Blistering Barnacles – by post!

Anelasma squalicola - note the small cirri & the root-like filaments on the peduncle (Adapted from fig. 2 & 3, Pl. IV, Cirripedia by Charles Darwin. Source: Biodiversity Heritage Library/MBLWHOI Library)

Anelasma squalicola – note small cirri & root-like filaments (from fig. 2 & 3, Pl. IV, Cirripedia by Charles Darwin. Source: Biodiversity Heritage Library/MBLWHOI Library)

The Natural History Museum of Denmark has just discovered a collection of barnacles sent by Charles Darwin to Japetus Steenstrup in 1854 as a thank you gift – a “very inadequate return” (Letter 1589, 7 Sept [1854]) – for the many cirripede specimens that Steenstrup had sent him. Among these was one which turned out to be of the highest significance from an evolutionary viewpoint.

 

How important was Steenstrup’s gift to Darwin? Important enough that when he thought the parcel might be lost in the post, Darwin was ready to put an advertisement in the Times newspaper and offer a “large reward” for its recovery, for he had been told it contained a specimen of Alepas squalicola, “the cirripede of all others in the world,” he wrote “I wish most to dissect”. (Letter 1273, 1 Dec [1849])  All was not lost, however. It turned out the parcel had been packed within another box sent to a dealer in minerals and was merely delayed. A relieved and happy Darwin reported its arrival to Steenstrup: he was particularly pleased, he said, “to see the Alepas” (Letter 1297, 25 Jan [1850]).

 

What Darwin found when he was finally able to to investigate this “curious cirripede” was an anatomy that seemed to show the process of evolution in progress. He renamed the species, giving it its own new genus, Anelasma squalicola, and only just stopped short of giving it its own family (it was more than fifty years later when that happened). Among many anomalies, Darwin observed a few key points. Adult barnacles fix themselves to a substrate (anything from a rock to a ship to a turtle or a whale) using cement produced by specialised glands. Anelasma attaches itself to small sharks (dogfish), but Darwin couldn’t find any cement or cement glands. A further anomaly was the fact that its stomach was completely empty. What he did find was a network of filaments branching out from the peduncle – the flexible stalk by which it was attached.  Of these filaments he observed, “it is particularly difficult to understand their growth, for it is not possible, after examining them, to doubt that they continue to increase, and send off sub-branches, which it would appear probable, penetrate the shark’s flesh like roots” (LC 1851, p. 173). Darwin didn’t speculate too much in his official description, beyond noting the features that clearly intrigued him.

 

Fourteen years later, having read a description of an even more bizarre parasitic barnacle, Darwin wrote to Fritz Müller, “Until reading your book I knew nothing of the Rhizocephala; pray look at my account & figures of Anelasma for it seems to me that this latter Cirrepede is a beautiful connecting link with the Rhizocephala.” (Letter 4881, 10 Aug [1865]). Anelasma appeared to be mid-way between a traditional filter feeder and a true parasite, retaining organs of the former, but adapting to a new parasitic way of feeding with its modified peduncle.

 

None of these observations would have been possible without the single, rare specimen sent by Steenstrup that almost got lost along the way!

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