Acorn Barnacle

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.

Known Predators:

• nudibranchs (Nudibranchia)
• dog whelks (Nucella lapillus)
• humans (Homo sapiens)
• shanny (Lipophrys pholis)

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.

Range length: 2.5 to 7.5 cm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

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

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.

Habitat Regions: temperate ; tropical ; saltwater or marine

Aquatic Biomes: reef ; coastal

Other Habitat Features: intertidal or littoral

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.

Biogeographic Regions: indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea

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.

Animal Foods: zooplankton

Plant Foods: phytoplankton

Primary Diet: herbivore (Algivore); planktivore ; detritivore

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.

Crustaceans have various sensory resceptors, mainly setae over the body. Photoreceptors are also generally present.

Communication Channels: visual ; tactile ; chemical

Perception Channels: tactile ; chemical

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

US Federal List: no special status

CITES: no special status

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.

Development - Life Cycle: metamorphosis

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.

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.

Positive Impacts: body parts are source of valuable material

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.

Ecosystem Impact: creates habitat

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.

Key Reproductive Features: simultaneous hermaphrodite; sexual ; fertilization (Internal ); ovoviviparous

Parental Investment: pre-fertilization (Provisioning); pre-hatching/birth (Protecting)

When the tide rises, the plates covering the aperture open, and the thoracic appendages (known as 'cirri') are extended into the water current and used to filter particles of food from the water (2). During winter, barnacles do not feed, but rely on stored reserves (3). This barnacle is a hermaphrodite; individuals, although possessing both male and female reproductive organs, function as a male or a female (3). There is a single breeding season during autumn (2). Functional males extend the penis, which is much longer than the body, out of the shell wall and seeks a nearby functional female (3). After fertilisation, the embryos are stored within the barnacle's body, until they develop into 'naupilus' larvae. These are released into the water from February to May, and live in the water column feeding on plankton for several weeks. They undergo six moults, before developing into a second larval form known as a 'cyprid' larvae. This stage is specialised for seeking a suitable site for settling. They search the substrate with their antennae; once a suitable site has been found they release a substance that fastens them to the rock. This typically occurs in spring and early summer. They then undergo metamorphosis into the adult form (2). Sexual maturity may be reached in the first year after settlement, but it usually takes 2 years (3).

No conservation action has been targeted at this species.

Barnacles are well-known intertidal organisms. They were not firmly accepted to be crustaceans until the 1830s; before this time, the calcareous shell and sessile habit of the adults resulted in confusion with molluscs (2). Our current knowledge of barnacles is largely based on an 8-year period of research by Charles Darwin (3). Adult acorn barnacles are sessile; they have a cone shaped shell-wall comprising a number of calcareous pates. In this species, Semibalanus balanoides, the shell-wall consists of 6 greyish-white plates. The opening at the top of the 'cone' is diamond-shaped, and can be covered by movable plates when the tide goes out. This protects the animal inside from desiccation (2).

Inhabits rocky shores of all exposures, and forms the distinct greyish-white 'barnacle zone' on the shore with species of Chthamalus barnacles. When the two occur together, Semibalanus balanoides tends to occur lower down the shore than Chthamalus. Fewer barnacles occur where seaweeds dominate the shore, as the sweeping action of the seaweed fronds removes newly settled barnacles from the rock (2).

Found in the north-east Atlantic from Spitzbergen to north-west Spain, as well as the Pacific coasts of North America reaching south to British Columbia, and on the Atlantic coast south as far south as Cape Hatteras. It is common around all coasts of Britain, but is rare or absent from the south-west of Cornwall and the Isles of Scilly (3).

Common and widespread (2).

Not currently threatened.

In Great Britain and/or Ireland:
Animal / parasite / ectoparasite
adult of Hemioniscus balani ectoparasitises mantle cavity of Semibalanus balanoides

Animal / predator
Thais lapillus is predator of Semibalanus balanoides
Other: major host/prey

The acorn barnacle is the most prevalent barnacle along the North Sea coastline. These barnacles look like flat cones. The animal itself lives in this six-sided cone. Within the cracks and crannies of rocks, dikes or whatever they land on, they are often so squished in between their neighbors that they are forced to grow upwards. An unusual fact: larvae of the acorn barnacle living in waters which freeze near the North Pole, are caught in the ice and also freeze. However, they can survive life as an ice cube for several weeks; once they defrost, they just move on with life.

Semibalanus balanoides (common barnacle, common rock barnacle, northern rock barnacle) is a common and widespread boreo-arctic species of acorn barnacle. It is common on rocks and other substrates in the intertidal zone of north-western Europe and both coasts of North America.

northern Gaspe waters, downstream and upstream part of middle St. Lawrence Estuary, southern Gaspe waters (Baie des Chaleurs, Gaspe Bay to American, Orphan and Bradelle banks; eastern boundary: Eastern Bradelle Valley), Magdalen Islands (from Eastern Bradelle valley to the west, as far as Cape North, including the Cape Breton Channel), lower St. Lawrence estuary, Prince Edward Island (from the northern tip of Miscou Island, N.B. to Cape Breton Island south of Cheticamp, including the Northumberland Strait and Georges Bay to the Canso Strait causeway), upper North Shore (between Sept- Iles and Pointe des Monts), middle North Shore (from Sept - Iles to Cape Whittle, including the Mingan Island), lower North Shore, South slope of Anticosti Island, western slope of Newfoundland, including the southern part of the Strait of Belle Isle but excluding the upper 50m in the area southwest of Newfoundland, and the Southwestern slope of NL; Cobscook Bay

This species lives in the intertidal zones of the boreo-arctic regions of the Atlantic and Pacific oceans.

intertidal, infralittoral and circalittoral of the Gulf and estuary