Semibalanus balanoides

Geographic Range

Semibalanus balanoides is found in the north-east Atlantic from Great Britain to southwestern Spain. It is also found on the Pacific coast of north America as far south as British Columbia and on the Atlantic coast as far south as Cape Hatteras. However, it is absent from the Biscay coast of France. (White, 2004)


Acorn barnacles are found more readily in tropical tidal zone marine environments, but may also thrive in cooler areas. These species attach to almost any roughened surface such as rocks, whales, piers, ship hulls and sea turtle shells. An acorn barnacle primarily determines its habitat based on water movement in the area. Such factors as light and contour of the surface are important as well. (Dando and Burchett, 1996)

Physical Description

Semibalanus balanoides are modified shrimp that have six pairs of biramous legs. They are surrounded by a series of overlapping calcereous (limestone) plates and are protected by another set of plates which close over the top opening of the animal when it is threatened. Semibalanus balanoides range in length from 2.5 cm to 7.5 cm and their shells are commonly yellow, orange or pink. These animals are monomorphic and have no eyes but are sensitive and responsive to sudden shading that may be a potential predator. Additionally, these animals are called "acorn barnacles" because the pyramid shape of their body resembles the fruit of the oak tree.

Key identification features found on this species include six grey-white plates on the shell wall and a membranous shell base. The opercular aperature, or opening, is diamond shape and the tissue inside is white to pinkish-white. The rostral plate, or shell plate on the head, is broad. (Anderson, 1994; Author unknown, 2003; White, 2004)

  • Range length
    2.5 to 7.5 cm
    0.98 to 2.95 in


One barnacle may produce up to 10,000 eggs that are stored in a sac outside the body but within the shell cavity. They hatch to produce nauplius larvae with three pairs of legs, one pair of antennae and one eye. There are six instars which result in further molting to form a more complex larvae. Barnacles molt depending on feeding rate and temperature. Each instar lasts three to five days.

After these instars, the napulis larvae changes into a cypris larva that has a bivalve carapace and eyespots. The cyprides larva does not feed and may spend nearly two weeks finding a substrate to attach. When settled the larve begins to "walk" on the surface with its antennae to find a barnacle of its own species. It then attaches to the surface with cements excreted by the antennae. Metamophosis begins with the sheding of the bivalve carapace and eyespots. It will resemble an adult acorn barnacle 24 hours later.

Additionally, the larva will begin to build its calcium carbonate walls. The body of the acorn barnacle will shed the sac that encloses it periodically as the exterior shell increases in size. The range of lifespan for these animals is one to seven years. (Anderson, 1994; Author unknown, 2003; Dando and Burchett, 1996; White, 2004)


Semibalanus balanoides are hermaphrodites that reproduce by internal fertilization. This fertilization differs from other intertidal invertebrates that release gamates into the marine environment. Cross fertilization in S. balanoides occurs through the insertion of a long extensible penis into the mantle cavity of another acorn barancle that is within close proximity. It is essential that the barnacles live no more than three to five centimeters apart to mate. For acorn barnacles, the time of breeding and rates of development vary depending on their latitudinal location. However, S. balanoides is a northern species that normally breeds over winter at optimal temperatures of 14 deg C. One barnacle may produce up to 10,000 eggs. (Anderson, 1994; Author unknown, 2003; Dando and Burchett, 1996; Anderson, 1994; Author unknown, 2003; Dando and Burchett, 1996; Anderson, 1994; Author unknown, 2003; Carefoot, 1977; Dando and Burchett, 1996; Little and Kitching, 1996; Museum Victoria Australia, 1996)

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • protecting


The range of lifespan for these animals is one to seven years.


Semibalanus balanoides is unlike any other crustacean because the adult is sessile. Although acorn barnacles are hermaphroditic, they cannot fertilize themselves. Therefore, cross fertilization occurs through the insertion of a long extensible penis into the mantle cavity of another acorn barancle that is within close proximity. It is essential that the barnacles live no more than three to five centimeters apart to mate.

Predators to acorn barnacles vary from humans to dogwelks and nudibranches. In response to a potential threat from a predator, S. balanoides will withdraw into its calcareous plates for protection. (Carefoot, 1977; Little and Kitching, 1996; Museum Victoria Australia, 1996)

Communication and Perception

Crustaceans have various sensory resceptors, mainly setae over the body. Photoreceptors are also generally present. (Brusca and Brusca, 2003)

Food Habits

Acorn barnacles are suspension feeders that kick their six pairs of cirri to capture food and transfer it to the mouth. The cirri capture food by forming a net that catches microscopic plants and particles floating in the water, such as plankton. (Anderson, 1994; White, 2004)


Predators to acorn barnacles vary from humans to dogwelks and nudibranches. In response to a potential threat from a predator, S. balanoides will withdraw into its calcareous plates for protection. (Carefoot, 1977; Little and Kitching, 1996; Museum Victoria Australia, 1996; White, 2004)

Ecosystem Roles

Several species use Semibalanus balanoides as a host. The protozoan Pyxinioides balani parasitizes the gut and Epistylis horizontalis parasitizes the gills and mantle. Trematode metacercariae are in or near the barnacle guts. The isopod Hemioniscus balani also uses the barnacle as a host. (White, 2004)

  • Ecosystem Impact
  • creates habitat

Economic Importance for Humans: Positive

Recently, there has been great interest in barnacle cement for its possible use in both dentistry and bone surgery due to its incredible properties. For example, a layer of cement three thousandths of an inch thick over one square inch will support a weight of 7,000 pounds. At high temperatures the glue will not crack and it does not dissolve in most strong acids, organic solvents, or water. Through its usage, filings could be placed on teeth and bones could be mended. (Author unknown, 2003; Author unknown, Date unknown)

  • Positive Impacts
  • body parts are source of valuable material

Economic Importance for Humans: Negative

Barnacles have been refered to throughout history as the "bane of boaters." This name is primarily asociated with barnacles because their attachment on boat hulls and bottoms can cause a significant decrease in a vessel's speed. For example, two to three inches of barnacles on the bottom of a ship can weigh more than one hundred tons. Removing barnacles from ship's hulls costs ship owners up to $125 million annually. Additionally, their prevelence on marine structures such as piers and pilings can be burdensome. (Author unknown, 2003; Author unknown, Date unknown; Museum Victoria Australia, 1996; White, 2004)

Conservation Status

Currently, there are no active conservation programs associated with S. balanoides.

Other Comments

Acorn barnacles, which are the most common barnacle, have existed since Jussasic times, and fossils of barnacles have been discovered that are over 150 million years old. Historically, Europeans believed that barnacles were formed from the molted feathers of sea birds. Up until 1830 when zoologist V.J. Thompson discovered that barnacles were related to lobsters, most thought that barnacles were molluscs. Additionally, much of the knowledge of barnacles is the result of research by Charles Darwin who published a book on the subject in the 1840's. (Author unknown, 2003; Author unknown, Date unknown)


Renee Sherman Mulcrone (editor).

Leslie Lott (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.


Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


an animal that mainly eats decomposed plants and/or animals


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


An animal that eats mainly plants or parts of plants.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton


structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.


non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


Anderson, D. 1994. Barnacles: Structure, Function, Development, and Evolution. New York: Chapman and Hill.

Author unknown, Date unknown. "Barnacle" (On-line). Rhode Island Sea Grant Fact Sheet. Accessed 10/30/03 at

Author unknown, 2003. "Barnacle" (On-line). The Columbia Encyclopedia. Accessed 10/21/04 at

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Carefoot, T. 1977. Pacific Seashores: A Guide to Intertidal Ecology. Seattle: University Of Washington Press.

Dando, M., M. Burchett. 1996. SeaLife: A Complete Guide to the Marine Environment. Washington D.C.: Smithsonian Institution Press.

Little, C., J. Kitching. 1996. The Biology of Rocky Shores. New York: Oxford University Press.

Museum Victoria Australia, 1996. "Biology of Barnacles" (On-line). Crustacea Library. Accessed 10/22/04 at

White, N. 2004. "An acorn barnacle, Semibalanus balanoides " (On-line). Marine Life Information Network for Britain and Ireland. Accessed October 21, 2004 at