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[♪ INTRO]
If I asked you to picture a crab, chances are you would imagine a creature with a flat,
hard shell, legs sticking out to the sides, and some pinchy claws up front.
And you wouldn’t be wrong. Except what if I told you that a lot of critters that
look like that are… technically not crabs.
King crabs, coconut crabs and porcelain crabs all look pretty crabby, but they’re all examples of
crustaceans who have undergone carcinization: the convergent evolution of a crab-like form.
Convergent evolution is when organisms independently evolve similar traits as
a way of adapting to similar conditions -- even though they’re not immediately related.
The crab-like body plan is so popular in crustaceans that it has evolved at least five
separate times throughout evolutionary history -- only one of which we consider to be “true” crabs.
And evolutionary biologists have a whole word for it.
So what makes it so great to be a crab -- and could we see
even more crustaceans going all crabby over time?
Researchers believe the most important reason
many crustaceans have evolved a crab-like shape was to protect their pleon.
The pleon is the back section of a crustacean that contains their abdomen. Within are a lot
of their most important bits: intestines, parts of reproductive organs, and so on.
In order to keep things safe from predators who might be inclined to take a bite,
it’s worthwhile for these organisms to tuck the pleon safely away.
Think of a lobster or shrimp -- their delicious pleon is oh-so-exposed.
But the crab-like body plan takes the pleon
and folds it up against the underside of the animal to keep it safe.
And we see this in the fossil record about the same time as certain fish began
to increase in diversity, so it seems like it helped them hide from hungry predators.
This is because their new body shape allowed them to spend more time on the bottom,
where they would be less likely to be munched on.
Crustaceans whose bodies became more crab-like mostly lost their ability to swim in the water
column, because when you ditch the part of the body that the tail fan attaches to,
it turns out you also lose the tail fan -- which helps with swimming.
Their bodies flattened out as well.
However, a folded pleon and flatter body gives a crustacean
a lot more mobility and speed on the ground.
The crab-like body shape lends itself to running and burrowing,
and can easily move sideways as well as forward -- all great tricks to escape would-be predators.
Crab-like bodies also have less abdominal mass compared to other crustaceans
with a more elongated body shape.
This body plan requires less energy to build and maintain overall. Combined with
the increased speed and mobility, it makes you wonder why all crustaceans aren’t crab shaped.
Even though there are a lot of crab-like critters out there, the
only true crabs are crustaceans in the suborder Brachyura.
The Brachyurans include almost 7000 species that occupy a wide range of
habitats, ranging from marine, to freshwater and terrestrial.
These true crabs range in size from the tiny pea crab, at just a few millimeters,
to the massive Asian spider crab whose legs span several meters.
However, that brings us to the many crab look-alikes.
Why don’t all these crab shaped animals get to be a part of team Real Crab?
It’s because there are a lot of things taxonomists have to consider when classifying an organism.
Shared ancestry carries more weight in taxonomy than appearances -- so if something evolved
to look like a crab convergently, but isn’t directly related to OG crabs… it’s not a crab.
First are critters in the order Cyclida,
who evolved a crab-like body plan before true crabs were even around.
Yet these critters are more closely related to small parasites called fish lice
than they are to true crabs, and are believed to have coexisted
with the ancestors of true crabs for a while before going extinct.
True crabs’ close cousins belong to the group Anomura,
and several species in this group have a crab-like body.
In fact, the crab body plan is believed to have evolved
at least three separate times within Anomura.
King crabs, porcelain crabs, coconut crabs, and the hairy stone crab
are all species in this group that have a crab-like body plan, but are not true crabs.
King crabs and hermit crabs do
share a common ancestor with each other -- just not with true crabs.
Porcelain crabs and hairy stone crabs, on the other hand, are most closely related
to squat lobsters -- crustaceans that are not crabs and also… not lobsters.
Unlike most of our crabs, true and otherwise, some species of porcelain crab are able to use
their folded pleon to swim, flapping it to propel themselves through the water column.
Squat lobsters themselves are considered by some researchers to be half-carcinized.
Their pleon is partially curled under, making them look as if they are squatting.
In other words, they may be yet another traveler on the evolutionary road to being a crab.
Convergent evolution doesn’t make crabs exclusively.
It’s something we’ve seen in many other species throughout evolutionary history.
Researchers believe the pressures of similar environments have caused carcinization to occur
multiple times in crustaceans. The benefits provided by a squat,
compact crab-like body are simply too good to pass up.
Enough so that nature just keeps on declaring that it’s time for crab to shine.
Thanks for watching this episode of SciShow. While you’re here, I want to tell you about the
new charity project called the Awesome Socks Club! It was started by Hank and John Green,
and if you sign up, you can get a fancy new pair of socks each month of 2021.
Each pair was created by a different designer, and -- Hey, do any of them feature crabs? No?
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in Sierra Leone, which is one of the most dangerous places to be pregnant in the world.
But here’s the catch: you can only subscribe between now and December 11th.
So if you’re interested, click the link in the description!
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