Don’t forget: Our 2023 calendar featuring some of our favorite “main characters of
paleontology” from classic Eons episodes is available now at ComplexlyCalendars.com!
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.