Do fish know what they look like?

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1:05The blue ones are the Neon Tetris. That big orange one is a sword tail. Oh yeah, wow. Hello and welcome to CrowdScience from the BBC World Service, the show that goes fishing every week for answers to your science questions. That wee one's an endler. the wee one with the orange tail. Oh yeah, right in the background. I'm Anand Jagatia and our listener for this episode is talking to us on video call from the Czech Republic where he lives with his pet tropical fish. Would you like to see a frog? Yeah, go on then. Always. Do you see him there? Oh yeah, that is very cool. A wee flesh-coloured fella.

1:50Right, back to what we're meant to be doing. The question... There's something about the way these bright, colourful creatures swim around his tank that inspired Ian to reach out to us with a question. The fish are agitated just now, but when they're relaxed, all these ones with the blue stripe will sort of shoal all together. And if I add new ones, they swim with others in the tank of the same breed who were there before. But they know what fish to swim with, so they obviously somehow recognise the other fish. But unless the fish knows what it looks like, how does it recognise another one that looks like it?

2:34So you're wondering how can a fish recognise another member of its species and then also how does it know what it looks like to compare that to? You said that perfectly. Thanks Ian for showing us the critters in your aquarium. and for your great questions. To answer them, first we're going to explore the senses and cues that fish use to recognise other individuals. And then we're going to tackle the slightly more existential issue of whether a fish can recognise itself. Now of course, before we dive in, we should say that there are many, many different species of fish, from sharks to eels to goldfish.

3:16More species, in fact, than every other type of vertebrate combined. So rather than lumping them all together, we're going to look at the abilities of a few specific species that are similar to the ones living in Ian's tank. Social fish that live in shallow tropical waters, like reefs and lakes. And to see some of them for myself, I'm heading to the Blue Reef Aquarium in Portsmouth in the south of England. Meeting me here is someone who can tell us more about why fish hang out in groups in the first place. My name's Lauren Nadler. I am a fish biologist based at the University of Southampton School of Ocean and Earth Science.

3:56Thanks for meeting us here, Lauren. So we've come to a tank in the aquarium that features a fish that you know a lot about. So can you tell us why they're special? So one of the most common species in this tank is a species called the blue-green chromis, or chromis veridus. So this is a small fish that is found in the Indo-Pacific So in this tank there's a few different species There's these orange ones that look a bit like goldfish There's these stripy ones And then which ones are the blue-green chromis? Is it like this one? Yeah, exactly So it's all of these blue-green ones So this is a species that is always found in groups You've probably heard the collective term for a group of fish like this A shoal or a school And while people often use those interchangeably they actually mean slightly different things.

4:41So a shoal is basically a general term for any animal that socialises in any way. So if it's a group of three or more individuals, then it can be called a shoal, even if they're kind of slightly doing their own thing in the same area. But a school is a particular type of shoal, and that is when the species actually engages in very coordinated behaviour, moving around their environment. They're almost acting like one big animal. they're communicating so rapidly and consistently. That's a really useful distinction to know about and will save me looking foolish in front of marine biologists in future.

5:15A lot of marine biologists use it wrong as well. Oh okay or maybe it will help me to feel smug in front of them. There you go. So in this tank here the blue green chromis fish are they a school or a shoal? I'm going to say probably a shoal because they are broadly hanging out but they're not like one sort of big coordinated mass? Yeah so in this tank there's no predators and so they don't really have to come together to protect themselves. Being in a group allows them to get greater protection from getting eaten by a predator. There's the idea of safety in numbers that if there's one predator and there's a hundred individual fish only one of them is going to get eaten so your chances are one out of a hundred but if you're by yourself and there's one predator your chances are probably higher.

6:01So one of the main reasons that fish form shoals or schools is to reduce the chances of becoming someone else's dinner. And it seems there are even physiological benefits to shoaling behaviour, as Lauren investigated in the blue-green chromis during her PhD. I wanted to see if there was something called a calming effect. So basically, is being in a group going to make the individual feel calmer, less stressed than if they were by themselves. And so I looked at them in the lab, so I measured basically their metabolic rate. So how much energy are they using just to exist, basically. And what I found is that when the individuals were in a group versus when they were alone, their metabolic rate was 25 % lower.

6:43If you think about 25 % being how much less food you would need to eat just to live your daily life, that's a lot. So they need a lot less food being in that social group than if they were isolated. To test this, Lauren put individual blue-green chromis fish inside a special tank that was within another tank, meaning that while they were physically isolated, they still received some of the signals of being in a group. Individuals were put into a chamber around their testing chamber, essentially. So there's sort of a 360-degree view of those fish swimming around them, but then also the smell from that chamber was kind of forced into the inner chamber.

7:22So they had both the visual cue of the fish swimming around them, but then also the really strong smell of them as well. Interesting. Okay. So you were able to show that having those cues reduced their metabolic rate, and that was the calming behaviour. But you were also able to kind of indirectly help us answer Ian's question, which is that fish are clearly using visual information, but also olfactory information. you can demonstrate that in the lab, that it changes their physiology. If you give them those signals, they recognize their group mates and they feel calm. And it's something I'd like to study more, and we're doing more work to follow this up in a lab.

8:00So if we only give them visual or only give them olfactory cues, how does that then change this calming effect? I mean, I guess it shouldn't be a surprise that smell is a sense that works underwater. Like, obviously, as humans, we're smelling with our noses above ground. but in the water those same chemicals I guess dissolve and fish can sense that presumably. Yes definitely they actually have quite a good sense of smell. So it's not just about looks, smell also plays a role in recognition between fish and it's especially important for fish swimming in a big shoal where visual information will have its limits.

8:37The goal is actually to look similar because for that idea of evading a predator they want to look as similar as possible because then it makes it harder for a predator to hone in on a particular individual to attack. If you're the one that sticks out you're probably in trouble right? Exactly we actually call that the oddity effect there's actually a lot of work on this idea of the oddity effect in socializing fish or schooling fish in particular and so when they all look the same how do you then tell who's who and that's when smell becomes really important so generally individuals will have a very unique smell profile that will help with this social recognition side.

9:12That seems quite surprising to me. I mean, why would they need to recognize specific individuals within their joel? So it's an interesting one because we look at these fish and we think, oh, they're just sort of randomly swimming around. But it's actually much more non-random than we think, because each individual in the group will take on a very specific role. Who is going to lead the charge if a predator comes in? Who is going to take the food first and we find it. That makes total sense. You can't really have social interactions if individuals can't recognise each other, because then it's just like every interaction is just starting all over again.

9:51Yes. So Ian, to answer the first part of your question, not only can fish use visual information to recognise the rest of their group, they can use odour cues to distinguish specific individual fish, which is really important for all kinds of social dynamics. But you also wanted to know if a fish can somehow compare itself to others to see if they're the same species. To do that, does a fish have to know what it itself is? It often comes down to this oddity effect that you're able to compare yourself, often by some sort of internal template to others around you. And by doing so, you can therefore associate with individuals that look and behave much in the same way as you.

10:36This is Cullen Brown, leader of the fish lab at Macquarie University in Sydney, Australia. He told me that animals do have internal templates for sticking with their own kind, but they don't always accurately reflect themselves, as demonstrated by a different kind of animal, birds. And people will probably be aware of the visual template that young chicks develop when they first encounter their mother. I mean, it's not always their mother chickens and ducks end up following people around them or even coke cans or depending on whatever object they happen to visualize during that template window once a chick hatches kind of locks onto the first thing it sees and then instantly imprints on that and just thinks that that's that thing is its mother and normally that probably that thing probably will be its mother but sometimes it might be a human being or yeah as you said an inanimate object and i guess that's interesting because that sort of shows that the way that an animal behaves in terms of the other individuals it hangs out with isn't necessarily related to what it looks like or what it is right there's a mechanism there that could have you know fish might hang out with fish that look like it but it might have nothing to do with how the fish kind of understands what it looks like and then you can actually manipulate that as well there's been all sorts of studies that show that fishes also prefer to hang out with other fishes no matter what species they are if they've been eating similar things well i've been eating this and i think that's tasty and you've been eating that and so we must be similar or share similar interests or what have you so those sorts of things have been shown not just in terms of diet but also in terms of habitat preferences as well of course different stream habitats and ocean habitats smell differently and and fishes can clearly differentiate between those things it's hard for us to imagine because our sense of smell is not terribly good, but in the oceans, the sense of smell is very, very important.

12:28I guess the equivalent would be, you know, you drink in the same bars that I drink here, we're probably similar to each other. We both smell like old beer and stale smoke. Yeah, yeah. Let's hang out. So fish don't just socialise with fish that look like them. They also prefer to be around fish that smell like them. And that's not always because they belong to the same species. it can be because they've been feeding on the same things in the same places. But how exactly do fish generate this internal smell template so that they can match it to others? Well, a clue might come from a group of small fish with spines on their backs called sticklebacks.

13:10Primarily during the 80s, there was a lot of work done looking at sticklebacks and how they might be able to recognise not just what a stickleback is but familiar versus strange sticklebacks and even kin, so brothers and sisters and cousins and things like that. And at the time, the most likely source of that smell was from the MHC complex, which is a series of genes that are responsible for combating diseases and things like that. So individuals can compare their self-template, their own MHC template, with that of the smell of others around them. There's no doubt that they know what they smell like.

13:53So I guess with the smell, because the fish can smell itself, there's a kind of constant comparison there, right? So, OK, does what I'm smelling smell like me? OK, that is probably my species of fish. Yes, exactly. It seems that fish are actually pretty discerning. They can tell if another fish is the same kind as them, if they're related to them, even if they've been to the same habitats or eaten the same food as them. And they can use that information to keep track of exactly who's who within their social group. But some fish species have even more remarkable powers of recognition. And back at the aquarium, producer Emily accidentally stumbled across one of them while she was out collecting sounds for this episode.

14:45No way! Oh my gosh! That's amazing! So I've just been standing by this tank, I was just holding my microphone up to get some sounds of the bubbles and suddenly this jet of water just sort of squirted out the top of the tank and I looked down and realised these are archer fish. They just did it again. It's like if you've ever been in a swimming pool and taken a mouthful of water and shot it out at your friends. That's what these fish are doing. Turn your back for a second and your producer wanders off to go and play with the animals. Honestly. But what Emily had observed was the hunting behaviour of archer fish, tropical fish that take down their prey living above the surface by shooting jets of water at them.

15:40This means they have particularly good vision, which also comes in handy for recognition. There were some tests done with archerfish because they use water as a tool to dislodge insects above the surface. But in this particular case, they were actually testing to see if archerfish could recognise human faces and differentiate between human faces. Surprise, surprised that a lot of fish could do that. More recently, there was another test where they actually used 3D renditions of people's faces, and they trained the fish using a front view of human faces, and then asked them to differentiate between different human faces as the faces were rotated.

16:25And the fish could continue to differentiate between human faces, even when the face had been rotated by 90 degrees. In the test that Cullum is describing, archerfish had to choose between pictures of humans by, you guessed it, spitting water in their faces. Charming. And they were really good at recognising the same face among lots of others, even from different angles. Which is super impressive. But can fish recognise themselves? That's what we'll be looking at next. This is summer at its peak. Whole Foods Market Summer Fruit Fest is your invitation to eat the season. Fresh, organic, and bursting with flavor.

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18:04You're listening to CrowdScience from the BBC World Service, the show with the distinguishing feature of answering your science questions. I'm Anand Jagatia, and this week our question comes from listener Ian, who wants to understand how fish figure out which are their own kind. Part of the answer seems to be that they can compare their own unique odour to the smell of other fish, to judge how similar they are. But Ian also wanted to know if fish somehow know what they look like. And the easiest way to test that is to see how they react to their own reflection. Can they work out that what they're seeing in the mirror is actually themselves?

18:46We wanted Ian to see what would happen if he tried it with his own fish. We have the mirror. What I'll do first of all, put it against the glass. Oh, there's one. Paying attention. in that big orange one. The orange one, definitely paying attention there. Yeah. Most of the fish in the tank seemed totally indifferent to the mirror, but one of them did seem to swim up to its own reflection several times. That big orange one has come to the mirror, definitely. I'm surprised. I'm thinking, oh, Anand's wasting his time with his blinking mirror business. OK, granted, Ian's little experiment wasn't particularly rigorous, but it was inspired by a classic technique called the mirror test, used by scientists as one way of measuring whether animals recognise themselves.

19:39There's evidence for only a handful of species being able to pass this test, including bottlenose dolphins, chimpanzees, magpies and elephants. Also part of this exclusive club is a tiny species of fish. So you can see there's a little small fish that's swimming through all the other species. Here's Lauren Nadler again, introducing it to us at Blue Reef Aquarium. It's one fish that you can see is kind of on its own. There aren't other individuals in the tank that look like them. Is it that sort of, that thin one there? It's that one there. It's got kind of a long narrow body with sort of blue stripes along the tail.

20:17and so that's called a cleaner wrasse. They basically pick actual parasites off of the fish that come up to them. The fish that actually come to get cleaned are called clients. Cleaner wrasse clean other fish by picking parasites off their skin. They're really useful so they're put into lots of the fish tanks here to help keep the other occupants healthy. Yes, you can see that there's a cleaner wrasse that's kind of gone under the gill of that fish so he's actually probably picking off parasites from the gale filaments themselves. These fish have a really important role. I mean, it's hard not to anthropomorphise them, but they kind of have a job.

20:54Yeah, exactly. And so you will actually, on some reefs, you'll see a line of larger fish that are just waiting to be cleaned at these stations. So it is funny that they're waiting for the car wash or something like that. Yeah, yeah, the car wash. It turns out that cleaner wrasse are kind of the perfect fish to take the mirror test. And that's because simply showing a fish a mirror isn't enough. You need to observe some kind of behaviour that unequivocally demonstrates it's recognised itself. This test was with the idea that if you could see your reflection and you saw a mark on your own body in the reflection and then you touched your own body, that is to say you touched the mark where it truly was, rather than your reflection, that meant you understood that the reflection was you, and then, in turn, that meant that you were self-conscious.

21:52This is Alex Jordan, an evolutionary biologist at the Max Planck Institute of Animal Behaviour in Germany. And as he explains, cleaner wrasse are already well-equipped to deal with the mark element of the mirror test. Its whole lifestyle is around seeing small blemishes on the skin of clients and removing them. And so we thought of any species, this would be a great one because A, it can see small marks on the skin, and B, it would care about those marks and may even be motivated to remove them because that's what it does in its natural life. And so we did this test and lo and behold, they passed the test.

22:32They were presented with the mirror. They initially met that reflection with aggression. That aggression tailed off over the next days. They started to do strange, unusual, idiosyncratic behavior, swimming upside down. and then when we put a mark on their bodies where they couldn't see it directly, they could only see it in the reflection, they would go down to the ground or the rocks and they would scratch themselves at that location and then go back to the mirror and inspect the reflection again. This, to me, was very clear evidence that this fish passed the mark test. The cleaner wrasse became the first fish to pass the mirror test by recognising and attempting to remove a mark on itself.

23:20To date, it remains the only fish to have done so. It's a surprising result, and back in 2019, when the study was published, it caused its fair share of controversy. Here's Cullen Brown again. The consensus response from the scientific community was not that we've underestimated fish, but this mirror self-recognition test must be broken. And of course, they got bombarded with all sorts of scientific responses saying, oh, but you didn't control for this and you didn't test that, blah-dee-blah-blah. And some years later, I think we're up to about the third or fourth, perhaps even the fifth iteration of them trying to come up with any other possible explanation for the results.

24:03And there simply isn't another explanation. So I would argue that now fish are probably the most well-studied animal on the planet in terms of the mirror self-recognition test, which is quite flabbergasting. According to Cullum, the evidence that cleaner wrasse can recognise their own body in a mirror is pretty incontrovertible. You could say that it means they're self-aware or self-conscious. But these terms are slippery in meaning, and they can start to unravel when we apply them to animals apart from ourselves. Back to Alex again. Humans love science that reflects themselves. That may be very egocentric, certainly anthropocentric.

24:47And so when we, as scientists, say something like, this animal is self-conscious, it just explodes in the imagination. Well, does that mean that it also remembers that embarrassing thing that it said yesterday and is tormenting itself? does it also think of what it's going to be like when it's got kids and it's you know having a midlife crisis no well i mean certainly there's no scientific evidence for that but that's what we think of when we say self-conscious and i think we allow chimpanzees in our imagination to have those traits we think they're very very intelligent in many ways so it doesn't jar that thought Then when it was a fish, all of a sudden there was this cognitive dissonance, which I love, because it's said to everybody, what assumptions have you been tolerating when we throw these words around?

25:44So what does it mean that cleaner wrasse can pass the mirror test? My conclusion was not that this was evidence that these fish that passed the test was self-aware and self-conscious in the grand sense of having a mental representation of self, but rather they were solving this task in simpler ways. What is the mirror test doing? Well, it's asking an animal over a period of time to understand the function of an unknown object, an unknown tool, and learn the function of it. And of course, framed like that, of course, an animal should be able to do that. Animals are continually experiencing new objects in their environment, and they have to learn about those objects.

26:30I don't think the mirror test is an adequate mechanism to uncover something as deeply philosophical and complex as the concept of self-consciousness. And that's true for a fish, for a dolphin, for a chimpanzee, and also for a human in my opinion. I think what Alex is saying is that we shouldn't throw out the results of the mirror test just because a fish has passed it. As we've heard throughout this episode, fish are enormously diverse and can be social, intelligent creatures. But we should also be wary of assuming that recognising yourself in a mirror equates to the kind of self-consciousness that we possess as humans.

27:13There are many ways to be self-conscious, from being aware of your own body to being aware of your own thoughts. This is a bit of a can of worms which we haven't got time to get into right now. But if you're interested in going down that particular rabbit hole, we dedicated an entire episode to animal consciousness back in 2020. Find that wherever you get your podcasts. But to wrap up this episode, Ian, your questions have taken us from the way fish shoal and school to avoid predators, to how they use smell to recognise specific individuals, as well as reflecting on what it means to be self-aware.

27:53Thanks so much for getting in touch. Now over to you for the credits. That's it for this edition of CrowdScience. This week's question was from me, Ian, in the Czech Republic. If you have a question you'd like the team to do a deep dive on, please email crowdscience at bbc.co.uk. The program was presented by Anna Jagatir and produced by Emily Bird. Thanks for listening. Bye.

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