Thomas Ling: 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 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 me to reveal more about mantis shrimp and their remarkable abilities. So, what actually is a mantis shrimp?
Dr Martin How: 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 are 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.
TL: How many species of mantis shrimp are there?
MH: There are a lot of species of mantis shrimp – several hundred. And they're as diverse as you can get. 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.
TL: I guess the people that have heard about mantis shrimps would have heard about their extraordinary punch. What can you tell me about this?
MH: Yeah, mantis shrimp have got one of the strongest punches underwater. It's one of the fastest 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. It hits so hard that it vaporises, the water in between the club and the object is 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.
TL: What is so special about these bubbles? Why are they so damaging?
MH: So what happens is that the intense pressure that's created between the club and the object causes the water to essentially vaporise 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 both boat propellers when the propellers are spinning too fast.
TL: Do all mantis shrimps punch?
MH: There are two types of mantis shrimp. There are the smashers, which have got the punching arms and there are the spearers. Now spearers have got arms that look a bit more like a praying mantis arm. And so they're essentially the barbed spears that they use for the spearing soft prey. So they'll tend to hunt things like fish or squid or things like that, whereas the smashers will hunt hard-body prey like crustaceans and shellfish. 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, well, you know, well, 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.
TL: Do both mantis shrimp types hunt in different ways?
MH:They do differ little bit. So the spearers 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 trap door. And then when an unsuspecting fish swims overhead, they'll shoot out and spear this fish and pull it down underground where they'll devour it later. Whereas the smashers tend to be a bit more of a foraging species. 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.
TL: Do mantis shrimp attack each other?
MH:They do. They have a very interesting territorial and courtship process that 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 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 one of the male rivals will do is they'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 these are very evenly-sized matched animals then that can escalate to cause damage and even death. So these dangerous animals, not just for their prey, but also for each other.
TL: One of the most amazing things about the mantis shrimp that people might have heard of is their amazing vision. What can you tell me about this?
MH: This is a fascinating topic and one that several researchers have spent their entire careers looking at because they're really incredible animals, they have the most complex visual system 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 – red, green and 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 colour channels. That's way more than any other animal, including several in the UV, so they can discriminate different ultraviolet colours, 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 polarisation 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 polarised 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.
TL: What exactly is polarised light?
MH: 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. 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.
TL: How exactly do mantis shrimp use this polarised light?
MH:It's very useful underwater because the background underwater is very polarised, whereas the objects in front of it tend to be unpolarised. And so what that does is give 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 mantis shrimp 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.
TL: So mantis shrimp use this light to communicate with each other?
MH: Yes, it's essentially a secret form of communication because most of their predators can't see the polarisation of light. Most big fish are a bit more like us in that they can see usually see brightness and a bit of colour, but they can't see any polarisation. So if you can signal to the other members of your species in polarisation, then that's basically like a secret spy signal.
TL: What secret messages do they send to each other?
MH: They'll send the same sort of messages that colour is used for. So generally it's 'I'm a big scary mantis 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.
TL: As mantis shrimp have 12 photoreceptors, do they see more colours?
MH: 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 colour vision system that can see sort of huge amounts of information. And in our early experiments, we've tried testing how well these animals can see colour. And actually they're not particularly good at telling colours apart, surprisingly. And so we, for example, can tell very fine differences between different shades, for example, of green, whereas a mantis shrimp has very blocky colour vision. So it's sort of almost like it's been sort of pixilated into groups of colours. So whereas we can see thousands of colours, we think mantis shrimp can probably only see 12 different colours, a colour that corresponds with each one of their photoreceptors. So this is sort of a different way of seeing colour completely from our own.
TL: Is it true that mantis shrimp eyes more independently?
MH: They have spectacularly mobile eyes, their eyes can move in more degrees of freedom than any animal that we know of. One of the strangest things to watch is how they can rotate their eyes around the 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 colour vision system around to effectively scan their environment. Their eyes act a bit more like light colour printers or colour scanners than normal eyes.
So you can imagine they've got a strip of sensors that sense colour and you have to sort of remove that, sweep that across the visual scene to paint colour onto it.
TL: Do they also use this vision to find mates as well?
MH: 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 mantis shrimp. And they have incredible 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 colour 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 make telling the difference between males and females and so on.
TL: How can you smell in the ocean?
MH: 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 is 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.
TL: How long do mantis shrimp live for?
MH: A very good question. We don't know. So 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, you know, these things can be very long-lived, particularly the big ones. It is quite difficult to tell how old a mantis shrimp is, though, 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 they get larger in the cartilage. But in crustaceans, you can't really do that very easily.
TL: Is it true that some people have had difficulty keeping them in tanks?
MH: I haven't actually heard of them smashing out of an aquarium. I think that might be a bit of an old wives' tale. They certainly pack a punch and they also go for weak spots. So we often will keep them in perspex-lined enclosures in our aquarium 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 to to damage their environment. But then the other thing that sometimes aquarists 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.
TL: I take it they're not great pets then?
MH: Oh, they're fantastic pets. They're very easy to look after, you just have to treat them with respect.
TL: True or false: mantis shrimp are monogamous.
MH: True and false. So some mantis shrimp 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 that 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.
TL: Do they raise their young together?
MH: They raise the eggs. 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 stomach 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.
TL: What is your favourite mantis shrimp fact?
MH: I think my favourite 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 polarisation. We heard about polarised light earlier, which is light where the waves go in one plane: say, up and down or left and right. So,you can have vertical or horizontal polarised light. Circular polarised light is when the lightwaves travel a bit like a corkscrew. It can be a left-hand corkscrew or right-hand corkscrew. And this 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 polarisation is because it's a very physics-y sort of term, but it's a type of light that we can't see and no other animals can see. But mantis shrimp can. And not only can they see it, but they actually make signals to each other. So that is a true secret communication channel.
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