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British Beasts | Barnacles

Barnacles are hermaphrodites with the longest penises in the animal kingdom

The only way the lowly barnacle is going to impress most people is that it has, relatively speaking, the longest penis in the animal kingdom.

 At four times its body length, in human terms this is roughly equivalent to the average 1.8m bloke having a penis about 7m long! Doesn’t bear thinking about. But there is a good reason for barnacles to have this apparently excessive manhood. Living fixed to the rock for the whole of its adult life, a barnacle’s choice of mate is limited to other barnacles within reach. The longer its penis, the greater the chances of impregnating more than one neighbour and producing offspring. 

Barnacles are hermaphrodite, and can fertilise each other. The resulting embryos are brooded inside the parent’s shell for a while, eventually being released as distinctive little larvae that actively swim in the plankton. Millions of them contribute to the dip in visibility in British waters in spring, as an explosion of life makes our seas temporarily murky. Barnacle larvae feed on tiny algae (phytoplankton), moulting their skins several times as they grow.

When they are ready to settle on the sea bed, a larva picks a spot near an adult barnacle. This has two advantages. Firstly, if adults are there already, it’s obviously a suitable spot for barnacle survival; secondly, it’s critical for mating that it settles within penis-reach of another barnacle. So the larvae often fill in the gaps between adult barnacles, creating densities of up to 10,000 barnacles per square metre on exposed seashores. Once a barnacle larva has decided where to settle, it cements its head firmly to the rock, and that’s it fixed for life.

A barnacle’s other body parts are equally specialised for its unique lifestyle. Although they don’t look much alike, barnacles are related to crabs and prawns, all having a basic crustacean structure. The major difference is that crabs and prawns use their jointed legs to walk around, while adult barnacles, which cannot walk, use them instead to collect food. A hairy, fan-shaped foot emerges from the shell to filter particles of food from the seawater, after a few seconds pulling back inside to transfer food to the mouth. Watching a dense bed of barnacles feeding is an extraordinary sight, with thousands of little fans waving rhythmically. The side of a slipway at high tide is often a good place to see this.

Their tough shells protect barnacles from many predators. A few fish, like the shanny, are adept at nipping off a barnacle’s foot as it comes out to feed. The chocolate-chip se slug (see DIVE, August 2001) is a particular enemy of barnacles. Less than 2cm long, this sea slug sometimes occurs in ‘swarms’, which chomp their way through large areas of barnacles, leaving only bare rock covered with white scars where barnacle shells were once attached. When you see this local devastation, it brings home the importance of barnacles as an attachment for all sorts of other life. Barnacles are often first colonisers on new wrecks, settling on smooth surfaces and on materials that the larvae of other animals find unattractive. Once the barnacles have settled, their calcareous shells provide a good surface for many other animals to attach. However if the underlying barnacles are killed, everything else goes too. So, even if their main use to divers is as a handy non-slip surface on a shore dive, ecologically speaking the lowly barnacle is a pretty important creature.



BeAst at a glance  

ACORN BARNACLE, SEMIBALANUS BALANOIDES

Known haunts: Rocky seashores, reefs and wrecks.

Best place to see: Abundant on wave-exposed and current-swept shores and sea beds all around Britain.

Likely to appear: Present all year.

Distinguishing features: Barnacles are tiny crustaceans living inside a conical, calcareous shell made up of several plates joined together, cemented to the rock. A hole at the top is sealed by more plates, which are moved aside for its fan-shaped foot to emerge when feeding. En masse, barnacles form a whitish zone on exposed seashores. There are around 14 ‘acorn’ barnacle species (ones without stalks) in British waters, identified mainly by the shape, size and number of their shell plates.

Size: Most are less than 1cm across.

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