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We may think of a lot of critters with crab-like body plans as crabs, but, technically, many of them are other types of crustaceans. So why do they share so many physical traits?

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This episode is sponsored by Awesome Socks Club,   a sock subscription for charity!

Click  the link in the description and sign   up between now and December 11th to get a  new pair of fun socks each month in 2021. [♪ 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?   ... Well, I’m sure they’re lovely anyway.

Also, 100% of the after-tax profit will go  to decrease maternal and child mortality   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! [♪ OUTRO].