Instant Genius - The bizarre biology of the mantis shrimp

Episode Date: April 5, 2021

What is your favourite animal? If you know anything about the mantis shrimp, it might well be your top pick. Dwelling in shallow tropical waters, these mysterious predators not only wield one of the strongest punches in nature, but also a one-of-a-kind visual system that scientists are only just making sense of. One of these scientists is Dr Martin How from the University of Bristol. He joins us on this week's episode of the Science Focus Podcast to reveal more about mantis shrimp and their remarkable abilities. Let us know what you think of the episode with a review or a comment wherever you listen to your podcasts. Subscribe to the Science Focus Podcast on these services: Acast, iTunes, Stitcher, RSS, Overcast Read the full transcription of this episode [this will open in a new window] Listen to more episodes of the Science Focus Podcast: Everything you ever wanted to know about… the deep sea with Dr Jon Copley Andrew Hunter Murray and Dan Schreiber: Is there really no such thing as a fish? Brad Lister: Are we facing an insect apocalypse? Neil Gemmell: The genetic hunt for the Loch Ness Monster Neil Shubin: How do big changes in evolution happen? Mark Lynas: Could leaving nature to its own devices be the key to meeting the UK’s climate goals? Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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in your app store. Hello and welcome to the Science Focus podcast. I'm Thomas Ling, staff writer at BBC Science Focus magazine. What is your favourite animal? If you know anything about them, the mantis shrimp might well be your top pick. Dwelling in shallow tropical waters, these mysterious predators not only wield one of the strongest punches in nature,

but also a recently discovered one-of-a-kind visual system that has left scientists' stumped. One of these scientists is Dr Martin Howell from the University of Bristol. Today he joins me to reveal more about mantis shrimp and their remarkable abilities. So I guess I should start from the top and ask, what is a mantis shrimp? Well, a mantis shrimp is neither a mantis nor a shrimp. It's its own group of animals, its own order called the stomatopods. And they're a very ancient lineage of crustaceans.

They diverged from other crustaceans about 400 million. years ago. So yeah, very interesting and diverse group that is very different from any other crustaceans that we know about. And how many species are there of mantis shrimp? There are a lot of species of mantis shrimp, several hundred, and they're as diverse as you can get. So the smallest adults are about an inch long and then the longest are almost a foot long. So they're pretty diverse in their range of body shapes and certain colour patterns. as well. They really look very different. I guess the people that have heard about mantis shrimps would have heard about their extraordinary punch.

What can you tell me about this? Yeah, mantis shrimp have got one of the strongest punches underwater. It's one of the fastest feeding movements that we know of. What they do is they ratchet up a muscle in their arms and then release a trigger, which releases this incredible force of the blow. And as it hits, it hits so hard that it vaporizes the water in between the club and the object it's hitting, creating cavitation bubbles, which are very destructive. So it's a very powerful thing, not something you want to accidentally have whacking your finger. So what is so special about these bubbles then? Like, why are they so damaging?

So what happens is that the intense pressure that's created between the club and the object causes the water to essentially to vaporize, to steam. And that causes tiny microbubbles. And the pressure that is then re-released after these bubbles collapse creates a second shockwave effectively. It's the same force that destroys boat propellers when the propellers are spinning too fast. What about the other kind of weapons that some mantis shrimp will? They understand sort of punching isn't their only way to attack a prey? Yeah, so there are two types of mantis shrimp. There's the smashes, which have got the punching arms, and there's the spearers.

Now, spears have got arms that look a bit more like a preying mantis arm. And so they're essentially, they're barbed spears that they use for spearing soft prey. So they'll tend to hunt things like fish or squid or things like that, whereas the smashes will hunt hard-bodied prey like crustaceans and shellfish. Both soft-shell and hard-shell animals are full of protein. So this is a high-energy diet, which is what they need for this very active lifestyle that they have. They're very active predators. And so the soft-bodied animals will give you protein,

but you need a different system for, dealing with those. And so the spears are really good for impaling soft bodies, while the hammers are very good for smashing hard, protective layers. So using these different weapons, do they hunt in different ways, or do both these type of mantis shrimp hide and then leap out at their prey? Yeah, they do differ a little bit. So the spears tend to be ambush predators. And what they'll do is they'll build a burrow, and then they'll camouflage themselves in the entrance of this burrow looking very inconspicuous, sometimes with a layer of mucus covered with sand. So it's essentially like a trapdoor.

And then when an unsuspecting fish swims overhead, they'll shoot out and spear this fish and put it down underground, well, they'll devour it later. Whereas the smashes tend to be a bit more of a foraging species. So they'll go out and they'll ramble around their environment until they find a suitable prey item, and then they'll smash it to pieces. So do the mantis shrimp use their spears and their punches against others of their kind? They do. So they have a very interesting territorial and courtship process, which involves these weapons. So for example, one of the most dangerous things for a mantis shrimp is to get caught up in a territorial dispute.

and what they do is they have an escalating combat. So rather than going full on trying to destroy each other, they'll escalate gently with something that we call bottom bashing. So what the male will do, what one of the rivals will do is it'll curl its tail around in front of it, and then its rival will just give it a little tap, and then they'll take that in turns, tapping each other, escalating and escalating until one of them backs down. Now, if those are very evenly size-matched animals,

then that can escalate to cause damage and even death. So these are dangerous animals, not just for their prey, but also for each other. I guess one of the most remarkable thing about the mantis shrimp that people might have heard of is their amazing vision. So what is it that's so special about mantis shrimp parties? This is a fascinating topic and one that several researchers have spent an entire career looking at

because they're really incredible animals. They have the most complex visual system really known to science. It's really incredible. So one of the ways that it differs from other vision is its diversity of colour. So we, for example, have three different colour channels, a red, green and a blue in our eyes, which we use for discriminating the rainbow of colours that are visible to us. Most other animals have something similar. A few species will have a few more colour channels.

but a mantis shrimp just does them all to dust. It has 12 different color channels. That's way more than any other animal, including several in the UV, so they can discriminate different ultraviolet colors as well as the visible range. And then layered on top of that, they also have sensitivity to a property of light

that we can't see, which is the polarization of light. So this is to do with the angle of light waves. And we don't usually think about it unless we wear Polaroid sunglasses to screen out polarize glare. But these animals can actually see that as an extra channel of information. It's like an extra colour that's visible for them, but not for other animals.

So what exactly is polarised light? So the polarisation of light is a tricky thing to describe. But essentially, if you think of light as a wave, we think of it as an electromagnetic wave in the spectrum. Those waves, as they travel through space, they have an orientation. So the wave could be a vertical one or a horizontal one or any angle in between. between. And beams of light where all the waves are lined up are called polarised. And that happens when light reflects off objects or scatters in the air. And so we tend to use polaroid filters to try and filter out this polarised light, either in the form of polaroid sunglasses or in

Polaroid filters that photographers sometimes use to screen out the glare from the sky, for example. Fishermen love to use polaroid sunglasses because it helps them to screen off the reflected light from water so you can see fish underwater. So imagine having these polaroid sunglasses built into your own vision so you can see that as an extra channel of information. And that's what these animals do. So how is that good to they use this polarised light then? It's very useful underwater because in the background, in general, underwater is very polarised, whereas the objects in front of it tend to be unpolarized. And so what that does is it gives you an extra bit of contrast that you can use for detecting either predators or prey. And so that's one of the main reasons that

they use it. But then the other thing that they do is they actually signal to each other using polarised patterns. So we're familiar probably with the colourful patterns that these Manchester have, but some of these colours are also very strongly polarised. And that means that they can see that as a polarised, essentially a colour that's polarised, whereas other animals can't. So can they effectively use polarised light to communicate with each other?

Yes, and it's essentially a secret form of communication because most of their predators can't see the polarisation of light. I think most big fish are a bit more like us, and that they can see, usually they can see brightness and a bit of colour, but they can't see any polarisation. So if you can signal to your other members of your species in polarisation, then that's basically like a secret spy signal. So what sort of messages do they send to each other using their secret spy channel?

They'll send the same sort of messages that colour is used for. So it'll be generally, I'm a big scary man to shrimp and you need to stay off my territory. or it'll be I'm a healthy mantis shrimp come and reproduce. Those are generally the two messages that are portrayed by most of these colour patterns. As mantis shrimp have 12 photoreceptors, do they effectively see more colours than humans do? That's an interesting topic and one that we've been working on for a while. So you'd expect with this incredible diversity of different sensors for colour, you'd expect them to have an incredible color vision system

that can see huge amounts of more information than we can. And in our early experiments, we've tried testing how well these animals can see color. And actually, they're not particularly good at telling colors apart, surprisingly. So we, for example, can tell very fine differences between different shades, for example, of green, whereas a mantis shrimp has very blocky color vision. So it's sort of almost like a, like it's been sort of pixelated into groups of colors.

So whereas we can see thousands of colors, we think mantisrim can probably only see 12 different colors, a color that corresponds with each one of their photoreceptors. So it's sort of a different way of seeing color completely from our own. And is it true their eyes move independently as well? And if so, how does that help them out? They have spectacularly mobile eyes. Their eyes can move in more degrees of freedom than any other.

that we know of. One of the strangest things to watch is how they can rotate their eyes around the eye stalk. That's something that no other animals really do. And one of the reasons why we think they do that is that they need to move their color vision system around to effectively scan their environments. Their eyes act a bit more like color printers or color scanners than normal eyes. So you can imagine they've got a strip of sensors that sense color. And you have to sort of move that, sweep that across the visual scene to paint colour onto it. Do they also use this unique colour vision to find mates as well? Yes, they'll use it for recognising each other and for finding mates,

and they'll use it for finding prey in particular. They also use their vision for constructing their burrows. So one of the species that we spend a lot of time looking at is the peacock man. to shrimp. And they have incredible sort of builder, builder tendencies to pick the right materials from their environment and construct the entrances of their burrows. And they'll pay particular attention to the color and texture of the different things that they're picking up. But as well as vision, they also have a very keen sense of smell. And so they'll use smell also to identify each other, potentially telling the difference between males and females and so on.

How can you smell in the ocean? It's much easier to smell in the ocean because you don't have to worry about things being carried on the wind. If you think of chemicals actually being carried by the water and then you're detecting, it's essentially like tasting the chemical from the water. So when I say smell, it's probably more akin to what we think of as taste. So I guess a big question is, how long do mantis shrimp live for? A very good question. We don't know.

it'll vary massively depending on the species. So the small, kind of one inch long species will probably be quite short-lived. But the very big ones, we had one living in the lab for about 20 years. It was collected by my previous boss, and that was living in the lab for 20 years, and it was still going last time I saw it.

So these things can be very long-lived, particularly the big ones. It's quite difficult to tell how old Amanda. shrimp is, though, because because they shed their exoskeleton when they grow, there's not really the same sort of way of kind of counting the rings in hard structures like the cuticle that you could do in other animals. So, for example, in fish, you can count the number of rings in the bones as the bones get larger in the cartilage. But in crustaceans, you can't really do that very easily. So you said then about sort of keeping a mantis shrimp in the lab. Is it true that some people have had difficulty keeping them

because they can punch out of their casing? I haven't actually heard of them smashing out of an aquarium. I think that might be a bit of an old wife's tail. They certainly pack a punch and they also, they go for weak spots. So we often will keep them in perspex lined enclosures in our aquaria, and they'll find a weak point, and then they'll work at it and work at it until they've broken their way through.

So they can definitely use their punch to damage their environment. But then the other thing that sometimes aquaists finds is if you put a lump of coral in your prize aquarium at home with all your lovely coral fish, you might find that over the next few weeks, your fish start disappearing for no reason. And then it turns out actually that there's a

mantis shrimp living in that coral head. So they can be very damaging to all the other animals in the aquarium if you're not careful. So probably not the best pet then? Oh, they're fantastic pets. They're very easy to do it after. You just have to treat them with respect. Okay. True or false?

Mantis shrimp are monogamous. True and false. So some mantis shrimp are and some mantis shrimp aren't. And so in particular, the ones that I'm thinking, of that are monogamous are these very large zebra striped mantis shrimp. And these are the ones that grow to about a foot long. You find them on the barrier reef in Australia. And they form these huge underground burrow networks in monogamous pairs. And what will happen is that the male will take the role of catching fish and the female will take the role of generating the brood. And the

male will feed the female with the fish that he catches. And they have this amazing kind of pair bond that lasts for years and years and years as far as we know. Very difficult to study in the wild, but I think from observational studies and from how we collect them, we find that they're always in monogamous pairs. So do these pairs raise children as well? They raise the eggs. So with the mantis shrimp, they have a larval phase.

So what happens is that the female will rear the eggs and she'll look after the eggs very diligently. She carries them on her sort of stomach surface, on a ventral surface, and she'll aerate them constantly and keep them clean. And then once they hatch, they'll then disperse up into the water column as larvae. They become part of the plankton. And they'll float around in the ocean for days or weeks, and then they'll settle back to the ground as new baby mantis shrimp.

Okay. I guess I should probably now ask, what is your favourite? mantis shrimp fact? I think my favorite mantis shrimp fact is that mantis shrimp are the only animals that we know of that can see a very strange property of light called circular polarization. We heard about polarized light, which is light that has the waves go, say, up and down or left and right, so you can have vertical or horizontal polarized light. Circularized light is when the light wave travels a bit like a corkscrew.

So it can be a left-hand corkscrew or a right-hand corkscrew. And it happens very rarely in nature. And so it's a really unique form of vision and one of the most amazing things about this incredible group of animals. Now, it's really hard to understand what circular polarization is because it's a very physics-y sort of term. But it's a type of light that we can't see. No other animals can see, but mantis shrimp can.

And not only can they see it, but they actually make signals to each other on their cuticle that is circular polarised. So that is a true secret communication channel. Thank you for listening to this episode of the Science Focus podcast. If you've enjoyed this episode, please leave a review whatever you're listening to us. More amazing facts about the mantis shrimp and their remarkable vision on sciencefocus.com where you can also find the latest science news and stories. Thank you for listening to the Science Focus podcast from the BBC Science Focus magazine,

with the UK's best-selling science and technology monthly, available in print and in several digital formats throughout the world. Find out more at sciencefocus.com or look out for us in your app store. This podcast is sponsored by Name, Audio and Focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name audio believes you can have digital precision with analogue warmth. Alongside French acoustic specialist focal, Name creates high-end audio systems combining innovation with craftsmanship, so you can listen to music, just as the artist intended. Discover more at namea Audio.com.