Don’t Let Their Name Fool You—Sea Slugs Are Awesome

Across their 10,000 species, sea slugs sport striking colors, external gills, and even the ability to regrow a body from a severed head.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Flora Lichtman?

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• 0:26Ira Flatow?

  Hey, it's Ira Flatow, and you're listening to Science Friday. Today on the show, we're spotlighting an unappreciated group of sea creatures, sea slugs. Now, I know that name is not very glamorous, so they probably evoke an image of garden slugs, you know, those gray, slow-moving snails, right? But I can tell you from personal experience, when I had a saltwater aquarium, that my sea slugs were some of the most colorful, beautiful creatures in the tank. And you know what? It's not just their beauty that is impressive. First of all, their diversity is vast, with an estimated 10,000 different species. Some are smaller than a quarter, and one species can weigh more than your terrier. I'm talking

• 1:11Ira Flatow?

  30 pounds. And as I say, they come in vivid neon colors with patterns that rival the most beautiful butterflies, come complete with feather-like external gills and tentacles, not to mention their usefulness in brain research. Understanding their neural networks was the basis for a Nobel Prize. Joining me now to take us inside the slimy science of sea slugs is my guest, Dr. Patrick Krug, sea slug researcher and professor of biological sciences at Cal State LA, based, of course, in Los Angeles. Welcome to Science Friday. Thank you for having me. I was always enamored with sea slugs when I had them in my tank. For listeners

• 1:53Ira Flatow?

  who may not be familiar with them, what exactly are they?

• 1:57Speaker 02

  Sea slugs are a group of mollusks, so they're related to the snails and slugs that I think most people are familiar with, either from their shells or from having maybe a less charismatic experience with a garden slug, which are kind of notoriously icky from their mucus and the slime that they create or from eating the vegetables in your garden. Sea slugs are relatives that live in the ocean and they have evolved ways to make a living without relying on the protection of a heavy shell.

• 2:30Ira Flatow?

  Huh. What's the craziest thing a sea slug can do?

• 2:34Speaker 02

  Wow, that's more of a top 10 list than a one-off. Their biology is really fascinating because so many different groups have evolved different ways to make a living without relying on a shell that involve very specific partnerships with other organisms, usually their food. So there are sea slugs that feed on fish and they have to grab a hold of a little goby fish to suck the fluids out of it and ride around on the back of a fish. There are sea slugs that are free swimming and live their life in the

• 3:19Speaker 02

  open water in the middle of the ocean that are called sea butterflies. There are sea slugs that can suck up the chloroplasts from seaweed that they feed on and keep them alive in their own bodies and become photosynthetic and can go months without eating and just live off of sunlight. There are incredible adaptations that different sea slugs show to kind of survive in an ocean where everything is trying to eat you all the time.

• 3:49Ira Flatow?

  Yeah, I understand that there's a sea slug that can pop off its head and regrow its entire body. Is that right?

• 3:59Speaker 02

  Yeah, this was a rough one for me. I got to to read that paper and I had collected many crawling severed heads of slugs over the years and I always thought I had done it like I had damaged the slug in collecting. And as soon as I read this work, it was like, oh, I was meant to discover this. Damn. This has now been shown in in many different species that in the group that I work on, embarrassingly enough, when their bodies become sufficiently old and and riddled with parasites, they can pop off their head, ditch their old decrepit body and regenerate an entire

• 4:44Speaker 02

  new body, full new clean set of organs and reboot themselves in a matter of a week or two and get a whole new start on life with their same brain. But everything else is new and they're they're the most advanced organisms that we know of that are capable of that level of regeneration.

• 5:07Ira Flatow?

  That that's crazy.

• 5:09Speaker 02

  Oh, it is crazy and and creeperific, having just gone through Halloween season, to think about crawling heads that are in the process of regrowing a whole body. But I think regenerative medicine has a lot to learn about how they can do that and what it might hold in store for human medicine.

• 5:27Ira Flatow?

  And as you say, there's nothing at that level in biology that can do that.

• 5:33Speaker 02

  Most of the other organisms that are capable of that kind of regeneration, so something like a sea star or a flatworm, they're at a simpler level of organization. But sea slugs are a little bit more like us. They have the same kind of bilateral symmetry, a left and a right. They have the same complete digestive system and basically all the organ systems that we have. So they're a lot more like us. And so to be able to regenerate a whole body from just a head, it kind of holds, I think, a lot of potential insights for what's at least possible.

• 6:07Ira Flatow?

  Let's dive in a little deeper. Sea slugs, especially the nudibranchs, the beautiful ones that I used to have in my tank, they really are colorful. They have these wild body shapes. If you Google nudibranchs, you won't be disappointed. We also have some pictures at sciencefriday.com/cslugs of the nudibranchs. Land slugs, as you mentioned before, are related, right? Why are the land slugs so ugly, should I say? And the sea slugs so colorful?

• 6:41Speaker 02

  Yeah, I mean, I think blah is the word you're probably looking for.

• 6:45Ira Flatow?

  Thank you. I didn't mean to insult them very much, but...

• 6:48Speaker 02

  I know. It's sad, but true. So, it's a great question. The land snails and slugs, the group we traditionally called pulminates, meaning the lunged snails and slugs, because they're air breathing, they evolved out of the sea slug lineage. So, your garden slugs evolved out of the sea slugs when they moved on to dry land and colonized terrestrial and even freshwater environments. And the land snails and slugs became one of the big success stories in the history of animals. There's probably something like 25,000 species of land snails and slugs.

• 7:28Ira Flatow?

  Wow.

• 7:29Speaker 02

  In comparison, maybe 10,000 species of birds, maybe 10,000 species of reptiles other than birds. So, 25,000, that's an enormous diversity of land snails and slugs. Admittedly, not so colorful, but hugely successful. The difference really is that the sea slugs have to contend with an ocean full of hungry fish that are trying to eat them all the time. And the solution that they have evolved to life without a shell is that almost every species lives in a strict partnership with their food. And that does two things.

• 8:11Speaker 02

  One, by being like, basically, your super picky four-year-old that only eats one thing. Each species of sea slug eats one kind of sponge, or coral, or seaweed.

• 8:24Ira Flatow?

  Really?

• 8:24Speaker 02

  And only that. And they mostly eat things that are toxic. Um, things that have a chemical that would poison you or stinging cells that would zap you. And the slugs can concentrate those toxins in their own body, or even take up those stinging cells to arm themselves against predators.

• 8:47Ira Flatow?

  So the predators know not to eat them?

• 8:50Speaker 02

  So the slugs have, much like butterflies or poison dart frogs, the slugs have evolved these brilliant colors to make it easy for predators to remember, oh God, I, I tried to eat that thing once and it made me so sick, never again.

• 9:06Ira Flatow?

  Mm hmm. Now, of course, one of the obvious questions would be why, why can they survive their own toxicity and not the predators?

• 9:14Speaker 02

  It's a great question. I don't think it's one that we actually know the answer to. They not only survive these toxins, they concentrate them in their own bodies, but they store them in special glands so they keep the, the toxic compounds away from most of their cells. And they become much more toxic than their own prey, but they can kind of, they can kind of release these toxins in a blast of super bad tasting mucus in the face of a would be predator, um, to be really repulsive.

• 9:46Ira Flatow?

  Uh, can we find anything useful in these toxins for our use medicinally?

• 9:52Speaker 02

  We, we do and we have. There are FDA approved cancer drugs now that were originally isolated from sea slugs, from their defensive chemistry, um, and from the food that they eat. And I think in the future, there will be even more. There is a lot of research that's been done on these compounds. And one of the kind of interesting quirks of this area of study is the slugs will often find a thing that we can't necessarily find easily, like a very tiny seaweed that's very hard for humans to collect, but the slugs will find it and store its chemistry and we can find the slug. So they are very good collectors

• 10:36Speaker 02

  of obscure things and they store the chemistry in a way that makes it easier for humans to find it and then to study its potential medicinal properties.

• 10:51Ira Flatow?

  We have to take a break, but when we come back, sea slug species that incorporate photosynthesis, yes.

• 11:10Speaker 03

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• 11:54Speaker 03

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• 12:07Speaker 04

  And from the Alfred P. Sloan Foundation, working to enhance public understanding of science, technology, and economics in the modern world.

• 12:23Ira Flatow?

  You study a group of sea slugs that can perform photosynthesis. They're not plants? How do they do that?

• 12:32Speaker 02

  Yeah, it's, um, they're sort of a crazy fusion of plant and animal in a way that would make any vegetarian green with envy. They do this amazing trick called kleptoplasty. So the theft of chloroplasts, when they eat seaweed, they suck the juice out of it. They kind of stab seaweed like, like a kid with a juice box. They puncture the seaweed and they suck out the juice. But instead of digesting it all, their gut cells suck up the chloroplasts into the slug's own cells, and they keep them alive.

• 13:12Speaker 02

  And those chloroplasts live in a protected vacuole, like a little sack inside the cell, and they can survive for weeks or in some cases for months in the slug. And they just carry on photosynthesizing and they feed the slug.

• 13:28Ira Flatow?

  That sounds like something like coral, right? Don't they have some symbiotic relationship with algae?

• 13:34Speaker 02

  They do. Many, yeah. So many marine animals have a photosymbiosis with a free living partner, like a dinoflagellate or a green alga of some kind. Lots of organisms like lichens, for instance, on land have evolved some kind of partnership with a photosynthetic organism that gives them sugars in exchange for a place to live. The slugs are kind of like a first step on a way to that partnership because they have to recharge by feeding again. So that it's not quite a stable photosymbiosis,

• 14:15Speaker 02

  but we're hoping that by studying it, we'll gain insight into how those longer term mutualisms evolve. And it might also help us understand ways that we could maybe improve our own clean energy technology. If, you know, we could develop better bio photovoltaic cells, for example, by being able to stabilize chloroplasts or cyanobacteria in a fuel cell. By learning how the slugs can do this, we might be able to benefit from it and develop better solar cell technologies.

• 14:47Ira Flatow?

  I want to go back to something you mentioned before. You mentioned that every sea slug species eats only one type of food. Now, to me, that would sound like that's not very beneficial if you have to depend on one kind of food to stay alive. What am I missing here?

• 15:06Speaker 02

  A student asked me that in class yesterday. It's a great question. So there's a cost and a benefit, right? The concept is specialization. You see it a lot with insects. So if you go to a tropical rainforest, for example, shake any bush and you're going to find one or more insect species that is in a highly specialized relationship with that plant species and only that plant species. And that is the reason why there are millions of species of insects on this planet, because they've evolved highly specialized relationships with plants. Sea slugs have evolved the same kinds of relationships in the ocean with the things that they eat. If you want to be able

• 15:50Speaker 02

  to cope with highly toxic compounds in your food, for example, that often requires that you really specialize on one thing, right? Because you're basically eating poison. And so those kinds of highly specialized relationships mean, well, you can't eat every poison in the world, but you can adapt to one poison and be able to eat something that no one else is eating, right? So it's kind of like going into a buffet. And if you're the only one who eats meatloaf, you get all the meatloaf. The reason why, as you alluded to it in the opening, there are

• 16:32Speaker 02

  perhaps 10,000 or more species of sea slugs, this incredible diversity and species richness, that's related to the specialization of each species, because they're not competing with each other for food. I see. If you, right, if you contrast that with something like sharks, right, there's less than 400 species of sharks in the ocean.

• 16:57Speaker 02

  They get their own week on the Discovery Channel. Everybody knows sharks, but there's not that much species richness in the group of sharks. And it's largely because they're all basically fighting for the same food. They're occupying the same niche. And even though they've been around for so long, they're kind of generalist feeders. And they don't have this high degree of specialization. Octopuses are like that, too. They're amazingly intelligent and super sophisticated, but they're kind of generalized feeders. And they end up in competition with each other. And I would argue that limits their diversity in a way that highly specialized feeders like sea slugs and insects, they can evolve a much greater species richness.

• 17:41Ira Flatow?

  So how do they get along with each other? How, for example, how do they reproduce?

• 17:47Speaker 02

  Elaborately and often.

• 17:51Ira Flatow?

  Should we go there? Go ahead.

• 17:55Speaker 02

  One of the other sort of fascinating areas of snail and slug biology is their reproductive systems are incredibly elaborate. Many groups of invertebrates like worms and other mollusks can often have fairly simple reproductive anatomy. But the snail lineage in particular has evolved extremely complicated reproductive systems for some historical reasons. And the upshot of that, all sea slugs are hermaphrodites. So they are all simultaneously male and female. And that leads to some sort of interesting complications.

• 18:40Speaker 02

  Any two slugs or more that encounter each other can always reproduce together.

• 18:47Speaker 02

  But it's also complicated, right? Because you have the option to be the male or the female or both. And so how they resolve that is different in different groups. Sometimes they they trade sperm and fertilize each other's eggs. Sometimes they get in these sort of violent tussles. In some groups, slugs will inject manipulative chemicals into their partner to try to basically make them relax and adopt one sex role. It's a little astonishing how how often some of these things evolve, too. Some slug groups have evolved hypodermic insemination, where they can inject sperm

• 19:31Speaker 02

  anywhere on the body of another slug in what's called hit-and-run mating. Wow. Yeah, kind of like, pardon me, do you have the time? Doink. Now you're pregnant and I'm crawling on my way. And this has evolved in a lot of different groups. There are flatworms that do it. Even bedbugs do a version of this. So it it crops up all over the animal tree of life when hermaphroditic organisms are able to evolve some of these mechanisms called traumatic mating. It has a tendency to show up. And so in different groups, you see all these kinds of different behaviors. The the really huge sea slugs you you mentioned, the sea hares,

• 20:16Speaker 02

  have this behavior where they lay egg masses that are laden with pheromones that humans have given names to, like seducin and attractin and enticin, which kind of tip the hat to what they do, which is these pheromones diffuse into the water and lure other sea hares to the area and turn them on. And get them in the mood to mate and then they lay more eggs, which release even more pheromones. And so it leads to group mating scenarios that pretty quickly snowball out of hand.

• 20:53Ira Flatow?

  Wow. Speaking of these sea hares, are they outliers in the sea slug world? How different are they from the others?

• 21:01Speaker 02

  They're kind of fascinating. They're outliers in their size, so they can grow to be the size of footballs or even basketballs in the case of the really big sea hares, like the black sea hare Apleja vicaria that we have here in California where I am. They are vegetarians. They eat seaweeds. So one of their very unusual characteristics is they can exude ink that can be like a bright purple in some species. It can be really beautiful, actually. And they exude other substances that can do things like they can blind the antennae of lobsters as a defense.

• 21:38Ira Flatow?

  No kidding.

• 21:40Speaker 02

  And they have these extraordinarily large, they're called neuromuscular junction. It's where the nerves touch the muscles. And they're large enough, you can actually see them. They can be like two millimeters in size.

• 21:54Ira Flatow?

  Yeah. Eric Kandel won the Nobel Prize for that.

• 21:57Speaker 02

  Yeah, about 25 years ago, that work was recognized.

• 22:01Ira Flatow?

  How big they were.

• 22:02Speaker 02

  Because it makes them so easy to study. You know, if you think about the size of a human nerve, th- those fibers are so tiny. It's difficult to manipulate them to like stick a needle or a probe into a nerve cell is very hard. But if that nerve cell is huge, it's just a much more tractable system to study. And the basic study was, you know, how did nerves record experience as memory? Basically, how does a sea hare remember how many times you've poked it? So not maybe the most sophisticated sounding problem, but it could have implications for understanding Alzheimer's disease and other neurodegenerative conditions by just basically

• 22:47Speaker 02

  understanding how do nerve cells work?

• 22:49Ira Flatow?

  Huh. You sound like you hate your job. How did you, how did you get interested in sea slugs?

• 22:57Speaker 02

  Oh, my poor parents. This is the only thing I ever, ever wanted to do from the time I was a little kid. I could have told you mollusks was the only thing I really cared about, but my, my sixth grade science teacher, Mrs. Marler, had, just like you, she had a nudibranch in an aquarium when I was 10 years old. I saw it and it was the most beautiful thing I had ever seen in my life. And from that moment, it's the only thing I ever wanted to do. And somehow I am lucky enough that I get to actually do it.

• 23:28Ira Flatow?

  You know, that's what they say. If you love what you do, you never work a day of your life.

• 23:34Speaker 02

  I mean, they're forgetting about the paperwork, but aside from that, yes.

• 23:37Ira Flatow?

  Yeah, yeah. Well, this is just fascinating, Dr. Krug. Thank you, Patrick, for taking time to be with us today.

• 23:44Speaker 02

  I hope your listeners take you up on the invitation to Google sea slugs. And I think everyone will be delighted with the images they see. And thank you so much for having me. It's been great talking to you.

• 23:55Ira Flatow?

  You're welcome. Dr. Patrick Krug, sea slug researcher and professor of biological sciences at Cal State LA. If you'd like to see some pictures of the sea slug and a video of a detached sea slug head regrowing its body, I want to see that. That's a no brainer. Go to sciencefriday.com/cslugs. That's sciencefriday.com/cslugs. This episode was produced by Shoshana Buxbaum. I'm Ira Flatow. Thanks for listening.