Acorn Barnacle - Balanus glandula
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence distantly related to crabs and lobsters.
Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages.
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.
Other members of the class have quite a different mode of life. For example, members of the genus Sacculina are parasitic, dwelling within crabs.
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.
Although they have been found at water depths up to 600m, most barnacles inhabit shallow waters, with 75% of species living in water depths of
less than 100m, 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.
Most barnacles are hermaphroditic, although a few species are gonochoric or androdioecious. Typically, recently molted hermaphroditic individuals are receptive as females. Self-fertilization,
although theoretically possible, has been experimentally shown to be rare in barnacles.
The sessile lifestyle of barnacles makes sexual reproduction difficult, as the organisms cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles
have extraordinarily long penises, up to 15 cm in length: the largest penis to body size ratio of the animal kingdom.
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.
Barnacles have 2 distinct larval stages, the nauplius and the cyprid, before developing into a mature adult.
A fertilized egg hatches into a nauplius: a one eyed larva comprising a head and a telson, without a thorax or abdomen. This undergoes 6 molts before transforming
into the bivalved cyprid stage. Nauplii are typically initially brooded by the parent, and released as free-swimming larvae after the first molt.
The cyprid stage lasts from days to weeks. During this part of the life cycle, the barnacle searches for a place to settle. It explores potential surfaces with modified antennules
structures; once it has found a potentially suitable spot, it attaches head-first using its antennules, and a secreted glycoproteinous substance. Larvae are thought to assess surfaces
based upon their surface texture, chemistry, relative wettability, colour and the presence/absence and composition of a surface biofilm; swarming species are also more likely to attach
near to other barnacles. As the larva exhausts its finite energy reserves, it becomes less selective in the sites it selects. If the spot is to its liking it cements down permanently
with another proteinacous compound. This accomplished, it undergoes metamorphosis into a juvenile barnacle.
Typical acorn barnacles develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives they are cemented to the ground, using their feathery legs (cirri) to capture plankton.
Once metamorphosis is over and they have reached their adult form, barnacles will continue to grow by adding new material to their heavily calcified plates. These plates are not moulted;
however, like all ecdysozoans, the barnacle itself will still molt its cuticle.
Barnacles and limpets compete for space in the intertidal zone. Barnacles are displaced by limpets and mussels, who 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 in length; other species may grow larger still.
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 by the latter group.