Barnacles are encrusters, attaching themselves permanently to a hard substrate. The most common, "acorn barnacles" (Sessilia), are sessile, growing their shells directly onto the substrate. The order Pedunculata ("goose barnacles" and others) attach themselves by means of a stalk.
Most barnacles are suspension feeders; they dwell continually in their shell – which is usually constructed of six plates – and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption.
Other members of the class have quite a different mode of life. For example, members of the genus Sacculina are parasitic, dwelling within crabs.
Although they have been found at water depths up to 600 m (2,000 ft), most barnacles inhabit shallow waters, with 75% of species living in water depths of less than 100 m (300 ft), and 25% inhabiting the intertidal zone. Within the intertidal zone, different species of barnacle live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined.
Since the intertidal zone periodically desiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they possess two plates which they can slide across their aperture when not feeding. These plates also protect against predation.
Barnacles are displaced by limpets and mussels, which compete for space. They also have numerous predators. They employ two strategies to overwhelm their competitors: "swamping" and fast growth. In the swamping strategy, vast numbers of barnacles settle in the same place at once, covering a large patch of substrate, allowing at least some to survive in the balance of probabilities. Fast growth allows the suspension feeders to access higher levels of the water column than their competitors, and to be large enough to resist displacement; species employing this response, such as the aptly named Megabalanus, can reach 7 cm (3 in) in length; other species may grow larger still (Austromegabalanus psittacus).
Competitors may include other barnacles, and there is (disputed) evidence that balanoid barnacles competitively displaced chthalamoid barnacles. Balanoids gained their advantage over the chthalamoids in the Oligocene, when they evolved a tubular skeleton. This provides better anchorage to the substrate, and allows them to grow faster, undercutting, crushing and smothering the latter group.
Among the most common predators on barnacles are whelks. They are able to grind through the calcareous exoskeletons of barnacles and feed on the softer inside parts. Mussels also prey on barnacle larvae. Another predator on barnacles is the starfish species Pisaster ochraceus.
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