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

Episode Date: December 8, 2025

Today we’re spotlighting an underappreciated group of marine creatures: sea slugs. Don’t let their humble name fool you. They come in vivid neon colors, with patterns that rival the most beautiful butterflies and feather-like external gills and tentacles.There are an estimated 10,000 species of sea slugs and they are incredibly diverse. Some are smaller than a quarter. And one species can weigh more than a terrier, up to 30 pounds. Not to mention their contributions to brain research—understanding their neural networks was the basis for a Nobel Prize in 2000.  Marine biologist Patrick Krug joins Host Ira Flatow to dive into the slimy science of sea slugs. Guest: Dr. Patrick Krug is a sea slug researcher and professor of biological sciences at Cal State LA.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

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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 great 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 very, 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 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, C-slug researcher and professor of biological sciences at Cal State, L.A., 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 who may not be familiar with them, what exactly are they? Sea slugs are a group of mollusks, so they're related to this.

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. Huh. What's the craziest thing a C-Slug can do? 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 gobi fish to suck the fluids out of it and they ride around on the back of a fish. There are sea slugs that are free swimming and live their life in the 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. Yeah, I understand that there's a sea slug that can pop off. its head and regrow its entire body. Is that right? Yeah, this was a rough one for me.

I got 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 many different species that in the group that I work on, embarrassingly enough, when their bodies become sufficiently old and riddled with parasites, they can pop off their head, ditch their old decrepit body, and regenerate an entire new body, full new clean set of organs, and reboot themselves in a matter of a way.

week or two and get a whole new start on life with their same brain. But everything else is new. And they're the most advanced organisms that we know of that are capable of that level of regeneration. That's crazy. Oh, it is crazy. 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. And as you say, there's nothing at that level in biology that can do that. Most of the other organisms that are capable of that kind of regeneration, so something like a C-Star or a flatworm, they're at a simpler level of organization. But C-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 ahead, it kind of holds, I think, a lot of potential insights for what's at least possible. Let's dive in a little deeper. Sea slugs, especially the nudie brunks, the beautiful ones that I used to have in my tank. They really are colorful. to have these wild body shapes.

If you Google Nudabongs, you won't be disappointed. We also have some pictures at ScienceFrider.com slash sea slugs of the Nudibrons. 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. Yeah, I mean, I think blah is the word you're probably. Thank you. I didn't mean to insult them very much.

I know, it's sad but true. So it's a great question. The land snails and slugs, the group we traditionally called pulmonates, 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. Wow. 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 huge. 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. One, by being like basically you're super picky four-year-old that only eats one thing, each species of sea slug eats one kind of sponge or coral or sea. Really? And only that. And they mostly eat things that are toxic, 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. So the predators know not to eat them? 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 tried to eat that thing once and it made me so sick, never again. Now, of course, one of the obvious questions would be, why can they survive their own

toxicity and that the predator? 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 toxic compounds away from most of their cells, and they become much more toxic than their own prey, but they can kind of release these toxins and a blast of super bad-tasting mucus in the face of a would-be predator to be really repulsive. Can we find anything useful in these toxins for our use, medicinally? We do and we have. There are FDA-approved cancer drugs now that were originally isolated from sea slugs,

from their defensive chemistry 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 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. We have to take a break, but when we come back, sea slugs species that incorporate photosynthesis, yes. You study a group of sea slugs that can perform photosynthesis.

They're not plants? How do they do that? Yeah, there's 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 kleptoplasti. So the theft of chloroplasts, when they eat seaweed, they suck the juice out of it. They kind of stab seaweed 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.

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. Is that sounds like something like coral, right? Don't they have some symbiotic relationship with algae? They do. Many, yeah, so many marine animals have a photosimbiosis with a free-living partner like a dynophagulet 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 photosimbiosis, but we're hoping

that by studying it will gain insight into how those longer-term mutualisms evolve and that it might also help us understand ways that we could maybe improve our own clean energy technology. If we could develop better biophotovoltaic 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. 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? 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 relationship 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 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 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 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 contrast that with something like sharks, right, there's less than 400 species of sharks in the ocean.

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. So how do they get along with each other? For example, how do they reproduce? elaborately and often.

Should we go there? Go ahead. 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.

Any two slugs or more that encounter each other can always. ways reproduce together. 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 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 often some of these things evolve too. Some slug groups have evolved hypodermic insemination where they can inject sperm 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 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 really huge sea slugs you mentioned, the sea hares have this behavior where they lay egg masses that are laden with pheromones that humans have given names to like seducin and attractin and enticing, 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 make. 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. Wow. Speaking of these sea hares, are they outliers in the seaslog world? How different are they from the others? 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 hair. A pleading. Vecaria 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. No kidding. 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. Yeah. Eric Candell won the Nobel Prize for that. Yeah, about 25 years ago, that work was recognized. How big they were. Because it makes them so easy to study. You know, if you think about the size of a human nerve, 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 seahare 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 neurogenerative conditions by just basically understanding how do nerve cells work. Huh. You sound like you hate your job.

How did you get interested in C slugs? Oh, my poor parents. This is the only thing I ever, ever wanted to do from the time I was a little kid. 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. You know, that's what they say. If you love what you do, you never work a day of your life.

I mean, they're forgetting about the paperwork, but aside from that, yes. Yeah, yeah. Well, this is just fascinating, Dr. Kroog. Thank you, Patrick, for taking time to be with us today. I hope your listeners take you up on the invitation to Google C-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. You're welcome. Dr. Patrick Krug, C-Slegg. 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 sciencefriady.com slash sea slugs. That's sciencefriady.com

slash sea slugs. This episode was produced by Shoshana Bucksbaum. I'm Ira Flato. Thanks for listening.