Barnacles are a bit weird. Even the common acorn barnacles that encrust rocks and jetties at the seaside can surprise you when you get in close enough and realise they are filter-feeding, upside-down crustaceans that build their own turret of a house, complete with opening and closing valves.
But another family of barnacles goes a step further into the world of peculiar adaptations. Go crabbing over the summer months and you’ll occasionally find a female crab in a condition referred to as ‘in berry’.
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This means she’s carrying a cargo of granular orange-pink eggs on the underside of her body.
However, sometimes this isn’t quite as it seems. If she’s got other commensal organisms such as barnacles or tube worms growing on her, and maybe looks a little more battered than usual, that lump on her underside probably isn’t an innocuous mass of eggs.
In fact, far from being the beginning of new life, that mass is the thing taking her own. It is a particularly dastardly species of parasitic barnacle called Sacculina carcini, which is also known, rather appropriately, as the crab hacker.
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Like any other barnacle, itstarts life as a plankton, but moults into a very different creature; a ‘cypris’ larva stage, when it looks more like a tiny, translucent turtle with swimming legs and, most importantly, odd hooked appendages up front.
If it is a female, this is the stage when it hooks itself onto its victim – in most cases the common shore crab. When firmly adhered in place, it moults again into a stage that sounds more like a character from Star Trek than a rock pool, and becomes a ‘kentrogon’.
With a long needle-like stylet, it drills through the crab’s armour. Once it has passed through, it all gets very strange indeed, as the barnacle effectively injects itself into its host. As an amorphous clump of cells, the barnacle has left the hard and crunchy world behind. It is now no more than a soft blob of tissue and passes down the hollow stylet into the crab’s bloodstream, taking up residence next to its gut and leaving the husk of its previous existence behind.
Here, it begins to subtly and insidiously hijack the crab’s life forces. Diverting food from the crab’s gut to its own development, it physically becomes a network of threads that ramify throughout the crab’s tissues, looking like the root system of a plant.
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As the ‘interna’– that is what the barnacle is now called – outgrows its crab-shaped crucible, it moves on to the next stage, which is the only time it becomes visible to the rock- pooling naturalist. It bulges out from the crab’s underside as a yellow-grey mass known as ‘externa’, which contains the eggs of the female barnacle.
What happens next is a little unclear, but free-swimming male cypris seem to be attracted to the female’s outward bulge and enter via a pore, joining the parasitic mass to become on-tap fertilisers of the thousands of eggs she produces. The larvae are then released into the surface currents, where they will waft off and start the whole dark, sinister cycle again.
