Common rock barnacle

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Onglets

Description

Distinguishing features

The common rock barnacle is an arthropod. It appears in two forms: at the larval stage, it is mobile and swims in open water. At the adult stage, it is immobile and attached to a rock.

In fact, the adult barnacle is nothing other than a modified “shrimp” that lives attached by its back to a rock and that has made itself a calcareous shell for protection (we say that it is sessile, or lacking in self-locomotion). Despite its six double limbs, at the adult stage it cannot move around (only the larvae are mobile). Its legs (which are called cirri) look like fine feathers and are used to filter food from the water. The barnacle’s shell is made up of four, six or eight calcareous plates, depending on the species.

Semibalanus balanoides, the most common species in the Gulf of St. Lawrence, has six plates. Its shell, in the shape of a small truncated cone, resembles a miniature volcano. The opening above the shell is blocked by two other calcareous plates, which open and close with the help of small muscles. These two plates make up what is called the operculum.

The calcareous shell is greyish white but may also contain some yellow, orange or pink. If the operculum is opened, the flesh inside is white or pinkish white. The adult barnacle has no eye.

In all, there are six barnacle species in the Gulf of St. Lawrence.

Reproduction

Although the barnacle is hermaphroditic (both male and female at the same time), it cannot fertilize itself. Therefore it needs a partner. The barnacle possesses a long stretchable penis, but given the fact that the barnacle cannot move, another barnacle has to be very close by, a maximum of three to five centimeters away.

Barnacles breed when water temperatures reach 14 °C, and can produce up to 10,000 eggs. The barnacle keeps its eggs close to itself, inside its shell.

Upon hatching, the larvae, which have three pairs of limbs, an antenna and a single eye, are released in the water and go off in search of planktonic life. These larvae will undergo a total of six metamorphoses, each one more complex than the last, and with each stage lasting three to five days. The pace of these metamorphoses depends on temperature and the abundance of food.

In the last stage, it will often take up to two weeks for the crustacean to find a spot to attach itself to – a rock, for example. It will use its antennae to explore the ground, and will discover other members of its species thanks to chemical sensors. The barnacle will then begin to produce plates of calcium carbonate (limestone).

Diet

To eat, the barnacle has to open its operculum. Little muscles allow the two calcareous plates to shift and let the thoracic limbs appear. These have the appearance of little feathers equipped with stiff hairs that serve to catch its tiny prey. The movement of the limbs, which resembles the motion of a fan, creates a water current that attracts plankton to the mouth.

The barnacle does not only eat small planktonic animals (such as foraminifera, copepods, amphipods, cnidaria and mollusks): it also eats plant plankton, grains of pollen, and sometimes, accidentally, it ingests grains of quartz (sand). The food is then transferred to the mouth, where numerous mouthparts will shred it before it is ingested.

Predators

The adult barnacle may have no eye, but it is sensitive to variations in brightness. Whenever there is movement and a shadow appears, it closes its operculum and takes shelter from a potential predator.

The barnacle’s main enemies are limpets, nudibranchs (kinds of sea slugs) and a gastropod called dog whelk (Nucella lapillus).

Habitat

The Nucella lapillus barnacle is a resident of the Atlantic coast, from the Arctic to Delaware Bay, including the Gulf of St. Lawrence. It can also be found around Great Britain and down to southern Spain, as well as in the northeastern part of the Pacific Ocean. It lives on rocky bottoms, large stones, dock beams, boat hulls and even the shells of turtles or on whales.

Barnacles are sometimes so numerous that they pile up on one another. They become shapeless, and grow only in height.

The primary factor determining barnacles’ location is the movement of the water (ocean currents facilitate access to food). Light and form of support (rocks, or others) are also important factors.

Ecology, behaviour

Barnacles play an important role in the ecosystem. In effect, they consume millions of eggs and planktonic larvae, in the process modifying the entire ecosystem by preventing certain animals from becoming too abundant. They compete with oysters, for example, in the consumption of plankton.

In every seaport on the planet we find several barnacle species from various distant regions of the world. Attaching themselves to the hulls of ships, barnacles travel around the globe, and if conditions are favorable, they can easily establish themselves in new waters.

French name

Balane commune

Scientific name

Semibalanus balanoides

Phylum

Arthropoda

Class

Crustacea

Order

Cirripedia

Family

Archaeobalanidae

Size

From 25 to 75 mm diameter

Life span

From 1 to 7 years

Status

Not rated

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