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This is a barnacle.

Chances are,   you've seen one before.

They’re found  in virtually all marine environments,   stubbornly attached to any surfaces they can find  - from rocks and boats to whales and turtles.

They may not look like much, but beneath that  shell lies an evolutionary mystery - one that   stumped the biggest names in natural  history for over a hundred years.

Some of the most influential biologists of their  day grappled with the challenge of working out   what they actually are and where they fit in the  tree of life, only to be proved totally wrong.

Others were almost broken by the  attempt – including Charles Darwin,   who embarked on an eight-year-long  personal side-quest to figure them out.

By year six, he wrote to a friend, “I hate  a Barnacle as no man ever did before”.

In the end, though, the mystery of the barnacles   would help him figure out part  of the mystery of life itself.

So what’s the deal with barnacles?

How  did they resist classification by some   of the biggest names in natural history?

And why did they make Darwin so mad?!

First, a quick barnacle 101.

This group of marine animals, known  scientifically as the Cirripedes,   is diverse, ancient, and wide-ranging.

Today, there are over 1000 known  species found across the world.

They generally exist as immobile filter  feeders that stay attached to surfaces,   and waft plankton and food particles  into their mineralized shells – though   some species live as parasites of other animals.

And we know from the fossil record that  they’ve been around for a very long time.

Definitive shelled barnacles first show up  in the mid-Carboniferous period around 320   to 330 million years ago, though  some are argued to be even older.

And we know of more than 400 extinct species,  mostly from fossilized shell fragments.

But even though they’ve been around  for hundreds of millions of years   and are common today, for much of  history we got them totally wrong.

In the mid-1700s, Carl Linneaus, often considered  the father of modern taxonomy – a.k.a.

the study   of classifying living things – was among  the first to try and place the group.

He saw barnacles as being similar to  some marine mollusk species that also   have mineralized shells and don't move much,  like oysters, clams, and mussels.

Which, fair.

So, he put barnacles in the mollusk group, too.

Then, in the early 1800s, the  French naturalist Georges Cuvier,   sometimes called the father of paleontology,  also turned his attention to barnacles.

He noted that, beneath their shell, they  seemed to share some traits with another   animal group called the articulates, which  included things like crustaceans and insects.

In the end though, when he published  his most famous work in 1817,   he also decided to classify barnacles as mollusks.

But then, in 1830, British naturalist William  Thompson entered the chat with a pretty bold   claim: those two world-renowned  naturalists had been fooled.

See, barnacles go through a free-swimming stage   as juveniles, before they stick  themselves to a surface for good.

Thompson observed this and realized that,  during part of their development, juvenile   barnacles were pretty much identical  to a juvenile stage of crustaceans.

So he figured that they must actually be  crustaceans, too, which makes them relatives   of shrimp, crab, and lobsters.

They just happen to  have an adult stage that resembles many mollusks.

It might not sound like much, but this was a   radical re-interpretation of  what barnacles actually are.

It’s rare in zoology for a  group of common, macroscopic,   living animals to be reclassified  at one of the higher levels of the   nested taxonomic system that we use – the  ones above family, genus, and species.

The higher up the taxonomic ranking that you  go, the more fundamental the differences get,   so it’s usually pretty obvious which group a  living organism belongs to at those higher levels.

And, as a result, the higher the  level of a proposed reclassification,   the more profound its implications are for  our understanding of the nature of a thing.

And this was one example of a deep and  significant shift of our understanding:   mollusks and crustaceans aren't even  part of the same animal phylum – the   rank right below kingdom, which tells  you if a thing is an animal or a plant.

It was the equivalent of scientists suddenly   announcing that birds are  actually a type of starfish.

The scale of this sudden reanalysis is partly what  triggered Charles Darwin to join the fray in 1846.

Barnacles had previously captured his  attention during his voyage around the   world on the HMS Beagle, when he came across  some species that left an impression on him.

So when they became a hot zoological topic  following Thompson’s radical proposal,   Darwin probably saw it as a great opportunity.

If he could bring some order to this  messy and confusing barnacle situation,   it would help him puzzle through some of the  evolutionary concepts he’d begun to think about… Like, the approach of trying to  group organisms on the basis of   common descent from a shared ancestor  by comparing their anatomical features.

Plus, if he could develop some expertise around  a poorly understood group like barnacles,   it would help him build his scientific credibility   before revealing his ideas about  natural selection to the world.

How hard could it be?

A quick side-quest  that would take a year or two, max.

In fact, it took eight.

Over those long years, he did  make several major insights   into the diversity and biology of  both living and fossil barnacles.

And in the process, he became recognized as  one of the world’s leading authorities on them.

But the strain of intensively studying such a   weird group of animals was almost  too much for him to take at times.

In one letter, he wrote: "I make no  perceptible progress… & I groan under my task.” And, on what must have been a particularly  rough day during the barnacle years, he said:   "confound & exterminate the whole  tribe… I can see no end to my work".

Despite being so frustrated by  these creatures that he found   himself wishing that they never  existed - a feeling familiar to   many graduate students - Darwin did  eventually see the end of his work.

His detailed analysis of modern and extinct   species is still important to  the study of barnacles today.

He helped confirm Thompson’s radical  reclassification of barnacles as a   branch of the crustacean family tree,  and they’ve stayed there ever since.

Today, we know them as a lineage  of crustaceans that embarked on a   totally different evolutionary journey  from any other members of the group.

While they did keep some crustacean traits, like  the free-swimming larval stage, cuticles made of   chitin, and segmented bodies, they also developed  some distinctly un-crustacean traits, too… For example, their mineralized external  shells and immobile adult stage where   they basically cement themselves  to a surface.

These two features   are typical of some mollusk groups, but  aren't seen in any other crustaceans.

And it seems that the reason this evolutionary  convergence with mollusks fooled some of the   biggest old-timey names in biology is that it  runs particularly deep, right down to their DNA.

In 2022, researchers in China published a new  study in which they compared barnacle and shelled   mollusk genomes to look for signs of convergent  evolution between them at the molecular level.

Despite having been on distinct evolutionary  journeys for hundreds of millions of years,   the researchers found several  instances of genetic convergence.

For example, they independently duplicated many of   the same families of genes, often  having to do with shell formation.

And some of the convergently evolved genes were   only seen in the two groups and not  in either of their closest relatives.

In fact, at the time the paper was  published, it was the most distant   reported example of two animal groups  showing genome-wide convergent evolution.

So it’s kind of understandable that  barnacles baffled biologists for so long.

Their physical convergence with mollusks is backed   up by an equally impressive level of  genetic convergence with them, too.

And this is something that none of those  big names could have ever come up with,   let alone investigated, seeing as they all lived  before anything was really known about genetics.

But, at least in the case of Darwin,  though the struggle to understand   the barnacles was long and painful,  it turned out to be well worth it.

Not just because the effort built him  some valuable scientific street cred.

But also because, in all  that stress and confusion,   came occasional glimmers of  evolutionary enlightenment.

For example, as he dissected, described, and  compared every barnacle specimen he could get his   hands on, he was struck by how much variation he  saw, even between individuals of the same species.

This realization about the extent  of variation between individuals   would later become a key pillar of his  theory of evolution by natural selection.

Variation is the raw material for evolution.

It’s what natural selection acts on,   favoring some variants relative to  others in a population because of the   survival and reproductive advantages they offer.

So, by studying barnacles in such detail for  so long, Darwin gained a deep appreciation   of the subtleties of biological variation  available for natural selection to act on.

And working on this enigmatic, ancient,  and diverse group allowed him to test   and refine critical aspects  of his revolutionary ideas.

While it seemed like just a detour from his  main project in the beginning, in the end,   we owe a lot of our understanding  of the natural world to Darwin’s   frustrating work on the mystery of the barnacle.

I can see no end - wait, I gotta do it... (In a faux-Victorian accent) I can see no end  to… how awesome this month’s Eontologists are!

Gale Brown, Juan M., Jacksy Weiss, Melanie Lam  Carnevale, Annie & Eric Higgins, John Davison Ng,   Colton, Chase Archambault, and Jake Hart.

By becoming an Eonite at patreon.com/eons you   can get fun perks like submitting a joke for  us to read.

Here’s one from Joey Litzinger.

Why did the plesiosaur cross the  ocean?

To get to the other tide!

Ha-ha-ha!

That was great, that was really cute.

And as always thank you for joining me   in the Adam Lowe studio.

Subscribe at  youtube.com/eons for more primordial ordeals.

Okay, I'm gonna do this one more time,   in my best Victorian man voice.

So, bear with me.

By year six, he wrote to a friend,   (in Victorian Man Voice) “I hate the  Barnacle as no man ever did before”.

You gotta   express, express the Victorian  pain of Darwin from deep within.