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MLA Full: "Diversity of Bodies & Sizes (but mostly crabs): Crash Course Zoology #3." YouTube, uploaded by CrashCourse, 29 April 2021,
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Even though animals can look very different on their surface, there are surprising similarities in how they’ve evolved to solve major problems - like how to support and move their bodies. Today, we’ll take a look at the various ways animals can grow, learn how and why some animals evolved to have a head and others evolved to not have a head, and examine the different types of skeletons - from those inside bodies (like our own), to skeletons outside bodies (like in insects), and even those made of water (like in jellyfish). Evolution is a wild journey that brings us so many different animals with a huge array of bodies and sizes - that is, until everything turns into a crab!

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As a wildlife ecologist, I’ve spent hours following bear tracks around Nevada and New. York, hoping for a glimpse of these furry, four-legged, foragers.

Though, if we look across the whole animal kingdom, we’d find all kinds of bears. Bears with four legs, eight legs, six legs, or no legs! Some weighing over 1000 pounds, and others that are microscopic with very few, or even no, organs at all.  Even bears that can walk on paws or suction disks, fly, or sway in the ocean currents.

I’m talking about grizzly bears, but also water bears, bear moths, and pore bearers. It’s not always clear why such a diverse array of animals became associated with an. Old English or proto-German word for a brown creature...  [or why we thought that pore bearer pun was a good idea!] Like water bears look kinda like 8 legged grizzlies.

But bear moths spend most of their lives as fuzzy caterpillars, and pore- bear-ers is a translation for the phylum Porifera that just contains sponges. None of those look like bears!   Turns out, “bears” come in all sorts of sizes and body plans, and vary a ton in how they move.  And we’ll see that even though animals can look very, very different on their surface, there are surprising similarities in how they’ve evolved to solve major problems -- like how to support and move their bodies!  I’m Rae Wynn-Grant, and this is Crash Course Zoology.  INTRO. An animal’s size and body plan shape their entire lives -- what they eat and how much, how they move, where they live, and their place in their environment.

Like the grizzly and black bears I study carry their 1000 pounds of muscle and fur around on four paws. They spend most of the fall fattening up for the winter, and unlike most four-legged mammals, they can stand up and sit like us! But animals come in a huge range of sizes, and most animals are actually very small compared to us.

A human like me weighs about 59 kilograms, which is about 59,000 times more than a house spider. And a blue whale can weigh 100 million times more than that spider. Even the animals that get big as adults -- and by “big” I mean more than a couple inches long -- spend a good part of their life being very small, undergoing drastic changes in body shape and lifestyle as they grow.   For instance, some fish, amphibians, reptiles, sponges, corals, and mollusks gain both weight and length for their entire lives -- this is called indeterminate growth.

Some animals have periodic growth, alternating between growing fast and slow (or not at all). Like animals with exoskeletons, or hard outer skeletons. During molting when an animal sheds its old skin or shell, these animals actually grow their new exoskeleton under their old one and then inflate it with fluid before it eventually hardens into their new larger size!

Other animals experience predetermined growth and stop growing when they hit a more-or-less maximum size. But once any animal reaches a certain size, they hit some physical limitations. Animals thicker than about 1 millimeter need extra plumbing like a cardiovascular system to move oxygen and waste around their bodies.

Bigger animals also need to eat a lot more to feed their 1000s or millions or even trillions of cells.  And big animals need more structure like bones and muscles to support them as gravity pulls on all their weight. That’s why the biggest animals, the whales, live in the ocean -- the water supports their bodies instead of legs.  Growing also means making new tissue, and animals have evolved a few different solutions. Many clades, or groups of animals with a common ancestor, add more cells.

Other animals grow by making each cell bigger but keeping the same number of cells, a trait called eutely. Other animals like echinoderms use a weird process called maximal indirect development and grow their adult form out of a special ball of cells that have been set-aside.  The larval or immature form is mostly made up of cells that already have all their development planned out for them, and a set amount of growing they’re going to do. But in the larvae, there’s also a small amount of set-aside cells that take over once the other cells get old and die off.

Then the ball of set-aside cells develop into the many cell types needed to create the adult form. But the growth that probably seems the wildest to us humans is colonial growth when animals get bigger by adding more complete, individual clones.  These colonial animals, like Siphonophores and Bryozoans, are made up of tons of little clones that work together, sort of like how a school of fish can coordinate and swim together.  But even though animals can grow in completely different ways, a lot of them can look quite similar. Basically, some body designs show up again and again.

Distantly related animals evolving similar traits independently is called convergent evolution, and it usually happens because different lineages face similar problems in their environment or take on similar ecological niches. One of the most stunning examples of convergent evolution is carcinization, a process that zoologist Lancelot A. Borradaile famously defined as "the many attempts of Nature to evolve a crab.” Let’s go to the Thought Bubble.

It all started with the cycloids, a group of arthropods that lived from the Carboniferous to the Cretaceous era. They had that flat crabby shape, a small abdomen, and a bunch of walking legs just like today’s crabs. It wasn’t until the early Jurassic period, tens of millions years after the first cycloids were around, that the first of what we think of as real crabs -- members of infraorder.

Brachyura -- showed up.  A little after that is when the fake crabs started showing up, with things like this early squat lobster all looking very crabby. And the crab fad kept happening over the Mesozoic era -- now we have hermit crabs, hairy stone crabs, horseshoe crabs, crab lice, and king crabs. None of which are descended from Brachyurans and so none of which are actual crabs!

One way you can tell is that most “fake crabs” have six walking legs instead of eight! But why? Probably because “crab” is a great body plan.

It’s tough and adaptable to life on land or in water, and their flat and round bodies fit into more places than a long lobster tail might. So crab-shaped animals have more evolutionary fitness, which means they tend to survive and pass on their genes more than non-crab shapes.  The real kicker is that the cycloids -- the ones who first came up with the crab body plan -- died out in the Cretaceous era, at a time when there were real crabs and fake crabs all over the place. One hypothesis is that the cycloids got outcompeted -- out-carcinized, out crabbed -- by both the crabs and “crabs” we know today!

Thanks Thought Bubble! Convergent evolution pops up when a similar solution works in different environments for different lineages. Animals’ bodies evolve to better suit a function -- even if it means turning into a crab.

Now it’s important to remember that evolution has no set goals besides passing on genes -- there’s no “body plan plan.” Often a simpler form can perform a function much better than a complex one! Cephalization -- evolving a head -- and in some cases, decephalization -- or evolving to not have a head -- are good examples of how sometimes simpler is better. But like what is a head, really?

Heads collect the sense organs needed to perceive the world, the mouth, and the nerve cells that coordinate them in the front of the animal, where they can react quickly to danger or prey. We know that the ancestor of all the animals that can be divided into symmetrical halves, which are called Bilaterians for their bilateral symmetry, had a head. And most animals are bilaterians.  But some animal groups have lost their heads -- literally -- because they became less useful.

Like bivalve mollusks, like clams, are bilaterians that stay rooted in one place and just filter water through their mouths to catch bits of food. Which you don’t really need a dedicated head for, so the “head pieces” like the central nervous system and sensory organs, are distributed around the clams’ body. Other animals with radial symmetry have bodies that are symmetric around a central point.

Most of these are echinoderms, and with the exception of sea cucumbers, they don’t have anything resembling a head. But heads or no head, bilaterally or radially symmetrical, for all these forms to be possible, they need some kind of structural support -- otherwise everything turns into a blob of cells. Phylum Chordata solves this with a notochord, a flexible rod that supports their body as embryos and sometimes as adults.

The notochord develops into the vertebral column, or spinal column, in vertebrates, which is where they get their name. All other animals are invertebrates, and they have several different types of support -- which is part of the reason they don’t form just one phylum. In general, skeletons are frameworks that support, shape, and protect soft tissues.

When you think of a skeleton, you probably picture an endoskeleton, an internal support structure made of mineralized tissues. Vertebrates have rigid endoskeletons made of bone, which gets its hardness from large amounts of calcium phosphate.  Invertebrates have endoskeletons made from other materials. Like echinoderms such as sea urchins have endoskeletons made from fused plates called ossicles, which are made of calcite.  Even sponges have an endoskeleton made of the flexible protein spongin and spicule crystals.

But some sponges also secrete an outer skeleton from cells on their skin, which leads us to exoskeletons -- skeletons that sit outside the rest of the body!   Mineralized exoskeletons show up in at least 18 clades, including some sponges, echinoderms, corals, and mollusks.  Other animals based their exoskeletons on long chains of sugar molecules, called polysaccharides.  Arthropods like insects, crustaceans, and arachnids use chitin to make their skeletons.  The third type of exoskeleton is actually made of water. Which sounds rather...flimsy. But hydroskeletons work because water is incompressible -- you can’t realistically squeeze it into a smaller volume like you could a marshmallow.  So as long as animals can contain water in a tube or sac, they’ve got the makings of a stable structure.  Invertebrates like worms and jellyfish use hydroskeletons to support their very flexible bodies.

It’s an especially great adaptation for living deep in the ocean.  Endo-, exo-, and hydroskeletons -- plus heads when animals have them -- are what give animals their shape. But animals aren’t statues like you’d see in a museum -- they move, and how animals move also influences how they look. Some animals move with the help of their environment -- spiders cartwheel down sand dunes, and.

Velella velella, a jellyfish-like colony of animals, use a sail to catch the breeze. These animals needed to evolve the right instincts and structures to take advantage of their surroundings. Other animals move under their own power with the help of cilia and muscles.

Cilia and flagella are hair- or tail shaped parts of cells that beat in co-ordinated waves to paddle microscopic animals forward. And how these tissues connect with the skeleton influences how an animal moves.  Moving an entire skeleton at once is harder, because they're usually rigid and heavy. Most animals solve this by turning their skeletons into a bunch of levers that pivot around joints as pairs of muscles contract and relax.  Even animals with hydroskeletons use muscles to control fluid pressure and bend their body.  Animals move their bodies in all sorts of ways, balancing where they want to go, how quickly, and how much energy it’ll take.

In fact, there’s a whole field of zoology called biomechanics that’s interested in how mechanical principles guide how animals are shaped and move.  But all this moving and growing takes a tremendous amount of energy, and we’ll talk more about where animals find that energy in our next episode. Evolution is a wild journey that brings us so many different animals with a huge array of bodies and sizes. That is, until everything turns into a crab.

Thank you to KiwiCo for supporting PBS. KiwiCo’s mission is to inspire kids to see themselves as makers by providing them with the tools and a foundation to become creative problem-solvers and critical thinkers. The crates include everything you’ll need in the box and cover a wide variety of topics from month to month like art, science, engineering, and geography.

Inside you’ll find the project materials of course, a blueprint (which are the instructions written for kids), and a magazine containing lots of additional content and experiments.  Go to or click the link in the description for more information.  Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE. It is shot on the Team Sandoval Pierce stage at Porchlight Studios in Santa Barbara, California and made with the help of all these nice people. If you’d like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.