Mr. Arthrobalanus

Today, Charles Darwin is mostly remembered for the theory of evolution, natural selection and his four most famous books"The Voyage of the Beagle", "On the Origin of Species", "The Descent of Man" and "The Expression of the Emotions in Man and Animals". Few realize he published some 21 major books (one he was a writer an editor of a multi-volume set which I have counted as one book) and numerous papers. Even without the theory of evolution and the four most popular works, Charles Darwin was an eminent scientist, well published in geology, general zoology and in the area that most concerns us... invertebrates.

Of the many books he published, one was about the structure of coral reefs, two were about the interactions of plants and insects; five were completely about invertebrates. Of those only one, "The Formation of Vegetable Mould, through the Actions of Worms, with Observations on Their Habits" was not about barnacles. He spent 8 years and published four monographs on the barnacles.



First the back story. Barnacles were, when Charles Darwin went off to university, understood to be molluscs. Linneaus and Cuvier both reasoned they were molluscs because of the morphology of their shells and the presence of a sea water filled "mantle" cavity. In 1830 John Thompson showed they were related to crustaceans when he published the results of watching the metamorphosis of a nauplius into a cyprid larvae and then into and adult barnacle.

By 1844 Charles had almost completed the description of the specimen collected on the Voyage of the Beagle, and he had completed the first draft of On the Origin of Species. 1844 was also the year that Vestiges came out, which caused Darwin to rethink publishing "On the Origin" in its current form, and which prompted his friend Hooker to remark in 1845: "no one has the right to examine the question of species who has not minutely described many."

Darwin returned to writing "Geological Observations on South America" and by 1846 had cleared his desk, except that "illformed little monster" he would later come to call Mr. Arthrobalanus, a microscopic barnacle. Darwin figured it would take a few months, at most a year to examine, describe and publish the results. Then he could return to the issue of revising and publishing "Origins". 8 Years later he finished with the barnacles, having written 4 monographs on them.

I had originally intended to have described only a single abnormal Cirripede, from the shores of South America, and was led, for the sake of comparison, to examine the internal parts of as many genera as I could procure.

Unfortunately Mr. Arthrobalanus turned out to be a vexing character – a microscopic, parasitic barnacle. Darwin requested barnacles from friends and colleagues around the word to try and understand how arthrobalaus fit in. He began looking at life histories and larval forms. With each new species loaned to him, he sank deeper into the world of Cirripedia. Loius Agassiz called out for someone to revise the group, since had not been evaluated since becoming part of the crustaceans, a move which required a complete re-evaluation of their anatomy. In Darwin he found a willing patsy author.

For years he studied barnacles from around the world. The modern barnacles he dissected, the fossils he disarticulated. Yep, he did the fossils too, since no complete re-examination of the barnacles would be complete without re-examining the multitude of fossilized species. Darwin both suffered and enjoyed the challenge of the work. He suffered from eye and mental fatigue dissecting each day, but he made discoveries that kept him excited:

I believe Arthrobalanus has no ovisac at all!, & that the appearance of one is entirely owing to the splitting, & tucking up to the posterior penis, of the inner membrane of sack.— I have just found a Cirripede with an indisputably abortive anterior penis; so that this chief anomalous feature (viz two penes) in Arthrobalanus is in some degree brought within bounds.—

In short order Darwin had trained himself to become a taxonomist. He relied on homology and embryology for both description and as a key to the evolutionary relationships between group members lines of descent. In the 8 years he worked on Barnacles he soon became the worlds leading authority on the subject and quickly met his friend Hooker's objection to Vestiges, since he had now described many indeed, in creating the first modern taxonomic description of the Cirripedia. This "detour" actually provided time to rethink "On the Origin" and provided him with invaluable experience describing species and their relations to create a more compelling argument of evolution.

Soon after publishing the first monograph on living cirripedia, Charles Darwin was awarded the Royal Medal of the Royal Society of London, for his work. Clearly Mr. Darwin was a friend of the invertebrates.

Oh, Darwin renamed Mr. Arthrobalanus to be Cryptophialus minutus a burrowing or boring barnacle, a rather curious and unusual group of barnacles that includes sexual dimorphism and reduced males.

The sources for all of this post are Darwin Online and the Darwin Correspondence Project, a pair of excellent resources for studying anything Darwin, especially with the PDF's of his Ciripedia Monographs (PDF files)!