Complex Animals: Annelids & Arthropods - CrashCourse Biology #23

Summary

This script from Crash Course Biology explores the significance of annelids and arthropods, two of the most successful animal groups on Earth. It delves into their evolutionary traits, such as segmentation and synapomorphies, and highlights the diversity within these groups, from earthworms and leeches to insects and crustaceans. The video script also touches on the importance of these creatures in maintaining soil health and plant life, emphasizing the interconnectedness of all life on our planet.

Takeaways

• 🪱 Earthworms process about 16,000 pounds of soil per acre per year, making plant life possible.
• 🦟 80% of known animal species are arthropods, with approximately a billion billion of them worldwide.
• 🐛 Segmentation is a key evolutionary trait in Annelida, Arthropoda, and Chordata, indicating complexity.
• 🧬 Annelids, such as earthworms, leeches, and lugworms, exhibit clear segmentation in their bodies.
• 🔬 Synapomorphies are traits that set one group of animals apart from others, such as segmentation in Annelids.
• 🐍 Arthropods have segmented bodies, exoskeletons made of chitin, and paired, jointed appendages.
• 🕷 Cheliceriformes, including spiders and scorpions, have long fang-like pincers, simple eyes, and lack antennae.
• 🐜 Hexapoda, primarily insects, have three-part bodies, three pairs of jointed legs, compound eyes, and antennae.
• 🦋 Complete metamorphosis in insects allows for a dramatic transformation from larva to adult.
• 🦞 Crustaceans, such as crabs and lobsters, have three body segments and some have developed bioluminescence.

Q & A

Why do the narrator's distant relatives earn their respect?

The narrator respects them because they are among the most successful organisms on Earth, processing large amounts of soil and being crucial for plant life and, consequently, human life.

What is the significance of annelids in cultivated land?

Annelids like earthworms process about 16,000 pounds of soil per acre each year, making plant life and human life possible by aerating the soil and enriching it with nutrients.

How do arthropods compare to humans in terms of numbers and complexity?

There are about a billion billion arthropods on Earth, and they make up 80% of known animal species. Although they outnumber humans, humans are far more complex in terms of physical and cognitive abilities.

What are the two traits that indicate an animal's relative complexity?

The two traits are the number of germ layers they develop as embryos and whether they have a coelom, a body cavity that holds their organs.

What is segmentation, and why is it important in animal evolution?

Segmentation is the repetition of anatomically identical units that can be modified for different purposes as animals evolve. It has proven to be extremely useful from an evolutionary perspective, allowing for diversity and specialization.

Which three major animal groups display segmentation?

The three major groups are Annelida, Arthropoda, and Chordata, which include the vertebrates.

What are synapomorphies and plesiomorphies?

Synapomorphies are traits that set a group of animals apart from their ancestors and other related groups, while plesiomorphies are very basic traits shared by animals with a common ancestry.

What is a key synapomorphy of annelids?

A key synapomorphy of annelids is their segmentation, as their bodies look like rings fused together, and they also have chaetae (bristles) for movement.

How do arthropods' segmented bodies contribute to their diversity?

Early arthropods had little variation between segments, but as they evolved, segments fused and specialized for different functions, leading to a great diversity of forms like scorpions, butterflies, and lobsters.

What are the defining characteristics of arthropods?

Arthropods have segmented bodies typically divided into a head, thorax, and abdomen, an exoskeleton made of chitin, and paired and jointed appendages.

Outlines

00:00

🌏 The Dominance of Earthworms and Arthropods

This paragraph introduces the earthworm and its significant role in soil processing, which is vital for plant and human life. It emphasizes the sheer number of arthropods on Earth and their diversity, highlighting that 80% of known animal species fall into this category. The speaker points out human complexity in activities like arm wrestling and playing the guitar, but notes that the foundation of such complexity started with a shared trait with these simpler organisms. The paragraph delves into the concepts of germ layers and coelom in embryonic development, and introduces segmentation as a key trait in the evolution of complexity, particularly in Annelida and Arthropoda phyla. It also touches on the shared ancestry of segmented animals, dating back to about 600 million years ago.

05:00

🐛 The Evolutionary Advantages of Segmentation in Annelida and Arthropoda

The second paragraph explores the concept of segmentation in more detail, explaining it as a trait that allows for the repetition and modification of anatomical units, which is crucial for evolutionary adaptability. It identifies Annelida as the earliest phylum to exhibit segmentation, with earthworms, leeches, and lugworms as examples. The paragraph also discusses synapomorphies, which are shared derived traits that distinguish a group from its ancestors, and plesiomorphies, which are basic traits shared by animals with a common ancestry. The speaker provides examples of synapomorphies in annelids, such as chaetae for traction, and delves into the different classes within Annelida, including oligochaetes, hirudinea, and polychaetes, each with their unique characteristics.

10:01

🦋 The Incredible Diversity and Evolution of Arthropods

This paragraph focuses on the Arthropoda phylum, which includes an immense variety of species due to the specialization of body segments. The speaker discusses the synapomorphies of arthropods, such as segmented bodies, exoskeletons made of chitin, and paired, jointed appendages. The paragraph then breaks down the phylum into four subphyla, starting with Cheliceriformes, which includes spiders, scorpions, and horseshoe crabs, and highlights their unique features like the cephalothorax and the absence of antennae. Myriapoda is next, with millipedes and centipedes, characterized by their numerous legs and terrestrial lifestyle. Hexapoda, mostly insects, is noted for its vast number of species and the unique evolutionary path that led to their current diversity. Lastly, Crustacea is mentioned, which includes marine creatures like crabs and lobsters, known for their complex appendages and bioluminescence in some species.

Keywords

💡Annelids

Annelids, also known as segmented worms, are a phylum of invertebrate animals characterized by their body segmentation. In the script, annelids like earthworms are highlighted for their significant role in soil processing, which is vital for plant life and, by extension, human life. The script also mentions that annelids are one of the three most diverse groups of animals that display segmentation, indicating their evolutionary success and relevance to the video's theme of animal complexity and diversity.

💡Arthropods

Arthropods are a phylum of invertebrate animals that include insects, spiders, and crustaceans. They are noted for having the highest number of known animal species, as mentioned in the script with the figure of 'about a billion billion arthropods.' The script emphasizes their success and diversity, attributing this to their segmented bodies and other shared traits, which are central to the video's exploration of animal evolution and complexity.

💡Segmentation

Segmentation is a biological trait where an organism's body is composed of repeated, anatomically identical units. The script explains that this trait allows for the addition and modification of body parts as species evolve, which is a key concept in the video's discussion of how complexity in animals has developed over time. Segmentation is a shared characteristic among annelids, arthropods, and chordates, which are all highlighted as diverse and successful groups in the animal kingdom.

💡Coelom

A coelom is a fluid-filled body cavity that holds and protects an organism's organs. The script mentions coelom as one of the two traits, along with the number of germ layers, that indicate an animal's relative complexity. The presence of a coelom is a step towards greater complexity, as it allows for more sophisticated organ arrangements and is a concept that contributes to the video's overarching theme of increasing biological complexity.

💡Synapomorphy

Synapomorphy refers to shared derived traits that set one group of animals apart from its ancestors and from other groups that came from the same ancestors. In the script, synapomorphies such as the presence of chaetae in annelids and the fused head and thorax in arachnids are used to illustrate the unique characteristics that define different animal phyla. This concept is crucial for understanding how different animal groups have evolved and diversified, which is a central theme of the video.

💡Plesiomorphy

Plesiomorphy is a term used to describe basic traits that are shared by animals with a common ancestry. The script uses the example of worm-shaped bodies in platyhelminthes, nematodes, and annelids to illustrate plesiomorphies, indicating a shared distant ancestor with a similar form. Understanding plesiomorphies helps in tracing animal lineages and is relevant to the video's discussion of evolutionary relationships among animals.

💡Chordata

Chordata is a phylum that includes vertebrates and some invertebrates, characterized by having a notochord, a dorsal nerve cord, and pharyngeal slits at some stage of development. The script mentions chordates as one of the three largest and most diverse groups of animals that display segmentation, like annelids and arthropods. This keyword is important for the video's narrative as it places humans within the context of evolutionary history and complexity.

💡Oligochaetes

Oligochaetes are a class within the phylum Annelida, which includes earthworms. They are characterized by having 'chaetae' or bristles, but only a few of them, as indicated by 'oligo' in their name. The script highlights the importance of oligochaetes in soil aeration and nutrient cycling, making them a relatable example of how certain animal groups contribute to ecosystem functioning, which ties into the video's broader message about the significance of different animal groups.

💡Myriapoda

Myriapoda is a subphylum of arthropods that includes millipedes and centipedes. The script points out their numerous legs and their terrestrial habitat, emphasizing the diversity and adaptability of arthropods. The mention of myriapods in the video underscores the theme of arthropod diversity and the various ways in which segmentation has been utilized in evolution.

💡Metamorphosis

Metamorphosis is a biological process where an animal undergoes a significant change in form from larval to adult stages. The script describes two types of metamorphosis in insects: partial and complete. Insects are highlighted as unique among arthropods for their ability to fly and undergo complete metamorphosis, which allows for dramatic changes in body structure. This keyword is central to the video's exploration of insect evolution and their remarkable adaptations.

💡Crustaceans

Crustaceans are a group of aquatic arthropods that include crabs, lobsters, and shrimp. The script describes crustaceans as having three body segments and various specialized appendages. The discussion of crustaceans in the video serves to illustrate the diversity within the arthropod phylum and how different groups have adapted to their environments, aligning with the video's theme of evolutionary success and specialization.

Highlights

Earthworms and other annelids are among the most successful organisms on Earth, processing about 16,000 pounds of soil per acre annually.

Approximately 80% of known animal species are arthropods, which include a vast diversity of creatures like insects, spiders, and shrimp.

Despite their numerical dominance, simpler organisms lack the complexity of humans in areas such as physical strength, musical ability, and cognitive tasks.

The concept of segmentation in animal evolution, where anatomically identical units are repeated and modified, is a key factor in increased complexity.

Annelida, Arthropoda, and Chordata are the three most diverse and largest groups of animals, all of which exhibit segmentation.

All segmented animals share a common ancestor that lived around 600 million years ago, highlighting a deep evolutionary connection.

In humans, segmentation is evident in anatomical structures such as vertebrae, ribs, and the folds in our brains, which have evolved from simple to complex forms.

Annelida, including earthworms, are the earliest phylum to display segmentation, characterized by their ring-like body structure.

Synapomorphies, such as chaetae bristles in annelids, are traits that distinguish a group of animals from their ancestors and other related groups.

Plesiomorphies, like the worm-shaped body of Platyhelminthes, Nematodes, and Annelids, indicate a shared ancestry among these groups.

Oligochaetes, a class within Annelida, are known for their role in soil aeration and nutrient cycling, contributing to plant growth.

Hirudinea, or leeches, are characterized by their powerful suckers and are mostly parasitic, feeding on blood and bodily fluids.

Polychaetes are marine worms with many bristles, known for their diversity and contribution to the beach environment through their castings.

Arthropods' success is attributed to their segmented bodies, which have evolved to become specialized for various functions, leading to great diversity.

Arthropods share several synapomorphies, including a segmented body, an exoskeleton made of chitin, and paired, jointed appendages.

Cheliceriformes, including spiders and scorpions, are distinguished by their lack of antennae and presence of fang-like pincers.

Myriapoda, such as millipedes and centipedes, are known for their numerous legs and terrestrial lifestyle, with millipedes being early land dwellers.

Hexapoda, primarily insects, are characterized by their three-part bodies, compound eyes, and antennae, showcasing the vast diversity within this group.

Insects have evolved the ability to fly, which has been crucial to their success, and have developed metamorphosis as a life cycle strategy.

Crustaceans, such as crabs and lobsters, have adapted to aquatic environments with unique characteristics like bioluminescence and multifunctional appendages.

Transcripts

Hey there, I'm just hanging out with some distant relatives here.

I've got to say I've got a lot of respect for these guys,

because they are some of the most successful organisms on the earth.

We think we run things on this planet, but we don't.

They do.

On one acre of cultivated land, annelids like this earthworm

process about 16,000 pounds of soil a year, which makes plant life

and our lives

therefore, possible.

And not only are there about a billion billion arthropods

like this in the world, that's 10 to the 18th power by the way,

but also 80% of known animal species are arthropods.

Now, don't get me wrong, even though bugs and spiders

and worms and shrimp totally outnumber us humans,

we are far more complex than them.

When it comes to arm wrestling and guitar playing

and long division, we'd totally school them.

But complexity like ours had to start somewhere.

And it started with a very special trait

that we share with these animals.

Can you see the resemblance?

Last time we talked about how, in the very simplest animals,

there are two traits that indicate an animal's relative complexity:

there's how many germ layers they develop when they're embryos

and whether or not they have a coelom, or body cavity

that holds their organs.

So it's in the next two phyla, Annelida and Arthropoda, where we

find the new big thing in animal complexity: segmentation.

Segmentation is the repetition of anatomically identical units

that can be added to and modified to serve different purposes

as animals evolve.

And evolution is the way to win it, folks.

In fact, the three biggest and most diverse groups of animals

in the world are the ones that display segmentation:

Annelida, Arthropoda and Chordata, which includes the vertebrates.

All segmented animals have a common ancestor

that probably lived about 600 million years ago.

That's how long ago it was when one of your grandparents

and one of the earthworm's grandparents

and one of the beetle's grandparents

all played on the same softball team.

Pretty crazy.

Segmentation has proven to be unbelievably useful

from an evolutionary perspective.

In humans, you see anatomically identical pieces repeated

along an axis from our butts to our heads.

They can be a little hard to see because they're so highly modified,

but think about our vertebrae: They're segments!

Our ribs are segments!

The cartilage around our trachea? Those are segments!

Even the folds in our brains are segments.

They're crazy-evolved segments, but segments just the same.

Among today's animal phyla, the earliest to display segmentation

is Annelida, which includes leeches, earthworms and lugworms.

See how their whole bodies look like rings fused together?

Segments!

In fact, the word Annelida comes from the Greek for "little rings,"

and when you look at any annelid, you see that they're all

really obviously segmented.

Now, this segmentation is a great example

of synapomorphy in annelids.

Synapomorphies are traits that set one group of animals apart from its

ancestors and from other groups that came from the same ancestors.

So unlike their flatworm and nematode cousins,

Annelids are segmented and they've also got little bristles

on their bodies called chaetae that provide traction

and help them move through the dirt.

These are both little extra somethings that annelids' have,

that less complex relatives don't have and that their common

ancestors didn't have.

Synapomorphies, literally "shared derived traits,"

are usually the defining traits of a phylum.

But you can also learn something about an animal's lineage

by comparing plesiomorphies, very basic traits

that are shared by animals with a common ancestry.

So, between the Platyhelminthes, the Nematodes and the Annelids,

one plesiomorphy is that they're all worm-shaped,

which tells you they have a common, distant, ancestor

that was wormy-lookin'.

So as we talk about these phyla and the classes within them,

notice how they're similar and different from each other.

For instance, within Annelida, there are three different classes.

Everybody's favorite, of course, is the oligochaetes, the earthworms

Their name refers to the synapomorphy I just mentioned:

they have "chaetae", or bristles,

but only an "oligo", few of them.

And they're everyone's favorite because they eat soil and crap

it out the other end, allowing air and water

to circulate in soil.

Plus their poo is rich in things that plants need to grow,

like nitrogen, calcium, magnesium, and phosphorus.

And now i'm going to go wash my hands.

Now on to the class Hirudinea, the leeches,

a lot of which are parasitic and, you know, eat

blood and other bodily fluids.

A synapomorphy of leeches are their powerful suckers

they've got them on both ends of their body,

the posterior one being used to anchor itself while the

anterior one that surrounds its mouth attaches to its host.

All leech species are carnivorous and they are hermaphrodites

like earthworms.

Now, the Polychaetes are bristly worms, hence a synapomorphy

of this class is their "poly" (many) "chaete" (bristles).

Almost all of these are marine species,

and they're really diverse, but the ones you've probably seen

evidence of are the lugworms, the ones that dig holes

at the beach and leave piles of castings on the sand.

Okay, I know you've had enough of worms.

Now we've got Arthropoda to talk about, and that's not

very easy to do because there are A LOT of them.

Like I said, they totally outnumber us.

Just to put things into perspective, there are more

insects in a square mile of rural land than there are

human beings on the earth.

One reason scientists think arthropods do so well has to

do with their—

you guessed it!

-segmented bodies.

Fossils of early arthropods show that there used to be

very little variation between segments, but as they evolved,

segments fused and became specialized for different

functions, which led to crazy amounts of diversity.

So much diversity that Arthropoda includes stuff like scorpions,

butterflies and lobsters.

Which...I know.

Seems like maybe a bit of a stretch.

But here are the synapomorphies that make them all arthropods:

1. They have segmented bodies that, in most cases, are broken up into

three segments: head, thorax and abdomen.

And check this out: Here, segmentation in arthropods

is a good example of a plesiomorphy.

It's a basic, ancient trait that they share

with annelids and us chordates, for that matter,

dating back to that softball game our forebears played

some 600 million years ago.

2. All arthropods have an exoskeleton: a hard outer shell

made out of chitin, which is a really tough carbohydrate that's

chemically similar to the cellulose that you find in plants,

and in order to grow bigger they have to shed it.

And be glad that you don't have to do that.

Because it looks like kind of a traumatic experience for them.

3. Finally, they've all got paired and jointed appendages,

which is actually where their name comes from:

arthropod means "jointed feet."

But it's not just their legs that are jointed. Some of them have

claws and jointed antennae, and they all have these

external mouthparts that are also jointed.

So that's what all arthropods have in common, but they are

grouped into 4 subphyla, based on how they differ

from each other.

First, and perhaps most terrifying, you have your Cheliceriformes,

which includes spiders and scorpions, but also

horseshoe crabs which are kind of nice,

and ticks, which aren't

and mites, which don't bother you at all,

probably...

they might.

Cheliceriformes comes from the Greek for "arm-lips," which--

Whatever, Greeks...

refer to their long fang-like pincers.

Unlike a lot of arthropods, they have simple eyes with

just one lens, not compound eyes like flies, and they lack antennae.

Most Cheliceriformes are landlubbers, but the fossil record

tells us that a lot of them were marine back in the olden days.

Sea spiders and horseshoe crabs are the only ones left now.

The largest class of Cheliceriformes are

the arachnids, the group that includes spiders, scorpions,

ticks and mites.

They have what's called a cephalothorax, which is a

head segment and a thorax segment fused together,

with eight legs and an abdomen behind.

Most arachnids are carnivorous or parasitic and they're

very skittery.

Just sayin'.

Next, Myriapoda, or "many feet," includes what you would expect:

the millipedes and centipedes, these are where the arthropods

were like "let's see how far we can take

this segmentation thing, shall we?"

All Myriapods are terrestrial and have antennae and sort of

scary jaw-like mandibles.

Millipedes are vegetarians, and they may have been some of

the very first animals to live on land, where there

were mosses and primitive vascular plants for them to munch.

Also, although millipedes have a crap-ton of legs, they don't have

as many as a thousand, as their name implies.

They've actually got anywhere between 94 and 394 legs,

depending on the species.

Centipedes, whose name is a little more apt as they generally have

between 20 and about 350 legs, are carnivores and have

poisoned claws to paralyze their prey.

So, if you're looking to cuddle with a Myriapod,

I'm going to advise you to go with a millipede.

But please, save some love for Hexapoda, or "six feet,"

most of which are insects.

The synapomorphies they share are three-part bodies,

consisting of a head, a thorax, and an abdomen,

three pairs of jointed legs that come off the thorax,

compound eyes, and two antennae.

Now, think of any random way you could put these

characteristics together, and you'll probably come up

with something that exists.

Because, you guys, there are so many damn insects out there

you have no idea.

There are more species of insects than there are

all other animal species COMBINED.

Which is why I'm taking this opportunity to do a...

BIOLO-GRAPHY, The Insects Edition!

Very little is understood about the evolution of insects,

but scientists think that they probably split off

from their crustacean cousins about 410 million years ago.

And for tens of millions of years, insects and some

little skittery invertebrates were about the only land

dwelling animals.

About 320 million years ago, thanks to the high oxygen

levels of the Carboniferous period, some insects grew to be

terrifyingly big, like the Meganeura, which looked like a

dragonfly with a two-foot wingspan.

But since an insects' size is restrained by

their respiratory systems, as oxygen levels went down,

these massive insects couldn't circulate enough oxygen

to keep their gigantic bodies going, and they died off.

The next major milestone of insect history occurred

around 120 million years ago, which is when

most flowering plants evolved, and with it the

sweet spirit of cooperation that insects and flowering plants

still share to this day.

In fact, some insects and flowering plants have

co-evolved really neat pollination strategies

so that they basically evolved together,

which I think is really sweet.

And as a result of insect pollination,

flowering plants are now the overlords of the plant world.

And thus, everything smells nicer and looks prettier.

Thanks insects!

Insects are the only arthropods that have developed

the ability to fly, and it has served them well.

Insect's wings are just extensions of the cuticle of the thorax,

so unlike birds and bats which have to sacrifice walking legs

in order to evolve wings, insects are just as graceful

on the ground as they are in the air.

But in order to be so awesome, insects had to develop

this crazy thing called metamorphosis.

In partial metamorphosis, the young, called nymphs,

look exactly like the adult of the species,

and undergo a series of molts which allows them to get bigger

and bigger, but they look basically the same the whole time.

Most other types of arthropods do this, and some insects,

including grasshoppers and cockroaches.

However, complete metamorphosis is the process unique to some

insects that lets them completely change shape.

Maggots turn into flies, mealworms turn into beetles,

caterpillars turn into butterflies. The baby insect, called larva-

I have one right here, this is a rhinoceros beetle larva-

pretty gross!

hangs out and eats until it's time to

build a little case around itself called a pupa,

this is the rhinoceros beetle pupa, which is exceptionally creepy

[creepy laugh]

and the when it comes out of the pupa, it's fully grown.

Rhinoceros beetle! It's like, soft at first and then it's chitin

hardens up and it becomes the adult.

So insects are basically wizards, but not as delicious as Crustacea,

the insects of the sea!

Crustaceans include crabs, lobsters, shrimp, and barnacles,

and like insects, they have three body segments:

head, thorax and abdomen.

Although some, have a cephalothorax like spiders do.

While most other arthropods have learned to love the land,

very few crustaceans have.

They have decided to put their energy into developing

other amazing characteristics.

For instance, lobsters and crayfish are like walking multitools:

they have 19 pairs of appendages, some of which are claws,

some are mouthparts, some are regular old walking legs.

Some shrimp have evolved bioluminescence,

which is pretty much the most amazing thing any animal

can do as far as I can tell.

And this Yeti crab... looks like a Yeti.

And we've covered most of the types of animals on earth in, what?

10 minutes?

Well hopefully now you can see the resemblance between these guys.

I love my caterpillar.

Thank you for watching this episode of Crash Course Biology,

if you want to repeat anything that we went over or

do a little bit of review there's a table of contents

that you can click on.

Thank you to everyone who helped put this together

including all of our little animal friends,

and Brandon from the University of Montana

for coming out and helping us out with this.

And if you have any questions or ideas,

please leave them below in the comments

and tell me what you think we should name this one.

That's all. Bye!