Semibalanus balanoides

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)

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)

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)

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)

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

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)

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 isopodHemioniscus balani also uses the barnacle as a host. (White, 2004)

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)

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)

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

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)