive. They are thought to detect chemical cues in the water that signal the presence of a whale, and they preferentially settle in areas of lower water flow on the whale's body, such as the head, chin, flippers, and the leading edges of flukes.

Not all whale species carry the same barnacle loads. The heaviest infestations are found on slow-moving baleen whales, particularly humpback whales, gray whales, and right whales. These species are ideal hosts because their relatively slow swimming speeds and rough skin textures provide favorable conditions for barnacle settlement and growth. Humpback whales are perhaps the most famously barnacle-encrusted of all cetaceans. Their long pectoral fins, head, and chin often carry thick colonies of Coronula diadema. The bumps on a humpback's head, called tubercles, are frequently surrounded by barnacles. A single humpback whale can carry up to 1,000 pounds of barnacles on its body. Gray whales host a different primary species, Cryptolepas rhachianecti, along with large numbers of whale lice (cyamids). Gray whales are often described as looking like living rock gardens because of the dense patches of barnacles and lice on their heads and backs. The mottled gray and white appearance that gives gray whales their name is partly due to barnacle scars and active colonies. Right whales carry barnacles on the rough, raised patches of skin on their heads called callosities. These callosities are colonized by both barnacles and whale lice, creating a unique pattern on each individual whale. Researchers use these patterns to identify individual right whales in photo-identification studies. Faster-swimming species like blue whales, fin whales, and minke whales tend to carry fewer barnacles. Their streamlined bodies and higher speeds create hydrodynamic forces that make it more difficult for barnacle larvae to settle and survive. Toothed whales such as sperm whales and killer whales generally carry very few or no barnacles, though some species of stalked barnacles (Xenobalanus) can be found on their fins.

The question of whether barnacles harm their whale hosts is one of the most frequently asked questions about this relationship, and the answer is nuanced. The prevailing scientific view is that barnacles are commensal organisms, meaning they benefit from the relationship while causing negligible harm to the whale. However, some researchers have proposed that heavy barnacle loads could have subtle negative effects. The potential costs of carrying barnacles include increased hydrodynamic drag and added weight. A humpback whale carrying 1,000 pounds of barnacles must expend slightly more energy to swim, dive, and breach. Studies have estimated that heavy barnacle and whale lice infestations could increase a whale's drag coefficient by a few percent. For an animal that migrates thousands of miles between feeding and breeding grounds, even a small increase in energy expenditure could theoretically have cumulative effects. However, given the enormous size of most whale species that carry barnacles, the added weight and drag are generally considered trivial. A 40-ton humpback whale carrying 1,000 pounds of barnacles is carrying less than 1.5% of its body weight in external organisms. This is roughly analogous to a 200-pound human carrying a 3-pound backpack. There is even some evidence that barnacles could provide minor benefits to whales. Some researchers have suggested that barnacle-encrusted skin could serve a defensive function. Male humpback whales use their barnacle-studded heads and fins as weapons during competitive bouts for mating access, and the rough, barnacle-covered surfaces could inflict more damage on rivals. Additionally, the unique barnacle and callosity patterns on right whales may play a role in individual recognition. Barnacle attachment does cause localized tissue responses in whale skin. The epidermis thickens and grows around the barnacle shell base, and there can be minor inflammation at the attachment site. When barnacles eventually detach or die, they leave characteristic round scars on the whale's skin. These scars are commonly seen on gray whales and humpbacks and are a normal part of the whale's skin ecology.

Whale barnacles do not live in isolation on their hosts. They are part of a complex micro-ecosystem that includes whale lice (cyamids), diatoms, algae, and bacteria. This community of organisms living on whale skin is sometimes called the whale's epibiotic community, and studying it provides valuable insights into whale behavior, migration patterns, and health. Whale lice are small amphipod crustaceans that are obligate parasites of cetaceans. Unlike barnacles, whale lice are true parasites that feed on the whale's skin. They are often found in association with barnacles, living in the crevices and folds created by barnacle colonies. On gray whales, dense patches of orange whale lice are commonly seen alongside barnacles, particularly around the head, blowhole, and genital slit. The relationship between barnacles and whale lice is itself a subject of scientific interest. Some species of whale lice live preferentially in and around barnacle colonies, using the barnacles as shelter from water flow. Others live independently on the whale's skin. The species composition and density of these epibiotic communities can vary with whale age, health, geographic location, and season. Scientists have also used barnacles as biological records of whale migration and habitat use. Because different barnacle species have different temperature and geographic preferences, the species assemblage on a whale can provide clues about where the whale has been. Isotopic analysis of barnacle shells can reveal information about the water temperatures and conditions the whale experienced during the barnacle's growth period. The study of whale epibionts has also contributed to our understanding of whale evolution and historical distribution. Fossil barnacles found on ancient whale bones provide evidence of whale-barnacle associations dating back millions of years, suggesting that this relationship is ancient and has been a feature of whale ecology for much of cetacean evolutionary history.

Beyond their ecological significance, whale barnacles have become valuable tools for marine biologists studying whale populations, movements, and conservation. The unique patterns formed by barnacles and callosities on individual whales are used as natural identification markers, much like fingerprints in humans. Photo-identification is one of the most important non-invasive research techniques in whale biology. For right whales, the pattern of callosities and associated barnacles and whale lice on each whale's head is unique and remains relatively stable over time. Researchers photograph these patterns and maintain catalogs that allow them to track individual whales across years and decades, monitoring their movements, reproductive success, and survival. For humpback whales, while the primary identification feature is the unique black-and-white pattern on the underside of the tail fluke, barnacle patterns on the head and fins can provide supplementary identification information, particularly for whales that do not frequently show their flukes. Isotope analysis of whale barnacle shells is an emerging research technique. Barnacles incorporate chemical elements from seawater into their shells as they grow, creating a chronological record of the environmental conditions the whale experienced. By analyzing oxygen and carbon isotopes in barnacle shell layers, researchers can reconstruct whale migration routes and estimate the water temperatures the whale encountered at different times. Genetic studies of whale barnacles have also revealed information about whale population structure. Because whale barnacle larvae must settle on a whale host to survive, barnacle populations on different whale populations may become genetically distinct over time. Comparing the genetics of barnacles from different whale populations can provide independent confirmation of whale population boundaries identified through other methods. Recent research has even explored using the chemical composition of barnacle shells to study historical whale diets and the productivity of past ocean ecosystems. As our analytical tools become more sophisticated, whale barnacles are proving to be surprisingly rich archives of ecological information, making these humble crustaceans far more than just passive passengers on the ocean's largest animals.