Barnacles Are Everywhere: The Discreet Tenants Living with Coral — and the Parasites Hijacking Crabs!

This article is reprinted in full from Academia Sinica's Research Portal — "Barnacles Are Everywhere: The Discreet Tenants Living with Coral — and the Parasites Hijacking Crabs!"

What Kind of Marine Life Is a Barnacle?

To most people, barnacles are those little volcano-shaped creatures — and a famously "extreme" seafood delicacy. Barnacles are found virtually everywhere in the ocean: intertidal zones, coral reefs, shallow seas, the deep sea, and even on the bodies of sea turtles and whales. But what exactly are they, and what role do they play in the ecosystem? Academia Sinica's Research Portal sat down with Distinguished Research Fellow Benny Kwok-Kan Chan, Director of the Biodiversity Research Center, to hear from a scientist who has spent years studying barnacles and is happy to share their fascinating story.

The coral-symbiotic barnacle shown here is *Berndtia purpurea*, whose sapphire-blue coloration is strikingly beautiful. It has a mutualistic relationship with coral. Photo | Benny Kwok-Kan Chan

Do You Know Barnacles?

Barnacles are marine animals of remarkable morphological diversity. Some look like tiny volcanoes; others resemble a goose's neck — their external features can differ enormously. What they all share is a pear-shaped nauplius larva with a pair of long lateral horns projecting from each side. Once grown, barnacles almost always attach firmly to rocks, ship hulls, or coral and are largely incapable of moving on their own. More than two thousand species have been identified to date. Benny Kwok-Kan Chan notes that although many barnacles have hard shells that make them look similar to mollusks, they are actually crustaceans — classified by taxonomists within the phylum Arthropoda, making them relatives of crabs and shrimp.

Want to see barnacles for yourself? Head to the beach!

The volcano-shaped barnacle is known as an acorn barnacle. Pictured here is *Tetraclita japonica*. Photo | Benny Kwok-Kan Chan

Acorn Barnacles: Blanketing the Intertidal Zone

Strolling along Taiwan's rocky intertidal shores, you'll easily spot clusters of conical, volcano-shaped organisms with a circular opening at the top, cemented firmly to the reef. These are "sessile" (stalkless) barnacles — as the name implies, they have no stalk connecting them to the rock; instead, they attach directly, surrounding their soft bodies with a hard calcium carbonate shell. Along Taiwan's Northeast Coast, eastern coastline, and the shores of Kenting, a common species is the Tetraclita kuroshioensis, recognizable by its greenish shell. This species was discovered and formally named by Benny Kwok-Kan Chan in 2007.

*Tetraclita kuroshioensis*, a common species along Taiwan's shores, features a gray-green, volcano-shaped shell. It was discovered and named by Benny Kwok-Kan Chan in 2007. Photo | Benny Kwok-Kan Chan

The barnacle living inside the volcano-shaped shell has six pairs of appendages called "cirri." Each cirrus is branched into two rami that look like antennae, covered in slender cilia. Three of the six pairs are longer and form a net-like "cirral fan" used to filter plankton from the water. Once prey is captured, the other three shorter, stouter pairs pass the food into the mouth.

The intertidal zone experiences tides. At low tide, when the rocks are exposed to air, barnacles — unable to move — simply shut the two pairs of opercular plates that seal the opening at the top of the shell, preventing excessive water loss from wind and sun exposure. At that point, all you see is the volcano-like shell. When the tide comes back in and the barnacles are submerged again, they reopen those plates, extend their slender cirri, and resume feeding on plankton.

The world of barnacles is remarkably diverse. Beyond the hard-shelled sessile barnacles, there is another group: the "stalked" barnacles, characterized by a long, neck-like peduncle.

Pictured is the sessile barnacle *Chelonibia testudinaria* (turtle barnacle). Beneath the shell, the barnacle body has six pairs of "cirri"; three of the longer pairs form a net-like "cirral fan" used to filter plankton from the water. Photo | Benny Kwok-Kan Chan

Stalked Barnacles: A Prized Ocean Delicacy

In Kinmen, the Matsu Islands, and rock crevices along Taiwan's eastern intertidal shores, you can find the stalked barnacle commonly known as "turtle claw" (Capitulum mitella). Unlike the acorn barnacle with its volcano-shaped shell, its body is clearly divided into a capitulum (head) and a peduncle (stalk). The capitulum is composed of hard tergum and scutum plates, while the peduncle is fleshy and covered in small scales. Because it resembles a turtle's claw, praying hands, or even a pen holder, it goes by many names in Chinese: turtle claw, turtle foot, Buddha hand, Guanyin hand, and pen rack, among others.

The stalked barnacle *Capitulum mitella* (turtle claw) also has a shell, but its appearance differs entirely from an acorn barnacle. In Kinmen and Matsu, turtle claw is considered a delicious seafood. Photo | Benny Kwok-Kan Chan

Compared to sessile barnacles and turtle claw barnacles that cement directly to rock, another group of stalked barnacles attaches to floating objects — buoys or debris — via a goose-neck-like peduncle. These are commonly called "gooseneck barnacles" and belong to the genus Lepas. There is also the genus Pollicipes (stalked barnacles found along the coasts of Spain, Portugal, and nearby North Africa), which are scarce and difficult to harvest in the wild, making them a prized and expensive seafood delicacy.

There is also an amusing historical footnote about gooseneck barnacles. "In the Middle Ages, Europeans had a grand misconception about Lepas anatifera," Chan recounts. At the time, Europeans had no idea that barnacle geese were migratory birds that breed in Arctic regions and only arrive in Britain and continental Europe to overwinter. Every winter, barnacle geese would suddenly appear, yet no one ever witnessed their breeding. Because gooseneck barnacles bore a passing resemblance to these birds, some people actually believed that barnacle geese hatched directly from gooseneck barnacles — which is why the bird was also known as the "barnacle goose."

Pictured is the gooseneck barnacle *Lepas anatifera*, attached to driftwood. In the Middle Ages, Europeans believed that barnacle geese hatched from these barnacles — hence the bird's common name "Barnacle Goose." Photo | Benny Kwok-Kan Chan

"Rhizocephala": The Devious Parasitic Barnacle

Sessile and stalked barnacles are self-sufficient — extending their cirri, sweeping the water, and working hard to catch plankton. But there is another group of barnacles that has evolved a thoroughly freeloading lifestyle. These barnacles mostly parasitize crabs (though other crustacean hosts exist) and are known as Rhizocephala. Taxonomically classified under the superclass Rhizocephala, over three hundred species have been described. They have no distinct body form and live as internal parasites inside crabs, isopods, and other hosts.

The adult rhizocephalan consists mainly of a root-like network (the interna) that grows inside the host's body, plus a sac-like reproductive organ (the externa) visible on the outside of the host. The root-like threads draw nutrients from the host and can simultaneously manipulate the host's brain and behavior. The degree of control varies by species: in Europe, some rhizocephalan species cause the host crab to stop molting and growing, and to become sterile.

In Taiwan, the observed pattern is slightly different: infected crabs can still move, forage, molt, and grow, but cannot mate or reproduce. After infection, the crab loses its own instincts and instead carefully tends to the rhizocephalan. Male crabs even undergo physiological feminization — their abdomen gradually changes from a narrow male shape to the broad, rounded form of a female — and they begin exhibiting brood-care behaviors toward the parasite's offspring.

The name "rhizocephalan" (蟹奴, literally "crab slave") might suggest the barnacle is the crab's slave — but the reality is the complete reverse: the crab ultimately becomes the slave of the rhizocephalan, entirely under its control.

The prevalence of rhizocephalan infection varies by region; in some parts of the Mediterranean, infection rates among crabs can be remarkably high! Next time you eat crab, don't get too excited if you spot a yellowish protrusion on the underside hoping it's roe — look closely, because it just might be the rhizocephalan's external reproductive sac (yikes).

A: A swimming crab already parasitized by a rhizocephalan; the dark area in the lower center of the body is the rhizocephalan's reproductive sac (externa). B: The sac-like external reproductive organ of a rhizocephalan on the surface of its host. Photo | Provided by Benny Kwok-Kan Chan

The Role of Barnacles in Nature

Setting aside the parasitic rhizocephalans, most barnacles are filter feeders that settle on a substrate after larval metamorphosis and use their cirri to catch plankton — while themselves serving as food for other marine life. Most barnacles live out their days in quiet coexistence with the world, and because their lifestyle is highly sessile, they are extremely sensitive to environmental change. The abundance and distribution of barnacles also influences ecosystem balance.

Do barnacles truly live peacefully with everything around them? Other animals might disagree — particularly whales and sea turtles, which often bear the burden of barnacle attachment. One or two barnacles may not matter much, but a large colony is a different story entirely. The added weight alone is significant, and the rough surfaces of barnacle shells dramatically increase drag, making swimming laborious for both whales and sea turtles.

The shell and head of a sea turtle covered by *Chelonibia testudinaria* (turtle barnacles). Photo | Provided by Benny Kwok-Kan Chan, photographed by Ceri Lewis

That said, we should be wary of a flood of misleading videos circulating online — for example, clips purporting to show polar bears covered in barnacles, or people standing on whales and scraping off barnacles with shovels. Chan emphasizes that these videos are entirely fabricated: "First, polar bears don't get barnacles growing on them, because they don't spend prolonged periods submerged in seawater — barnacles simply cannot attach and grow. And generally speaking, whales don't let people climb on them; moreover, whale barnacles are deeply embedded in the whale's skin and cannot be removed with a shovel."

Barnacles can also be a headache for humans. Any underwater vessel or structure left in the sea will eventually become encrusted with barnacles. Ships must burn more fuel to generate the extra power needed to overcome the increased drag; underwater infrastructure faces steep maintenance and cleaning costs. For these reasons, barnacles are classified as marine biofouling organisms.

Despite all the trouble they cause, Chan reminds us that barnacles also deliver some surprising benefits. Species such as Pollicipes, turtle claw, and giant barnacles are considered exceptional seafood delicacies in Spain, Portugal, Japan, and Taiwan.

Barnacles also secrete a cement protein to anchor themselves to rock — an adhesive whose bonding strength is unaffected by seawater, making it arguably the most powerful underwater glue known, surpassing all synthetic adhesives. If scientists can harness this as a research foundation, it may be possible to engineer more durable and effective underwater adhesives. Chan's latest research has even found that Chelonibia testudinaria (turtle barnacles) growing on sea turtle carapaces can dissolve their own cement proteins, relocate, and then secrete new cement proteins to reattach.

Chan's latest research has found that *Chelonibia testudinaria* (turtle barnacles) growing on sea turtle carapaces can dissolve their own cement proteins and relocate. Top image: frontal view. Bottom image: view from underneath. Photo | Provided by Benny Kwok-Kan Chan

In Dasi, Yilan, giant barnacles are a local seafood specialty, nicknamed "underwater volcanoes." Photo | Provided by Benny Kwok-Kan Chan

In Matsu, turtle claw barnacles (bottom) and mussels (top) are both prized seafood catches. Photo | Benny Kwok-Kan Chan

Wanella milleporae: The Barnacle Living on Fire Coral

Above, we've seen barnacles that parasitize other organisms and barnacles that encrust marine animals and ships — admittedly a bit troublesome. But barnacles also have their "cooperative" side!

If you're lucky enough to dive at a tropical island, you'll encounter vivid, colorful corals hosting symbiotic zoanthids and algae — the source of all that spectacular color. Look closely at the coral surface, however, and you may notice small, charming circular holes with slender cirri barely poking out. Those are coral barnacles — mutualistic tenants living symbiotically with coral!

A barnacle living in symbiosis with coral. Photo | Benny Kwok-Kan Chan

The star of this research is Wanella milleporae, one of the most common symbiotic organisms found on fire coral. But is fire coral actually a coral? Despite the name and a superficial resemblance to familiar corals, fire coral is not a true coral.

Common reef-building corals are stony corals, formed by relatively large coral polyps. "Fire coral belongs to the genus Millepora. Compared to stony coral polyps, fire coral is more closely related to jellyfish and hydroids — it is a hydrozoan, made up of tiny polyps that secrete calcium carbonate skeletons. These hydroids are extremely small and their nematocysts can deliver potent toxins," Chan explains.

When diving, if bare skin makes direct contact with fire coral, the toxins released by the hydroid's thread-like tentacles produce a burning sensation at the point of contact, and in more severe cases can cause blistering and peeling — hence the name "fire coral."

Fire coral occurs in both branching and encrusting forms. Direct skin contact while diving produces a burning pain. Photo | Provided by Benny Kwok-Kan Chan

In appearance, Wanella milleporae looks nothing like the volcano-shaped acorn barnacle or the stalked turtle claw barnacle. The body of Wanella milleporae is embedded within the fire coral, with only the circular opening of the shell plates exposed; the rest is covered by fire coral and is easy to miss unless you look carefully.

The fire coral host of Wanella milleporae harbors symbiotic zooxanthellae, giving it a yellowish-green to yellow-brown coloration. It is distributed in tropical and subtropical waters and is commonly found around Kenting and Green Island.

The body of *Wanella milleporae* is embedded within the fire coral, with only the circular shell plate opening exposed; the rest is covered by fire coral and easy to overlook (arrows). Photo | Benny Kwok-Kan Chan

Although fire coral is equipped with nematocysts for capturing plankton and repelling enemies with toxins, a few organisms still manage to live on its surface — including amphipods, snapping shrimp, and most abundantly, Wanella milleporae. Some researchers consider the relationship mutualistic: nitrogen and phosphorus produced by the barnacle's waste can supply nutrients for zooxanthellae photosynthesis. Others, however, question whether an excessive density of Wanella milleporae might damage fire coral skeletons, making the relationship more parasitic than beneficial.

"Based on our observations, many fire corals in the wild remain structurally robust even when densely inhabited by barnacles. Taking into account other studies as well, we still consider the relationship to be mutualistic," says Chan. But for Chan, an even more intriguing scientific question remains: exactly how does Wanella milleporae manage to break through and settle on fire coral without being attacked by its nematocysts? In the follow-up article "Living on Flames: The Secret of Wanella milleporae's Symbiosis with Fire Coral," we will uncover that mystery!

Benny Kwok-Kan Chan introducing barnacle specimens to the Research Portal team. Photo | Research Portal

Reporting & Writing | Lin Cheng-Hsun
Editor | Jian Ke-Zhi
Art Design | Tsai Wan-Jie