Barnacles are crustaceans which are attached to the substrate from the head downwards. The abdomen of Balanus is reduced at the posterior end of the thorax and calcareous shell plates are used to protect the enclosed body from predators. The shell of sessile barnacles consists of a ring of overlapping calcareous plates, whose number and arrangement differ between the species. The shell walls of each species varies, and can consist of solid calcite or be porose with empty canals running vertically between an inner and outer lamina (1). There are over 110 species and subspecies within the genus of Balanus (2).
As they have an exoskeleton, barnacles must moult in order to grow. They moult by shedding the cuticle lining the body and the mantle cavity at intervals. Increased feeding and temperature have both been noted to increase the frequency of barnacles (3).
Most barnacles are cross-fertilising hermaphrodites. Seasonally developed testes form spermatozoa. The penis is well supplied with blood and is extensible. A pair of ovaries are situated in the mantle, and an oviduct leads from each to the basal segment of the first cirrus. It opens to the exterior via an oviducal gland. Each gland secretes an elastic ovisac into which the eggs are deposited. As more eggs are laid, the sacs are able to expand into the mantle cavity (4). The greatly extended penis of a functional male barnacle makes random movements in order to locate a suitable female (5). Once contact has been made with a female, the movements of the penis become more fluid, and sperm is transferred into the mantle cavity of the female. The larva has been noted to prefer to settle in depressions and will orientate themselves along grooves (6). They also appear to prefer to settle out of strong light and are orientated with the anterior end towards the light (7). Barnacles prefer to settle near other barnacles, and settle in an aggregative, gregarious manner (8).
Balanus are found from the midlittoral zone to several hundred metres below sea level (9). They are known to attach to any hard surface including rocks, piers, ship hulls, oyster shells and mangrove roots (2).
Balanus are found from the Mediterranean coast to the North Norwegian shores (9).
Barnacles are selective feeders, rejecting particles at or near the mouth and on the cirral net. They also show evidence of chemoreception – their feeding activity appears to be stimulated by amino acids and its associated compounds in particular (10). The cirri are used to extract particles from the water column. When they draw back, the food is then scraped into the mouth (2).
Not evaluated under the IUCN Redlist (11).
Description written by Kathryn Woodward (2009)
(1) Bourget, E., 1977. Shell structure in sessile barnacles. Le Naturaliste Canadian, 104, pp.281-323.
(2) ZipCodeZoo, 2009. Balanus sp. [Online] Available at: http://zipcodezoo.com/Key/Animalia/Balanus_Genus.asp [Accessed 03 September 2009].
(3) Crisp, D.J. & Patel, B.S., 1960. The moulting cycle in Balanus balanoides (L). Biological Bulletin, 118, pp.31-47.
(4) Walker, G., n.d. A study of the oviducal glands and ovisacs of Balanus balanoides (L) together with comparative observations on the ovisacs of Balanus hameri (Ascanius) and the reproductive biology of the two species. Philosophical Transactions of the ROyal Society of London Series B, 291, pp.147-62.
(5) Munn, E.A., Klepal, W. & Barnes, H., 1974. The fine structure and possible functions of the sensory setae of the penis of Balanus balanoides (L). Journal of experimental marine biology and ecology, 14, pp.89-98.
(6) Crisp, D.J. & Barnes, H., 1954. The orientation and distribution of barnacales at settlement with particular reference to surface contour. Journal of Animal Ecology, 23, pp.143-62.
(7) Barnes, H., Crisp, D.J. & Powell, H.T., 1951. Observations on the orientation os some species of barnacles. Journal of Animal Ecology, 20, pp.227-41.
(8) Crisp, D.J., 1974. Factors influencing the settlement of marine invertebrates larvae. In P.T. Grant & A.M. Mackie, eds. Chemoreception in Marine Organisms. London: Academic Press.
(9) Wirtz, P. & Debelius, H., 2003. Mediterranean and Atlantic Invertebrate Guide. Hackenheim, Germany: ConchBooks.
(10) Crisp, D.J. & Southward, A.J., 1961. Different types of cirral activity of barnacles. Philosophical Transactions of the Royal Society of London Series B, 243, pp.271-308.
(11) IUCN 2009. IUCN Red List of Threatened Species. Version 2009.1. <www.iucnredlist.org>. Downloaded on 03 September 2009.