The Fantastically Weird World of Photosynthetic Sea Slugs | Michael Middlebrooks | TED

193,258 views ・ 2023-03-09

TED


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I'm going to talk to you today about sea slugs and solar power.
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My background and what I do -- I'm an invertebrate zoologist.
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So invertebrate animals are animals that don't have a backbone.
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So this is actually most of the animals on the planet.
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It ranges from things like insects to clams to sea sponges to worms.
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And a great other many things
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that we don't have time to talk about today.
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On our planet, most of the biological energy that we have available
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comes, ultimately, from the sun.
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The process from this being converted to solar energy,
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to biological energy is photosynthesis.
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You’re probably familiar with this as something that plants do.
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And all of the food that we eat ultimately comes from photosynthesis,
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either us directly eating plants or eating animals that eat plants.
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And that's really where all of the energy
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that all of the animals have comes from.
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However, there are a few animals that have managed to get around that
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and become photosynthetic themselves
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and are able to capture the energy from the sun,
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convert it into biological energy, right?
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So just like plants, they take carbon dioxide in sunlight,
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turn it into sugar and oxygen.
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The best and most famous example of this are the corals.
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The photos that we're looking at here
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are some some corals from the Red Sea in Egypt
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and from a reef in Cuba.
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All the photos I'm going to share with you today are photographs I've taken.
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And when I've gotten photos from the field,
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I put the location there, if you're interested.
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The photos from the laboratory won't be labeled like that.
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They have a black background on them.
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Corals are able to photosynthesize because of a special partnership,
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a symbiosis that they have
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with a single-celled algae called zooxanthellae.
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The algae live inside of cells of the coral,
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capture the sunlight and provide the corals with sugar.
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So effectively we have photosynthetic animals.
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This is unusual,
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but actually occurs in quite a lot of the corals
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and many of their relatives,
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and actually happens in a fair number of other animals as well.
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So we see this in things like some sea sponges,
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we see this in some flatworms,
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and we see it in other animals closely related to corals
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like sea anemones and jellyfish.
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So this is an upside down jellyfish.
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This is an organism that, just like the corals we were talking about,
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has these zooxanthellae inside of them and can photosynthesize.
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So this animal lives in like, shallow mangroves throughout the world
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and just lays on the bottom, capturing sunlight.
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I keep a few in my laboratory that I use for teaching,
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and I don't actually have to feed them.
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I just give them light from one of my aquarium lights,
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and I’ve managed to keep some of them there for two years.
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And I use them in my invertebrate zoology courses.
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I like the jellyfish, though,
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even though the coral is perhaps a more famous example.
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The jellyfish is nice because we can take a tentacle of a jellyfish
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and look at it under the microscope,
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and that allows us to see this process.
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So this is a micrograph, a microscopic photograph I've taken,
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of some zooxanthellae from the tentacle
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of the upside-down jellyfish Cassiopeia.
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And I did this back at my laboratory in Tampa.
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And all of those little golden brown spheres that we're seeing,
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each one of those is one of those algal cells.
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So they're loaded in there quite densely.
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There's a lot of them there.
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So these animals are able to photosynthesize that way.
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So it's quite remarkable that we have animals
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that are not doing the typical animal thing.
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The jellyfish can still feed, and in the wild, they do.
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But they can get most of their energy just from the sun.
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Fantastic.
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I primarily study a group of organisms called mollusks.
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Mollusks are probably familiar to some of you,
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if, because of nothing else,
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because of their shells and in some cases as food.
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So these include animals like snails and clams.
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Also things like octopus and squids
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and some other strange things that we won't have time to go into today.
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But the mollusks are a fantastically diverse group.
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They are the second most diverse group of animals on the planet
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after the arthropods,
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which win out because of the insects.
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But in terms of sheer diversity in body form,
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the mollusks, I would argue, are in fact the most diverse
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and range from things smaller than grains of rice
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to colossal squids that are absolutely enormous.
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And we have photosynthesis within our mollusks as well.
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So these are giant clams.
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They're fantastically beautiful animals.
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They live in tropical coral reefs in the Indo-Pacific, primarily.
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And when I say giant, there's a range.
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Some of the species aren't quite so large,
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but some of them are absolutely enormous, and they’re beautiful.
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And a lot of the coloration we're seeing in some of these
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comes from their symbiotic algae.
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They have the same zooxanthellae that we saw within the corals.
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Unfortunately, giant clams are rather difficult to see in the wild.
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They've been overharvested in many areas because people want their large shells
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and they are also eaten.
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That's a very interesting symbiosis too.
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But the group that I'm most interested in are called the gastropods.
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Gastropods are snails and slugs.
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So your typical garden snail that you may be familiar with
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and many of the seashells
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that you may have collected visiting the beach
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come from gastropods.
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So this is a tremendously diverse group of mollusks.
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They're the most diverse group of mollusks in terms of number of species.
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Quite a large number of them.
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And they're fantastically interesting.
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I wish we had time to go into more of them here.
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The one in the middle is a cone snail,
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one of the most venomous animals on the planet.
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And also a ...
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subject of a great deal of biomedical research
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studying the potential of its venom.
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But my real passion is slugs.
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My favorite thing to do is go scuba diving in a tropical coral reef
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and look for sea slugs.
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And a sea slug, ultimately, or any slug,
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is a snail that, over the course of evolution, has lost its shell.
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So a slug is just a snail minus the shell, right?
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And this has happened multiple times.
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This was not a single evolutionary event,
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but one that occurred over and over again.
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And so we have multiple unrelated groups of snails
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that have either greatly reduced or lost their shell over the course of evolution.
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There's a few of them shown here,
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just to show you some variety.
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This includes the head shield slugs.
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There are sea hares as well as the pulmonates, or air-breathing, slugs,
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which may show up in your garden
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and cause you a great deal of displeasure.
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Now, the most common question that I get
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when I tell people what I do for a living, studying slugs,
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is, “Why would you do that?
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Why study slugs?
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You could do anything.
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Why do that?"
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And I think the best way to explain this to you all is not to tell you,
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but to show you.
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So here are some of the slugs I've encountered in my travels.
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They are fantastically weird.
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They're fantastically beautiful.
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They do some very strange things.
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There’s a lot of really interesting biology going on with these animals.
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And there’s so much that we don’t know.
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Many slugs,
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the only scientific paper written on them is a species description.
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And there's many more out there that haven't even been described.
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So there's so much for us to learn and so much that we don't know.
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It's just wide open and so fascinating to me.
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Now, in the ocean, there are many types of slugs.
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I've sort of shown you that already.
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There's two groups that I'm going to tell you about now.
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One group is probably the most famous of them
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and certainly the most diverse in terms of total number of species,
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are the nudibranchs.
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And that name, “nudi,” means naked,
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but "branch" means gills.
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So it's referring to those feathery tufts that we're seeing on these animals.
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They're beautiful, they're diverse, they're interesting,
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and they're carnivores.
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They eat other animals, mostly -- they are slugs, right,
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so they’re not chasing things down.
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They eat other slow things.
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So sometimes other slugs,
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they also eat things like sponges and in some cases,
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relatives of our corals,
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like sea anemones.
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And some of these nudibranchs that do that
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are able to take those zooxanthellae that we were talking about earlier,
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put them inside of their own cells,
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and then they become photosynthetic.
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So they steal their photosynthesis.
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They're excellent thieves.
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Some of them even steal the stinging cells out of jellyfish and anemones
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and use them for their own defense.
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Absolutely fantastic.
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This is my favorite group of slugs.
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These are the sacoglossan sea slugs.
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They are also thieves.
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They are, you may notice, green, right?
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Some of them are green for camouflage,
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but many of them are green for a very different reason.
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These slugs are herbivores,
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and they have a special single little tooth,
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and they just poke one little hole into the algae,
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and then they slurp out the contents inside of it.
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And they take some of those contents and they digest them.
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But others, the chloroplasts,
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which are the organelles inside of a plant cell
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that allows plants to photosynthesize,
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the slugs take those and stick them inside of their own cells,
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and then they become photosynthetic.
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We call this kleptoplasty.
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"Klepto", as in to steal, "plasty" as in chloroplast, right?
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So they've stolen chloroplasts.
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And these slugs, it varies,
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some of them can only do this for a couple of days,
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but some can do this for many months, even complete their entire life cycle.
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Let's take a little bit closer look
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at the digestive track of one of these slugs.
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This is a photo by my colleague Nick Curtis.
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And this is showing us the digestive tubules of these animals.
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So their digestive track is highly branched.
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It goes in many different directions.
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And at the end of these branches,
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we have these cul de sacs that are loaded with chloroplasts.
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If we look closer at a single cell in one of these cul de sacs,
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also a photo by my colleague Nick Curtis,
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we see a single cell here.
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That's what we're looking at, that structure labeled N.
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That is the nucleus of the cell.
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But all of those structures labeled C and everything that looks like them,
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those circles, those are chloroplasts,
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and they're jammed in there so tight and so dense
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that there's more chloroplasts in that cell
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than you would find in the algae, at least in terms of density.
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This is wonderful.
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These animals have stolen photosynthesis,
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and you can see some of them kind of look like leafs, right?
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They're super green,
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they're fantastically photosynthetic.
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And how they do this is somewhat of a mystery.
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Taking a chloroplast and sticking inside of a cell
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is not enough to become photosynthetic.
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Chloroplasts need things that the algae provide to them
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that animals shouldn’t be able to do.
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And we've started to unravel some of this.
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And this is a slow process
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and something that we're just really scratching the surface of.
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But one of the things that we've discovered for the two slugs shown here,
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the emerald sea slug,
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which can photosynthesize for its entire adult life cycle,
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nine months after one meal.
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And the lettuce sea slug,
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which lives throughout the Caribbean
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and the primary one I study,
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photosynthesizes three or four months after a meal.
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Both of these animals are able to make chlorophyll,
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which is one of the chemicals
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that is needed for photosynthesis to occur.
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And animals should not be able to do this.
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But somehow these slugs have managed to do it.
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And so this is one of the things that I find really exciting
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and that we're trying to unravel.
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But there's so many things that we still don't know about this.
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So many questions we haven't even thought of yet to ask.
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What's going on here at the cellular level?
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What's happening at the molecular level?
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What's happening at the biochemical level?
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We're starting to get the answers to these questions.
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But there's so much we don't know yet.
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Why are some of them blue?
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I have no idea.
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But some of our slugs are occasionally this wonderful blue color,
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something we hope to someday figure out.
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So I hope you've enjoyed this introduction to sea slugs.
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I hope this leaves you curious to learn more
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about slugs and other invertebrates.
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There's just so much that we don't know and so much out there for us to learn.
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Thank you very much.
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(Applause)
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