Barnacles are hardy intertidal animals. They are sometimes mistaken for molluscs because of their exterior shell; but they are actually crustaceans, which makes them more closely related to crabs and shrimps than to limpets or clams. They are the only group of crustaceans with an adult sessile stage. More than 1,400 species of barnacle have been identified. At least five of these species inhabit intertidal areas in the Pacific Northwest. The two most common species in local waters are the acorn barnacle, Balanus glandula, and the little brown barnacle, Chthamalus dalli.
Description:
Adult barnacles have been described as "a little shrimp-like animal, lying on its head in a limestone house, kicking food into its mouth". The hard, cone-shaped shell, which is the most noticeable feature of the animals, surrounds the body of the barnacle. The shell consists of overlapping while or grayish-brown calcium carbonate plates. The size of the shell varies, depending upon the species. The larger B. glandula has a maximum shell diameter of 2.2 cm and shell height of 1 cm. In crowded conditions, their shell can grow into elongated columns. In contrast, C. dalli is much smaller with a maximum shell diameter of 6 mm and shell height of 4 mm, and they do not produce elongated shells. On top of the barnacle shell are separate plates called the operculum that cover the opening. In B. glandula the plates form a diamond-shaped operculum, whereas in C. dalli the operculum is oval-shaped.
Enclosed in the shell, the back of the adult, barnacle is cemented onto the substrate with one of the strongest known natural glues. The operculum must open to allow the barnacles' long jointed "legs" or cirri to sweep through the water to collect plankton for food. Their cirri also allow gases to be exchanged. Although well protected by their shell barnacles still need to sense changes in their environment. For example, adult barnacles have a single eye that is probably only able to detect differences in light intensity. When abiotic conditions are unsuitable or predators are present the barnacle can protect themselves by closing the opening with the operculum.
Enclosed in the shell, the back of the adult, barnacle is cemented onto the substrate with one of the strongest known natural glues. The operculum must open to allow the barnacles' long jointed "legs" or cirri to sweep through the water to collect plankton for food. Their cirri also allow gases to be exchanged. Although well protected by their shell barnacles still need to sense changes in their environment. For example, adult barnacles have a single eye that is probably only able to detect differences in light intensity. When abiotic conditions are unsuitable or predators are present the barnacle can protect themselves by closing the opening with the operculum.
Ecology:
LifBarnacles are exclusively marine animals. They have a worldwide distribution, which includes Antarctica and hydrothermal vents communities. All five species in the Pacific Northwest, including B. glandula and C. dalli, have ranges that extend from Alaska to California. Adult barnacles attach to rocks and other hard surfaces in exposed and protected sites. The larger, more competitive B. glandula primarily inhabits the mid-intertidal zone, whereas C. dalli, which can tolerate more desiccation, dominates the high intertidal zone, this means that they spends more than half of their life out of the water! Both species can occur in very high densities when conditions are optimal. For example, the small acorn barnacle can reach densities of 72,000 individuals/square metre. Together, both species form a distinct white band in the upper intertidal zone.
Barnacles are filter feeders that collect tiny plankton with their feathery legs, which form a net to scoop the plankton down towards the barnacle's mouth. In turn, adult barnacles are preyed upon by sea stars (e.g. Pisaster ochraceus) and predatory snails, such as whelks, which can drill a hole into the barnacle's shell.
Life History:
Most species of barnacle are hermaphrodites that may act as male or female barnacles. Unlike most sessile invertebrates that simply release their eggs and sperm into the water and leave fertilization to chance, barnacles will exchange sperm with a neighbouring barnacle. Being sessile, this presents challenges, as individual barnacles cannot leave their shells to mate. However, barnacles have a penis that can reach up to 20 times the length of its body allowing the barnacle to reach nearby mates. If a barnacle is out of penis-reach of another barnacle they can self-fertlize.
B. glandula and C. dalli produce several clutches of eggs per year, with a maximum brood size of 12,000 eggs. The eggs are brooded in a sac within the barnacle for up to six months. The eggs hatch into larvae, which are released into the open ocean to float with the currents for several weeks. During the pelagic larval stage barnacles are consumed by filter feeders, such as blue mussels (Mytilus spp.). The larvae go through several stages of development before settling to the substratum and metamorphosing in adults. Attachment space is often limited in the intertidal zone and barnacles will compete with other sessile organisms, including blue mussels, macroalgae, and other barnacles. Once successfully attached B. glandula can live up to 10 years while C. dalli only live for up to 3 years.
To learn more:
Barnes, R.D. 1987. Invertebrate Zoology, 5th edition. CBS College Publishing, New York, NY.
Hines, A.H. 1978. Reproduction in three species of intertidal barnacles from central California. Biological Bulletin, 154: 262-281.
Sept, J.D. 1999. The Beachcomber's Guide to Seashore Life in the Pacific Northwest. Harbour Publishing, Madeira Park, BC.