Barnacles and Relatives: Thecostraca

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ROCK BARNACLE (Semibalanus balanoides): SPECIES ACCOUNTS


Adult barnacles and their relatives have very unusual bodies compared to other crustaceans. In fact, it is only during the free-swimming larval stages that most species are recognizable as crustaceans at all. The first pair of antennae, or antennules (an-TEN-yuls), of the larvae (LAR-vee) is used for grasping. They help the larvae, or young animals, to attach themselves to a host animal or object, their final home as adults. The bodies of the adults are different because of their sessile (SHE-sihl) or parasitic (PAIR-uh-SIH-tik) lifestyles. Sessile barnacles are unable to move because they are permanently attached to other objects, and the parasitic species spend most of their lives feeding inside the bodies of other animals. Most adult barnacles and their relatives have very small heads, lack eyes, and have abdomens with no appendages underneath. The second pair of antennae is usually absent. Thecostraca (thee-koh-STRAH-kay), is divided into three groups: barnacles and their parasitic relatives, ascothoracids, and the y-larvae.

Barnacles are the only group of sessile crustaceans. However, some species are able to move because they attach themselves to animals or floating objects. Adults have white, pink, red, purple, yellow, or brown bodies and live on their heads, upside down, in cone-shaped shells. In most crustaceans, the carapace (CARE-eh-pes) is tough and shieldlike. The carapace of barnacles is soft, sacklike, and called the mantle (MAN-tuhl). The mantle surrounds the body and gives off a substance that forms the cone-shaped shell. The shell is made up of thick, protective plates that join together in the shape of a round or flat cone. The plates are either distinct or tightly joined (fused) together. Inside the mantle is a multi-purpose space, or chamber, where the mouth and reproductive organs are located. It is also where solid waste is released from the barnacle's body. The eggs are kept in this chamber, and the larvae develop there until they are released into the open sea. Adults have a six-segmented thorax with six pairs of cirri (SIH-ree). Cirri are long, bristly, tentaclelike limbs and are either branched or not. They are used together for breathing and like a net to catch bits of food floating in the water. In acorn barnacles, the shell is attached directly to a solid object, but those of gooseneck barnacles are located on top of a soft, flexible stalk. Barnacles without stalks measure up to 9 inches (230 millimeters) in length and 3.1 inches (80 millimeters) across. Stalked species measure up to 27.5 inches (700 millimeters) in length.

There are two small groups of parasitic crustaceans closely related to barnacles. They burrow into corals or into the shells of mollusks (mussels, clams, snails), or they invade the bodies of decapods and isopods. They do not live inside a shell and measure only a few millimeters in length. In one group, the bodies of the adults are completely enclosed inside a folded carapace. Adult females are much larger than the males. The tiny male lives like a parasite and is permanently attached to the female. In the other group, the adults look almost plantlike. They have rootlike bodies that spread out through the tissues of their hosts to soak up nutrients.

The bodies of ascothoracids are completely surrounded by a folded carapace. Their mouthparts are cone-shaped and used to suck the body fluids from their hosts. The thorax has up to six pairs of cirri, and the tip of the abdomen has a pair of appendages.

As their nickname suggests, the y-larvae are y-shaped. They are very small, measuring less than 0.039 inches (1 millimeter) and are free-swimming. The fact that they have grasping antennules and hooked mouthparts suggests that the adults are probably parasites. However, the y-larvae have never been seen clearly associated with their adults.


Barnacles and their relatives are found worldwide in oceans and estuaries (EHS-chew-AIR-eez), where rivers meet the sea.


All thecostracan larvae and some adult males of ascothoracids swim in open water. Adult barnacles are sessile and will usually attach themselves to rocks. They will also attach to other solid objects like shells, floating wood, wharf pilings, ship bottoms, and floating bottles or other trash. Most barnacles live at or just below the seashore, but some prefer living in deep-sea habitats near hot water geysers known as hydrothermal vents. Still others attach themselves to living animals, such as jellyfishes, crabs, other barnacles, sea turtles, sharks, and whales. The adults of parasitic species related to barnacles burrow into corals, the shells of mollusks, or live inside the bodies of other crustaceans. Most ascothoracids live on or inside the bodies of anemones, sea stars, and their relatives.


Most barnacles eat algae (AL-jee), bacteria, tiny animals, and bits of other organisms floating in the surrounding water. Some stalked species prey on larger floating animals by capturing them with a single limb, or cirrus (SIH-ruhs). Parasitic thecostracans eat the tissues and fluids of their hosts.


Most barnacles and their relatives can move about freely while they are larvae. Some ascothoracids still have the ability to swim as adults, attaching themselves to objects only temporarily when they are ready to feed. They attach themselves by the front of their heads using glue that comes from special glands in their antennules.

At low tide, barnacles on rocky shores survive out of water by sealing the plates in their shells to keep themselves moist and protected from high temperatures. The larvae are attracted to chemicals produced by the adults and usually settle in large numbers where there are other living or dead barnacles.

Many barnacles and their relatives are hermaphrodites (her-MAE-fro-daits), where individuals have both male and female reproductive organs. In species with both males and females, the male is often much smaller and permanently attached to the female like a parasite. These males seldom feed and lack internal organs, other than those directly involved with reproduction. However, most barnacles require both males and females to reproduce.

The eggs of barnacles and their relatives are brooded inside the chamber in the carapace or mantle. The eggs hatch into free-swimming nauplius (NAH-plee-us) larvae. Nauplius larvae use antennae and mouthparts—the only appendages they have—for swimming. They molt, or shed their external skeletons (exoskeletons), several times before reaching the next larval stage. This stage does not eat, and its body is completely surrounded by a folded carapace.


Native Americans once ate barnacles. Today, the rock barnacle is popular as seafood in parts of South America. Many other barnacles are considered pests. When attached to the bottoms of ships, barnacles can slow their speed by more than one-third. This increases the amount of fuel needed to get from place to place. Much time, money, and effort has been spent on the development of paints that will prevent barnacles from settling on ship bottoms.


Charles Darwin (1809-1882) collected a never-before-seen burrowing barnacle in a conch shell in Chile. In order to classify his discovery properly, Darwin began a careful study of all barnacles, including fossils. In eight years (1846-1854) he wrote four important books on barnacles, representing the first modern studies of the group. Darwin's work with barnacles, earthworms, insect-eating plants, and other organisms helped him to write his most important book, On The Origin of Species, in 1859.


Two species of thecostracans—barnacles Armatobalanus nefrens and Balanus aquila—are listed by the World Conservation Union (IUCN) as Data Deficient. This means there is not enough information to make a judgment about their threat of extinction.


Physical characteristics: Trypetesa lampas (abbreviated T. lampas) does not have a hard carapace. Females reach 0.78 inches (20 millimeters) in length. Their body is clear or yellowish and covered by a large, folded carapace. There is one pair of small cirri near the mouth and three uniramous (YU-neh-RAY-mus), or unbranched, pairs on the thorax. The tiny, bottle-shaped males have antennae and no other appendages. They measure 0.047 inches (1.2 millimeters).

Geographic range: They are found along the shores of the Pacific and Atlantic oceans north of the Equator.

Habitat: Adult females use the mantle and chemicals to bore into the shells of living or dead snails and hermit crabs.

Diet: Females flex their bodies to suck water carrying food particles inside the mantle.

Behavior and reproduction: Both males and females are required for reproduction. Adult males select and attach themselves to females while they are still in the non-feeding larval stage. Nauplius larvae molt four times before reaching the non-feeding larval stage. This stage has simple eyes and six pairs of thoracic limbs. Each simple eye has one lens and is used to find a suitable place to begin burrowing.

Trypetesa lampas and people: This species does not impact people or their activities.

Conservation status: This species is not considered threatened or endangered. ∎

ROCK BARNACLE (Semibalanus balanoides): SPECIES ACCOUNTS

Physical characteristics: Rock barnacles measure 0.19 to 0.59 inches (5 to15 millimeters) across. The shell is made up of six gray or white plates that are fused together. Two of the plates form a cover that opens and closes. Through this opening six pairs of biramous (BY-ray-mus), or branched, cirri fan out into the water or can be safely withdrawn inside. Tissue inside this opening is white or pinkish.

Geographic range: Rock barnacles are found in intertidal rocky shores in North America, Europe, and the Arctic.

Habitat: Rock barnacles attach themselves to rocks, pilings, and crabs. They prefer low tide areas sometimes splashed by water.

Diet: They eat bits of food floating in the water.

Behavior and reproduction: Rock barnacles feed only during high tide when the shell is covered by water. Food is collected from the water by the last three pairs of cirri that form a fan-shaped net. The cirri are extended and withdrawn inside the shell 33 to 37 times a minute as they collect food from the water.

Individuals are hermaphrodites. Eggs are fertilized in the mantle cavity and six thousand to thirteen thousand nauplius larvae develop there. The non-feeding larvae attach themselves to objects using their antennules. Rock barnacles live a total of three to eight years.

Rock barnacles and people: Rock barnacles are studied by scientists interested in learning how animals live and survive in harsh seashore habitats.

Conservation status: This species is not considered threatened or endangered. ∎



Anderson, D. T. Barnacles. Structure, Function, Development and Evolution. Melbourne, Australia: Chapman and Hall, 1994.

Brusca, R. C., and G. J. Brusca. Invertebrates. Second edition. Sunderland, MA: Sinauer Associates Inc., 2003.

Stott, R. Darwin and the Barnacle: The Story of One Tiny Creature and History's Most Spectacular Scientific Breakthrough. New York: W. W. Norton & Company, 2003.


"Earth Almanac. Hot Stuff. Pepper Paint Foils Barnacles." National Geographic (May 1994) 185, no. 4.

Starbird, E. A. "Friendless Squatters of the Sea." National Geographic (November 1973) 144, no. 5: 623-633.

Web sites:

Barnacles. (accessed on March 21, 2005).

Biology of the Barnacles. (accessed on March 21, 2005).

Introduction to the Cirripedia—Barnacles and Their Relatives. (accessed on March 21, 2005).

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Barnacles and Relatives: Thecostraca

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