If you look closely at a photo of a southern right whale (Eubalaena australis), you’ll see that few animals accessorize quite like they do. Their crusty white ‘bonnet’ (which looks more like a mustache, if you ask me) complete with their classic goatee and cumulus eyebrows truly set them apart as charismatic megafauna.

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But what are those white chunks of calcified skin? These so-called ‘callosities’ are rough patches of skin embedded with barnacles (Coronulidae) and lice (gross!) and are abundant on right whale species. Whales are actually living biomes, migrating islands of blubber and muscle that support many clingy crustaceans, including six fascinating barnacle species. Unlike their coastal counterparts that we like to chisel off of boat hulls, whale barnacles can only survive on the skin of whales.

What exactly is the ecological arrangement between these barnacle tenants and their mammalian landlords? Are they loyal companions, innocent bystanders, or annoying couch-surfers that just won’t leave?

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Believe it or not, barnacles are crustaceans, a taxonomic group that includes crabs, lobsters, and shrimp. However, they have traded in their walking legs and pinchy appendages for a crown-shaped, immobile shell of six to eight plates made of calcium carbonate and chitin. Inside these plates, the soft barnacle sits within a cuticle that is molted, or shed, periodically. Louis Agassiz, a 19^th century zoologist, described a barnacle as “nothing more than a little shrimp-like animal standing on its head in a limestone house and kicking food into its mouth.” I am here to convince you otherwise.

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Whale barnacles are a subgroup of acorn barnacles belonging to the family Coronulidae and are found almost exclusively on baleen (filter-feeding) whales. Whale barnacles deeply embed themselves into their vertebrate hosts; they actually pull whale skin up into their shell, bracing themselves for a year-long ride on a 20-ton ocean broncho.

Being filter-feeders themselves, these meta-filter-feeders use tentacle-like arms called cirri to passively filter food from water skimming by their blubbery hosts. Cirri are also used for copulation. Barnacles fully extending these feathery arms along with their penis until they detect a neighboring barnacle, then fan their cirri together to mate.

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Speaking of copulation, barnacles have also gone to great evolutionary lengths (no pun intended) to claim the longest penis-to-body ratio of any animal, ensuring they can fertilize their far-off neighbors. Most barnacle species are hermaphrodites, meaning that they can act as male or female. However, they need to reach a partner to successfully reproduce, which is how their impressively long penis earned its place.

Scientists only began studying the development and life cycle of whale barnacles in 2006 with Coronula diadema collected from a humpback whale fin (1). Barnacle embryos hatch to release larvae called nauplii (singular: nauplius), which go through six molting stages to reach their final juvenile stage, the cyprid larval stage. These young adults do not feed, and instead focus all of their energy on finding a suitable place to settle down. For whale barnacles, this means floating around in the water column in search of a suitable warm-blooded vessel. But the ocean is so large, how do they possibly find a whale? Whale barnacles locate their new blubbery island by following a chemical cue released into the water from their hosts. The exact cue is not yet known but is suspected to be a blood protein called alpha-2-macroglobulin (1). Once they claim a nice breezy patch of throat pleat, pectoral fin, or tail fluke, they become permanent residents; no mortgage necessary.

Scientists suspect that the whale barnacle lifespan only lasts one year, with whales shedding their barnacles annually at breeding grounds. Of the 14 Coronulidae species on the World Register of Marine Species (WoRMS), only six remain today. The remaining eight species are extinct, but their fossils remain and provide a wealth of information about past marine ecosystems. For example, scientists at UC Berkeley use ancient barnacles as ecological breadcrumbs to reconstruct migratory pathways that whales used millions of years ago (2). This can be used to reveal how previous climates impacted whale travel patterns and how our current whales may respond to our changing climate in the future.

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When you notice a barnacle-studded rostrum of a whale, you may notice different shapes and styles. It turns out that the six whale barnacle species still alive today are quite selective about their oceanfront real estate. Who are these tenants, and how are they different from each other?

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Tubicinella major and Cetopirus complanatus have only been documented in right whales (Eubalaena spp.) but use different strategies for bedazzling whale skin. Tubicinella are tall and (you guessed it) tubular, using ridges and growing new rings from their base to anchor themselves into right whale callosities (3). Cetopirus, on the other hand, embed themselves deeply into whale epidermis by hunkering down into a flattened pancake.

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Coronula diadema are a ribbed, barrel-shaped species with a preference for humpback whale (Megaptera novaeangliae) skin (4). They often share their humpback homestead with another barnacle species, Coronula reginae. This flatter species that has evolved to dive further into its host’s skin, submerging half of its shell. These species are known to expand their horizons and have been documented on sperm, right, blue and finback whales, but to a far lesser extent (5). Despite C. diadema being first described by Carl Linnaeus in 1767 and C. reginae by Charles Darwin in 1854, we acknowledge these species so rarely that they have not even been given common English names (or given to any other Coronulidae species, for that matter)!

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Cryptolepas rhachianecti are the pickiest whale barnacle of them all, found exclusively and abundantly on gray whales (Eschrichtius robustus) in the North Pacific (5). They tend to cluster on the head and back, resembling patches of honeycomb. They form a hexagonal maze for their crustacean partners in crime, the whale lice. These lice species have also evolved specifically to survive on whales, scrubbing algae and dead skin from their blubbery hosts.

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Perhaps the most promiscuous species of all is Xenobalanus globicipitis, inhabiting fins and flukes on both whales and porpoises across the North and South Atlantic, Mediterranean, and South Pacific (4). They would definitely win the gold medal for the oddest-looking whale barnacle. A distinctive star-shaped shell acts as an anchor, enabling them to evert their naked stalk-like body and hang off their host like a trapeze artist, extending their arms out to feed.

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Stalked barnacles even grow on the tusks of Blainville’s beaked whales (Mesoplodon densirostris). Male beaked whales have large teeth (i.e., tusks) that protrude up from their lower jaw. When colonized by barnacles, they resemble not one, but two party hats. It is suspected that females select mates based on their tusks (6), so how are barnacle-clad males viewed in the dating scene? Perhaps they are fun for a night on the town, but aren’t ready to settle down anytime soon.

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It is still up for debate whether the relationship between whales and their barnacles is mutualistic (i.e., both parties benefit), commensal (the whale host could care less), or just plain parasitic (the barnacles benefit from the whale’s sacrifice). Generally, whale barnacles are considered to be commensal riders. However, it is hard to imagine that a thousand pounds of barnacles would be inconspicuous to the humpback whale lugging them around (7). Gray whales have been observed scratching their crustacean patches on boat hulls, suggesting these passengers may also be a nuisance to their chaperones.

On the other hand, perhaps whale barnacles deserve a bit more credit. One theory suggests that barnacles benefit male whales in defense and mating, as the small-shelled wingmen provide a serrated surface on the whale’s body when it fights other males for mates (7). They may even act as armor against predators like the infamous killer whale, who may get a crusty bite of barnacle instead of blubber.

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February 21 is World Whale Day! While celebrating our magnificent ocean giants, don’t forget to take a minute to look a bit closer at their photos and appreciate the crusty but charismatic microfauna adorning their bodies. Are whale barnacles nature’s most successful freeloaders, skating through life being spoon-fed by their migrating chaperones? Or are they hostages of evolution; a species literally and figuratively clinging to their hosts for survival? We may never know.

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Jenn Cossaboon is a DVM/PhD student in the Integrative Pathobiology Graduate Group at UC Davis. She is interested in how contaminant exposures alter fertility and harm reproductive success in endangered species. Her current research focuses on endocrine disrupting chemicals and their effects on the developing fish ovary. She is also experimenting with homebrewing. Find her on Twitter @jenndocrinology.

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References

1. Nogata, Y., & Matsumura, K. (2006). Larval development and settlement of a whale barnacle. Biological Letters, 2, 92–93.
2. Taylor, L. D., O’Dea, A., Bralower, T. J., & Finnegan, S. (2019). Isotopes from fossil coronulid barnacle shells record evidence of migration in multiple Pleistocene whale populations. Proc Natl Acad Sci USA, 116, 7377–7381.
3. Seilacher, A. (2005). Whale barnacles: exaptational access to a forbidden paradise. Paleobiology, 31, 27–35.
4. Zullo, V. A. (1979). Marine Flora and Fauna of the Northeastern United States. Arthropoda: Cirripedia.
5. Scarff, J. E. (1986). Occurence of the barnacles Coronula diadema, C. reginae and Cetopirus complanatus (Cirripedia) on right whales. The Scientific Reports of the Whales Research Institute, 37, 129–153.
6. Dalebout, M. L. Steel, D., & Baker, C. S. (2008). Phylogeny of the beaked whale genus Mesoplodon (Ziphiidae: Cetacea) revealed by nuclear introns: Implications for the evolution of male tusks. Systematic Biology, 57, 857–875.
7. Ford, J. K. B., & Reeves, R. R. (2008). Fight or flight: antipredator strategies of baleen whales. Mammal Review, 38, 50–86.

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Main photo: A humpback takes a deep breath of fresh Antarctic air, its jaw and throat pleats inlaid with Coronula diadema barnacles. Photo and data collected under NMFS, ACA, and UCSC IACUC permits by Dr. Ari Friedlaender.

Edited by Jessica Schaefer