Cirripedia

The sessile habit and interlocking shell plates of barnacles led early investigators to believe they were molluscs. Not until the 1830s, when barnacle larvae were studied, were they correctly recognised as crustaceans. Barnacles are more abundant in temperate waters than in the tropics, but nevertheless range throughout the GBR on all hard substrata, including the carapace of sea turtles and the shells of other crustaceans. More than 40 species and 20 genera of barnacles are presently known from GBR waters. The body of most barnacles is enclosed by a somewhat tubular or conical opened topped chamber formed by a series of interlocking calcareous plates—usually known as the shell. The animal is positioned on its back, and thrusts its modified, feathery thoracic limbs, known as cirri, out of the shell to strain food particles from the water.

Unlike most other crustaceans, barnacles are hermaphrodites, meaning that each animal possesses male and female reproductive organs. The majority of barnacles in the reproductive phase develop a greatly elongated penis (up to 10 times their body length) that is used to reach out of the shell to fertilise a receptive neighbour. The fertilised eggs develop into free-swimming larvae that eventually settle on the substratum close to others of their species, identified by their unique chemical signature.

Three major types of barnacles will be encountered on the reef: balanomorphs (Fig. 23.1B), lepadomorphs (Fig. 23.1C) and rhizocephalans. In balanomorph barnacles, the so-called acorn barnacles, the shell is firmly fixed to the substratum, be it rock, coral, mangroves or wharf piles. Common balanomorphs on the reef flat rocks and boulders include species of Balanus, Chthamalus, Megabalanus, Tesseropora, Tetraclita and Tetraclitella. Not all barnacles are fixed to the substratum by the shell, however. The lepadamorphs, the goose-necked barnacles, are attached to the substratum by a flexible stalk, with common genera including Lepas and Ibla. Conchoderma virgatum is unusual in using sea snakes as a substratum. In contrast to the balanomorphs

Figure 23.1 A, ostracod (Cypridinodes sp.); B, balanomorph barnacle (Chthamalus sp.); C, lepadomorph barnacle (Lepas anserifera); D, opossum shrimp (Mysida); E, comma shrimp (Cumacea); F, apseudomorph tanaid (Tanaidacea); G, cirolanid isopod (Cirolana curtensis); H, sphaeromatid isopod (Cerceis pravipalma); I, parasitic isopod on fish (Anilocra apogonae); J, gammaridean amphipod (Leucothoe sp.); K, gammaridean amphipod (Grandidierella sp.); L, caprellidean amphipod (Metaprotella sp.). (Photos: A, D-G, I-L, R. Springthorpe, © Australian Museum; B, C, © GBRMPA; H, © N. D. Pentcheff.)

Figure 23.1 A, ostracod (Cypridinodes sp.); B, balanomorph barnacle (Chthamalus sp.); C, lepadomorph barnacle (Lepas anserifera); D, opossum shrimp (Mysida); E, comma shrimp (Cumacea); F, apseudomorph tanaid (Tanaidacea); G, cirolanid isopod (Cirolana curtensis); H, sphaeromatid isopod (Cerceis pravipalma); I, parasitic isopod on fish (Anilocra apogonae); J, gammaridean amphipod (Leucothoe sp.); K, gammaridean amphipod (Grandidierella sp.); L, caprellidean amphipod (Metaprotella sp.). (Photos: A, D-G, I-L, R. Springthorpe, © Australian Museum; B, C, © GBRMPA; H, © N. D. Pentcheff.)

and lepadomorphs that simply use the substratum as an anchor point, the rhizocephalan barnacles have an entirely different lifestyle as partially internal and partially external parasites of decapod crustaceans. Rhizo-cephalans have lost any resemblance to typical adult barnacles, including loss of the calcareous plates. A common rhizocephalan, Sacculina, infects crabs and is visible externally only as a fleshy mass beneath the abdomen of the crab. Rhizocephalans interfere with the reproductive cycle of crabs and through the effect of hormones, cause male crabs to slowly develop female characteristics, an effect known as 'parasitic castration'.

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