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Page last updated at 12:34 GMT, Tuesday, 22 December 2009
Barnacles become toxic to repel hungry predators
By Matt Walker
Editor, Earth News

Stalked ibliform barnacle (Chaetolepas calcitergum)
The rare and very unpalatable stalked ibliform barnacle

One species of a rare, ancient barnacle has extraordinarily high levels of a toxic chemical in its body, scientists have discovered.

Up to 7% of certain parts of the barnacle's body is bromine, with the chemical concentrated into the animal's most vulnerable parts.

The sessile crustacean likely hoards the chemical as a defence mechanism to repel predators.

The discovery is published in the journal Integrative Zoology.

The crustacean Chaetolepas calcitergum is known as a stalked ibliform barnacle.

Only six specimens are known of the species, which, because of its small size, is unlikely to be spotted by fishermen or researchers.

Ibliform barnacles, a type of goose barnacle, first arose in the Palaeozoic era, making them an ancient lineage that evolved before all other barnacles.

Barnacles are normally hermaphrodites
But ibliform barnacles often have two sexes, with dwarf males possessing two huge testes
In some species, dwarf males live within a larger hermaphrodite
Larvae may also live alongside dwarf males within the hermaphrodite parent

But like other barnacles, ibliformes are sessile, spending their adult lives attached to a substrate such as rock.

Because barnacles cannot move, they are vulnerable to drying out, and to predators from which they cannot flee.

In response, most barnacles have evolved strong protective shells.

However, Chaetolepas calcitergum has only a weakly mineralised outer coating, and is therefore vulnerable.

So it appears to have become toxic instead, in a bid to repel predatory gastropods that like to feed on barnacles.

Professor John Buckeridge and Dr Jessica Reeves of the Royal Melbourne Institute of Technology (RMIT) in Melbourne, Australia made the discovery while conducting a routine chemical analysis of a specimen of Chaetolepas calcitergum.

Chaetolepas calcitergum (line drawing)
Chaetolepas calcitergum (line drawing)

"The elevated levels of bromine were a surprise, I wasn't expecting this at all," says Prof Buckeridge.

The surface of the whole barnacle comprised about 1.5% bromine by dry mass.

But some regions contained up to 7% bromine by dry mass.

Because the toxic chemical is concentrated in the most vulnerable parts of the crustacean, the researchers strongly suspect it is used by the animal to defend itself against being eaten by predators.

"Bromine and bromine compounds are rather toxic. Their presence would deter predators or grazers from eating the host," says Prof Buckeridge.

The researchers now hope to investigate whether many other barnacle species also compensate for being sessile by having toxic bodies.

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