Why Do Cephalopods Make Ink?

ANNIE MINOFF: This is Science Friday. I’m Annie Minoff. The most wonderful time of year has arrived. And I am talking, of course, about Cephalopod Week. We are celebrating by highlighting octopus, squid, and cuttlefish science. What started as a response to Shark Week about a decade ago has now become a beloved SciFri tradition.

And one of the many things that makes cephalopod special is their ink. So why do they shoot ink at their predators, anyway? What is that ink made out of? And how did they evolve this incredible skill? Joining me to talk all about inking is my guest. Dr. Lauren Simonitis is a research and biological imaging specialist at Florida Atlantic University based in Boca Raton, Florida. Welcome.

LAUREN SIMONITIS: Thanks so much for having me.

ANNIE MINOFF: So let’s start with the basics. What do cephalopods use their ink for?

LAUREN SIMONITIS: So cephalopods use their ink for a variety of different reasons, but they all have to do with social activities, mostly. So one of the biggest ones is anti-predator. So during a predation event, an ink response is a great way to get out of that situation because ink is really dark in color, so it has a good visual response. Ink also has a lot of chemicals that smell really strongly or really badly. So it’s a chemical response. But it’s also a physical response because it’s really gloopy and it can stick all over them. So it’s this physical, chemical, visual trifecta of a predator weapon.

ANNIE MINOFF: I did not think of ink as gloopy.

LAUREN SIMONITIS: Yes. So ink is produced by the ink gland in cephalopods. But there’s also the funnel organ which produces mucus. And together, they release secretions from the ink gland and the mucus or funnel organ at the same time. And that produces this gloopy, black, smelly substance.

ANNIE MINOFF: OK. So you mentioned predation, which is I think the thing most people are aware of– that cephalopods might use their ink to avoid getting eaten. But what else might they use it for?

LAUREN SIMONITIS: Yeah, so there is some evidence that they can use it between animals of the same species. They can do different forms of ink. And that is also modulated by the amount of mucus they put in the ink. So if they have a ton of mucus in the ink, they can make these ink ropes where– think about if you pick up slime and you pull it for a really, really long time, it creates a nice thin line. They do the same thing with their ink.

And some papers have tied the behaviors that they do with the type of ink responses that they have. They can be used in mating displays. And some, in the deep ocean where light is limited, they can use it for this fluorescent type display where they can’t normally see each other too well.

ANNIE MINOFF: And you were talking earlier about shooting out this inky mucus combination to block maybe a predator’s view. But I also understand that sometimes they can use it to fool a predator?

LAUREN SIMONITIS: Yeah. It is a really cool thing. They make what’s called a pseudomorph. And basically, again, they put a good amount of mucus into it. One of the coolest things about them is their chromatophores, which allow them to change what they look like on the outside. So the color that they are. And what they can do is they’ll change to a really dark black color, and then they’ll ink this little black or dark brown ink mass. And then they change to a light color and jet away.

ANNIE MINOFF: No way.

LAUREN SIMONITIS: And that little ink mass stays. So from the predator’s view, they saw a black squid. Now they see this same black squid-like blob. So they’re going to go and attack that blob and get a mouthful of ink instead of a mouthful of squid.

ANNIE MINOFF: That is wild. So it’s like, don’t look at me. I’m going to jet off over here. The actual cephalopod is this inky stuff over here.

LAUREN SIMONITIS: Yeah, they’re very Houdini-like in that way.

ANNIE MINOFF: Wow. OK, so we talked a little bit about what the ink actually is. I know there’s this mucus involved. But what else is actually in this stuff?

LAUREN SIMONITIS: So there are a lot of really strong chemicals. So there’s a good amount of free amino acids. But one of the main things is melanin. And melanin is the same thing that gives our skin color, our hair color, our iris its color. And the melanin is what gives it that really dark black color. Melanin is a big molecule. It’s photorefractive. So that’s why it has that really dense coloring.

ANNIE MINOFF: And how do they make the ink?

LAUREN SIMONITIS: So within their ink gland, that’s where they generate it. So inside, they have a bunch of different layers of tissue. And they go through this process that have a lot of precursors of melanin. So some of those are– dopamine is one of them. Eumelanin is a form of melanin that they also make. A precursor to melanin. So they have these building blocks that they slowly make into melanin in their ink gland.

And then the ink gland is a little pouch off of their digestive system. And so whenever they are threatened or whenever they want to expel ink, that’s how they do it. They push it out from their ink gland. Because they’re so closely related, the ink gland and their gut, it’s thought that inking evolved as a way to make that excretory pathway more powerful. Because if you’ve ever picked up a frog or if you pick up a little dog and it’s really excited or really scared, it’s really often for animals to release waste as a form of a defense.

ANNIE MINOFF: It’s just a little poo when they’re just a little too excited.

LAUREN SIMONITIS: Exactly. So it’s pretty common. And we see it a lot in the animal kingdom– of animals just essentially flinging their waste products at predators. But then cephalopods, they’ve kind of given it a little extra oomph. We think that they added ink into their excretory pathway, and then it became its own excretory pathway through evolutionary history.

The other great thing about melanin is since it’s so big, a lot of chemicals can bind to it, such as heavy metals. So it’s thought that a lot of the excretory pathway that the ink started as was to detoxify the body because all of the heavy metals were sticking to the melanin. If they can jet off all of that melanin, they can detoxify their body.

ANNIE MINOFF: So it’s kind of like a cleanse.

LAUREN SIMONITIS: Exactly, yeah.

ANNIE MINOFF: An ink cleanse. Got it. And you mentioned earlier that this ink can have a smell. Could I smell it? And what does it smell like?

LAUREN SIMONITIS: So it’s kind of hard when we think about smelling in the aquatic environment versus the terrestrial environment. We both live in fluids, right? They live in water. We live in air. And the chemicals that we smell are transported on that fluid. So it’s a little hard to think about the parallels between smelling it in water and smelling it in air. But I can say from experience working with this, just opening up my vats of ink, it is so pungent that a lot of those odors do aerosolize and do become smellable in the air.

But I think one of the ways that we can really think about what it smells like and what that chemical cue is like is when we think about eating p or squid ink pasta. So as humans, we really like that bitter taste that squid ink has, which is why it’s incorporated in so many different cuisines in the world.

ANNIE MINOFF: And I don’t usually think about that squid ink pasta being smelly, though. When you open the vats of ink, can you describe what it smells like?

LAUREN SIMONITIS: So I get some of my supplies of my ink specifically for cuttlefish. I get them from food suppliers that sell giant vats of ink for squid ink pasta in that kind of stuff. And it kind of smells fishy. It kind of just smells really, really pungent. It’s just an overwhelming, kind of unpleasant smell. But then when you cook it and once it’s diluted– because I have essentially a giant vat versus the little teaspoon that you put in just to add flavor.

ANNIE MINOFF: I mean, not to make this a pasta conversation, but how do they get that ink that they then put in the pasta?

LAUREN SIMONITIS: Yeah. So unfortunately, it is not very animal-friendly.

ANNIE MINOFF: I was worried you might say this.

LAUREN SIMONITIS: Yeah. As far as I know with my research– I can’t say I can speak too much on the culinary side for it or the farming side. But I do know that they do have to kill the animals and harvest the ink glands. Because like I said, if you’re just inking the animal– you know, I always thought that they had them in little inking pens. And then they went in and collected their ink every day. But when you do that, they also have that mucus. And the mucus is not–

ANNIE MINOFF: Delicious?

LAUREN SIMONITIS: –ideal for what you want.

ANNIE MINOFF: OK. So some cephalopods may have been harmed in the making of your squid ink pasta.

LAUREN SIMONITIS: Yeah, unfortunately.

ANNIE MINOFF: So everyone associates octopuses and squid with ink, but cuttlefish also use ink. So I understand there’s at least one species of cuttlefish that uses its ink for a dramatic flourish. This is, of course, the male Andrea cuttlefish. And as I understand it, he shoots out this big plume of ink and then kind of pops through it to impress his mate. Like, ta-da. Here I am.

LAUREN SIMONITIS: Yeah, it is really adorable. I have seen videos of it, and I think it’s super fascinating. And the thing about these type of behaviors is, they are so– you have to be serendipitously seeing these behaviors because these are behaviors that normally happen in the wild and don’t always happen when we have these animals in a managed care facility like an aquarium.

So a lot of what we know about the ties between ink behavior and ink– what it looks like, happen from divers taking videos or fishermen seeing something happening out there and taking a video of it. And I’m sure that there are plenty of other cool inking behaviors that we just haven’t recorded and haven’t been lucky enough to see yet.

ANNIE MINOFF: And do other cuttlefish use ink in that way, too, or just the Andrea?

LAUREN SIMONITIS: As far as I know, that’s the only one I’ve heard of. Another really cool way that some cephalopods use ink is, it also acts as an antimicrobial agent. So there’s a lot of chemicals in it that stop microbial growth. And something in the ocean environment that is really prone to microbial growth is laying your eggs.

ANNIE MINOFF: Yeah, you don’t want any microbes near those eggs.

LAUREN SIMONITIS: Right. And you’re laying them on the substrate or on the sea floor. On seagrasses. And they’re just sitting there and waiting to develop. And you can kind of defend them, but keeping them from getting all those microbes and all that growth on them, all that biofouling is hard. But since ink has antimicrobial components and it also has this predator defense chemicals in it, it’s a really good thing to inject that into your eggs. So there are some species of cephalopods which their eggs are filled with ink.

ANNIE MINOFF: And what is special about the cuttlefish– their ink?

LAUREN SIMONITIS: So honestly, there’s not terrible amounts of differences between different cephalopod inks that have been studied. While we do know a lot about cephalopod ink compared to other inking animals, we still don’t have a very wide variety of ink samples from these guys.

ANNIE MINOFF: You just said other inking animals. What other animals ink?

LAUREN SIMONITIS: So one of the other more well-studied inking animals are these sea slugs. They all belong to the same genus, aplysia. And they actually eat red algae and they take the red algal pigment, they modify it in their body, and then they create some defensive chemicals in their body. And they mix them together and put them out into the environment as this inking response. And it’s this red-purply, really beautiful ink.

ANNIE MINOFF: Red and purple? Is that common? What kind of colors of ink are out there?

LAUREN SIMONITIS: So if we think about cephalopods, those are more of a black ink ranging to a bluish ink. Sometimes cuttlefish can be a little bit more brown. Aplysia– they make ink that is red or reddish-purple. It really depends on the color of algae that they’re eating. What’s interesting about aplysia, too, is that if you feed them green algae or even if you feed them, like, lettuce, they will still produce the defensive chemicals and they’ll still like produce something out of their ink gland. It just won’t be colored anymore. So the color really depends on the algae that they eat.

ANNIE MINOFF: Now, all of these animals that we talked about that produce ink, did they all evolve from a common ancestor?

LAUREN SIMONITIS: So, no, we do not have any common inking ancestor. So even though aplysia– the sea slugs, and cephalopods, they’re both gastropods, they don’t all descend from some common inking ancestor. Their ink gland production sites are not homologous, which means they’re not passed down through evolution.

But then we also get even more out of the box when we think about the other inking animals. So there’s a ctenophore. That ink that has been discovered recently by one of my colleagues. There are crestfish, which are this type of fish that, to my knowledge, we have no records of them inking. Nobody has seen them ink. But in necropsies, they have found ink glands within the crestfish.

ANNIE MINOFF: Interesting. So fish with ink glands.

LAUREN SIMONITIS: Yes. And then even weirder, if I can top that, is there are two species of whales, pygmy and dwarf sperm whales, that ink as a predator response.

ANNIE MINOFF: Interesting. Inking whales.

LAUREN SIMONITIS: Yes.

ANNIE MINOFF: So is it that all these species evolved this inking capability separately?

LAUREN SIMONITIS: Yeah, that is what initially really got me excited about this project. This was kind of the focus of my dissertation– was I joined my PhD lab under Dr. Christopher Marshall, and he had this collection of pygmy sperm whale ink. And he was like, do you want to figure out how does this work as a predator defense? And I was like, that sounds like the best project. Absolutely.

So I started doing research. And we have this really, really unusual ink, and I wanted to compare it to these other well-studied inks. Inks that we know more about. That’s why I picked an aplysia species, Aplysia californica– the California sea hare– and the common cuttlefish. And then I approached it from a lens of convergent evolution, which is exactly what you were just talking about where different species, different animal groups, evolve the same solution to a problem through different pathways.

ANNIE MINOFF: They all had the same good idea.

LAUREN SIMONITIS: Exactly. Yeah.

ANNIE MINOFF: And so you’re comparing all of these different inks. I think you mentioned the pygmy sperm whale. Cephalopods. What did you find in that comparison?

LAUREN SIMONITIS: So the first thing we did was look at the chemistry. And while that work had already been done on the cuttlefish and the aplysia, it hadn’t been done yet on Kogia, which is the pygmy sperm whales. So we were looking across the board what their chemical composition was like, and there wasn’t really a cocktail that every single inking organism has selected for and been like, this is what works.

But what we found is, if we grouped together the invertebrate inks– so if we look at the sea slug ink and the cuttlefish ink, they have a lot of taurine, which is a really strong amino acid. And then if we look at the cuttlefish and the pygmy sperm whale ink, they both have melanin-based ink. So we see comparisons and some similarities, but there’s not one formula that they all share.

ANNIE MINOFF: What is the biggest mystery out there about inking that you would like to solve?

LAUREN SIMONITIS: You know, we have these two theories about how it would evolve through this excretory pathway. Through this antimicrobial pathway. I would just love to find out how this evolved. And unfortunately, it’s a little hard. We do have some good fossil records of cephalopods.

ANNIE MINOFF: Do you get the fossilized ink?

LAUREN SIMONITIS: Yes. There is fossilized ink, and it’s been pretty consistent. We see those precursors to melanin in that ink. We see eumelanin. So we do know that it’s been pretty well-conserved within cephalopods, but we don’t really know that much about the other organisms. So I think really uncovering the evolutionary history of inking across the taxa are really interesting.

ANNIE MINOFF: Well, I wish you the best of luck uncovering that mystery. That is all the time we have. I would like to thank my guest, Dr. Lauren Simonitis, Research and Biological Imaging Specialist at Florida Atlantic University, based in Boca Raton, Florida.

LAUREN SIMONITIS: Thanks so much for having me. I had a great time.

ANNIE MINOFF: Me, too.

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