|General Biology Additional Information||Semibalanus balanoides has a membranous base, while Balanus crenatus has a calacareous base. Semibalanus balanoides is preyed on extensively by the dog whelk Nucella lapillus and the shanny Lipophrys pholis.
- Feeding: Semibalanus balanoides feeds by extending thoracic appendages called cirri out from the shell to filter zooplankton or similar sized organic particulates from the water (Rainbow, 1984). In the absence of any current, the barnacle rhythmically beats the cirri. When a current is present Semibalanus balanoides holds the cirri fully extended in the current flow (Crisp & Southward, 1961; Southward, 1955). Barnacles feed most during spring and autumn when plankton levels are highest. Little if any feeding takes place during winter, when barnacles rely on stored food reserves. Feeding rate is important in determining the rate of growth. Barnacles feed when they are immersed so barnacles low on the shore are able to feed for a longer time and consequently grow faster than those high on the shore (Barnes & Powell, 1953).
Moulting: Barnacles need to moult in order to grow. Frequency of moulting is determined by feeding rate and temperature. Moulting does not take place during winter when phytoplankton levels and temperatures are low.
- Growth: all barnacle species grow faster in early life and slower in later life. Growth rates recorded in the British Isles are given above (Anderson, 1994; Crisp & Bourget, 1985). Growth rate varies with a variety of biological and environmental factors, including current flow, orientation with respect to current, food supply, wave exposure, shore height, surface contour, and intra- or inter specific competition. Crisp (1960) concluded that un-interrupted current flow was the most important factor affecting growth and that growth was mainly determined by food intake. The influence of current, wave exposure and tidal level out-weighed latitudinal temperature influences in Semibalanus balanoides (Crisp & Bourget 1985). Individuals orientated with the rostral end, and hence the cirri, into the current flow gained a slight growth advantage over individuals of different orientation. Individuals that settled in pits grew slower than those on flat surfaces, perhaps since individuals in pits are removed from current flow, although should they out-grow the dimensions of the pits they grew normally (Crisp, 1960; Crisp & Bourget, 1985). At densities above 0.25/cm² barnacles compete for space, and, as soon as they touch, growth in diameter is replaced by growth in height, so that dry weight and volume continue to increase. However, at densities >1/cm² growth rate decreases with density. The presence of foliose species, e.g. filamentous algae, hydroids and bryozoans may also reduce growth, presumably due to reduced current flow over and food supply to the barnacles. (Crisp & Bourget, 1985). Growth is also reduced by the energy demands of reproduction and the presence of the cryptoniscid isopod parasite Hemioniscus balani.
- Parasites and epizoites: the midgut of Semibalanus balanoides is parasitised by the Gregarinid protozoan Pyxinioides balani while Epistylis horizontalis (a peritrich ciliate) lives on the gills and mantle (reviewed by Arvy & Nigrelli, 1969). Protozoan infestation may delay the release of nauplii. Metacercariae (a larval stage in the life cycle of trematodes) occur inside or near the gut of barnacles, e.g. Maritrema spp., a possible parasite of the turnstone (Arenaria intrepes morinella), terns or gulls, is found in Semibalanus balanoides (Rainbow, 1984; Arvy & Nigrelli, 1969). The cryptoniscid isopod Hemioniscus balani is a widespread parasite of barnacles, found around the British Isles, including Ireland, north to the Faroes and Oslo Fjord, and south to the Atlantic coast of France, as well as from Labrador to Massachusetts, New Scotland and Friday Harbour in the western Atlantic (Crisp, 1968). Hemioniscus balani is protandrous, the males becoming female after invading the host, eventually developing into a bloated, enlarged, star-shaped egg sac. An individual barnacle may contain up to 7 of theses parasites. Heavy infestation inhibits or destroys the gonads resulting in castration of the barnacle. (Rainbow, 1984; Crisp, 1968; Arvy & Nigrelli, 1969). The shell of British barnacles in the mid-shore may appear blackened due to the epizoic lichen Arthropyrenia sublittoralis (Rainbow, 1984). The crustose lichen Pyrenocollema halodytes can also grow on barnacle plates.