A fish can’t talk, but its eyes and ears can. Scientists have discovered that each layer of a fish’s lens reveals a different part of its life history, including what it’s eaten throughout its life. While you’ve probably never heard of fish otoliths, these ear bones tell us not only a fish’s age, but what rivers it has traveled. Understanding this could help wildlife managers know what habitats to protect to help imperiled species. In “Nature Tells Its Story, Part 1,” Unfold looks at the eyes and ears of fish.   

In this episode: 

Miranda Bell Tilcock, assistant specialist researcher, UC Davis Center for Watershed Sciences 

Carson Jeffres, senior researcher and fish biologist, UC Davis Center for Watershed Sciences

Audio transcriptions may contain errors.

Kat Kerlin Amy, have you ever heard people say that the eyes are the windows to the soul? 

Amy Quinton Yeah. Doesn't that mean you can tell a person's intent by looking at their eyes or what they're thinking or feeling? 

Kat Kerlin Yeah, something like that. I mean, with some people, it's like you can see the story of their life right there in their eyes. And it turns out that is even more true for fish. 

Amy Quinton Excuse me?

Kat Kerlin Yeah, fish. A fish's eyes really can tell you its life story, or rather its eyeballs can 

Amy Quinton Its eyeballs? OK, that's a little gross, but I'm intrigued. So do go on.

Kat Kerlin Well, to be even more specific, scientists called the lenses of a fish's eyeballs archival tissue. So think of it like tree rings, which can tell you the age of the tree and the climate where it grew. Fish eyeballs are just as revealing. I learned this from Miranda Bell Tilcock with the UC Davis Center for Watershed Sciences. 

Miranda Bell Tilcock You can actually look in the fish eyes by peeling away these different layers that they are made up of and identify what habitats these fish were feeding on throughout their lives and what kind of food that they were consuming throughout their lives. 

Amy Quinton OK, that is truly amazing. How would anyone know that was Miranda one day, just like, hey, I'm going to cut into the fish's eyeball to see what it's eaten for lunch? 

Kat Kerlin Well, it's a little more complicated than that, of course. 

Amy Quinton I suppose we should unfold it? 

Kat Kerlin We will. And it's important because knowing where fish has been and what it's eaten can make a big difference in how we go about trying to protect and restore their habitat. So we are going to be peeling back the layers one by one of fish eyeballs. 

Amy Quinton Ewww, I think you just lost the audience, Kat. 

Kat Kerlin Nah. We're calling this episode of Untold 'How Nature Tells Its Story: Part One.' 

Amy Quinton Coming to you from UC Davis, this is Unfold, a podcast that breaks down complicated problems and unfolds curiosity driven research. I'm Amy Quinton. 

Kat Kerlin And I'm Kat Kerlin. So, Amy, to really understand the eye-popping experience of fish eyeballs, I had to travel to the Yolo Bypass. This is just outside of Davis in California's Central Valley. For more than 100 years, the Yolo Bypass has diverted floodwater from the Sacramento River away from Sacramento and Davis. It is not just a flood control project. It's also part of the Pacific Flyway for migratory birds. 

Miranda Bell Tilcock Also in the wintertime, what is less known about the bypass is that this is also really important habitat for juvenile Chinook salmon to use and grow really large on their way out to the ocean. 

Kat Kerlin Miranda has done a lot of work here among the rice fields of Knaggs Ranch studying how fish use the floodplain. Today, she's brought me something very special. 

Miranda Bell Tilcock So first I'm going to put some VapoRub on my upper lip because these fish are in different forms of decay and decomposition. 

Kat Kerlin She reaches into a container, her hands unfold and out clunk two orbs into a plastic tray. 

Miranda Bell Tilcock I have brought you guys some salmon eyeballs, 

Kat Kerlin Specifically, large frozen adult Chinook salmon eyeballs. 

Miranda Bell Tilcock Well, just like a snowflake, every eye is different, but they all have different amounts of tissue still attached to them. And they're very, very large because salmon are very visual predators. And so they have nice large eyes so they can see their prey. 

Kat Kerlin I decided to take a closer look.

Miranda Bell Tilcock On some of them you can actually see this little white pearl-like structure that's starting to pop out of the top part of the eye there on like the thin part of the skin. 

Kat Kerlin Miranda is here to examine their lenses. And the best way to do that is by dissecting the eyeballs. 

Miranda Bell Tilcock So I have forceps and a pair of dissecting scissors and what I'm doing as I'm cutting open the eye. This part is not very glamorous or nice-looking. 

Kat Kerlin Nothing about this is very glamorous or nice looking. 

Miranda Bell Tilcock And so I like to move it then to a different tray. And then I take two pairs of forceps here and these are going to act as my hands. And then you're going to peel the lens. And first there's this kind of sticky sac that is on the lens that you need to remove.

Kat Kerlin Here on the Yolo Bypass she's using a magnifying glass to find the lens. But with most juvenile salmon, she uses a microscope in a lab to find the lens because they're so tiny. And apparently you need to peel the layers of the lens carefully, otherwise it comes off in chunks. 

Amy Quinton Oh, whoa, Kat, like that is totally disgusting. Need you describe it in so much detail? 

Kat Kerlin But I haven't gotten into the detail yet. Let me finish. Miranda says it's actually not disgusting. Once you find the lens. 

Miranda Bell Tilcock They're coming off and they kind of look like a little flower petals when they're coming off here. 

Amy Quinton A flower petal. Really? 

Kat Kerlin Really. 

Miranda Bell Tilcock Yeah. Once you get past the gruesome of pulling the lens out of the eye, it's really not that bad. And then, you know, you're also putting the I back away so you can't really see it anymore or smell it anymore. So then it's really not bad at all. And then I think at this stage it looks quite beautiful. It's like a little pearl onion and that you can begin to pull apart the layers. 

Amy Quinton Yeah, I'm glad you can't smell it anymore. OK, flower petal or pearl onion. Either way, this is surprising, disgusting and yet really interesting. But why is she doing this? 

Kat Kerlin Well, Miranda says each layer of the lens can help us understand a lot. 

Miranda Bell Tilcock Each layer really represents a different part of that fish's life history. And you can actually go back in time similar to like a tree rings on a tree and go back in time and look at those isotope values like carbon, nitrogen or sulfur things that you would find in the food webs of the habitats that they have been in throughout their lives and actually see what these fish were eating and where they were actually rearing. 

Amy Quinton Who knew? I mean, like, how did anyone know to do this? 

Kat Kerlin Well, it had been done in marine species before, but Miranda is the first to try it in a freshwater species, and she's really credited with advancing the technique. 

Amy Quinton So the big question, why is it so important that we know what fish like to eat and where they've been? 

Kat Kerlin Carson Jeffres, a fish biologist with the UC Davis Center for Watershed Sciences, has a good answer for that. He was also out on the Yolo Bypass with us and he's quite the pro at dissecting fish eyeballs himself. 

Carson Jeffres The floodplains were an important part of the salmon habitat historically and lots of that is gone. And if you spend a lot of money restoring a habitat, you want to know if your money was spent well. And the problem is you can't ask a fish where it's been and have it tell you it's story. But the work that Miranda has done has shown that you can look at their eyes and see whether they've been using these habitats or not. 

Kat Kerlin Carson says that's critically important given that salmon are such an imperiled species. 

Carson Jeffres Salmon are not doing great in California. They're not doing great really in the lower 48 in the U.S. But, you know, it's constantly in crisis management. I think the goal is to ultimately have a place where they're at a sustainable level and we're no longer managing for crisis, but managing for sustainability. 

Amy Quinton Yeah, you know, I suppose if fish could tell us where they've been and what habitat they've used, it would be a big fish tale anyway. 

Kat Kerlin Very funny. What's cool, too, is that salmon aren't the only fish whose eyes reveal their history. 

Miranda Bell Tilcock This is applicable to any fish that has eyes, really. And, you know, you can always learn more about these fish. Any fish, really. It's not unique to salmon. It's not unique to the Central Valley. There are people who are doing it in Florida and the Gulf of Mexico on marine species. There's also people who have done work on squid using the same technique, which is really cool and something that I would definitely be interested in seeing someday, a squid eye. 

Amy Quinton You know, I've seen a squid eye Kat. 

Kat Kerlin What? 

Amy Quinton Yeah, I put my fishing hook right through it and out the other eye when I was deep sea fishing in the Gulf. Sweet huh? 

Kat Kerlin And you call me gross? OK, it's just not the same thing. Anyhow, Carson recently took this technique to study fisheye lenses in Brazil. 

Carson Jeffres They just constructed the world's fourth largest dam and hydroelectric facility on a tributary to the Amazon. And we're using this to go back in time to look at what the food web was before the dam was constructed. And we were able to look into their eyes and hear their story of what they were eating before it was all different. 

Amy Quinton That is wild. 

Kat Kerlin Yeah. And Carson and Miranda also work on a project that looks at fish ears for their history. Otoliths. 

Carson Jeffres From the dead fish we'll take the eyeball we'll take otoliths, which are the ear bones inside of the head. And those can tell you which rivers the fish were in during their out migration that we use isotopes in the ear stones. And those are more like tree rings in that they put on a ring every day. And we can go look at those rings and see which river it was in.  We can look at the eye lenses and tell which habitat within that river it was in and then put that whole story together. 

Amy Quinton So they study the eyes and ears of fish. 

Kat Kerlin Exactly. That's actually the name of their project, the Eyes and Ears Project. 

Amy Quinton So can scientists learn about humans using the same technique? 

Kat Kerlin Miranda says that so far the technique hasn't been applied to mammals, let alone humans. 

Miranda Bell Tilcock You know, I've never peeled a mammal eye before and I've definitely never seen a human one. But I know for birds and frogs, I've done those and those were epic failures. Their eyes are definitely very different. 

Amy Quinton Birds and frogs and fish. Oh my. I wonder what her lab freezer is like. 

Kat Kerlin Oh, yes. Miranda is quite the eyeball collector. 

Miranda Bell Tilcock So I do have a bag of rockfish eyes in the lab freezer because my husband doesn't like it when I keep eyes in our freezer. My daughter caught a largemouth bass one time in a lake and I saved the eyes and the ears for that as well. There are the tuna eyes that Carson brought me back from Hawaii, which are really cool because they were huge. 

Amy Quinton Yeah, that sounds really appealing. 

Kat Kerlin Speaking of peeling, Miranda actually offered to let me peel a fish eye while we were out on the flood plain. 

Amy Quinton The glamorous life of a science reporter. 

Kat Kerlin How could I possibly resist? 

Miranda Bell Tilcock You want to just kind of like hold it right there with the forceps and then kind of just start cutting into it at the top part of the eye. 

Kat Kerlin The top here. 

Miranda Bell Tilcock Yeah. 

Kat Kerlin WelI these are like well, frankly, more rubbery and hard than I thought. 

Miranda Bell Tilcock Yeah, they're pretty. These are really tough. 

Kat Kerlin I feel like I've destroyed it already. 

Miranda Bell Tilcock Oh no you're good.

Kat Kerlin OK. What do I do now? 

Miranda Bell Tilcock So why don't you trade out the scissors for another pair of the forceps. And then you can start opening up the eye. 

Kat Kerlin Miranda was super patient and supportive, which was important because I was not the quickest learner. OK, ok, I'm going to calm down. Grab that. 

Miranda Bell Tilcock Yeah, hold it open and then.. 

Kat Kerlin Grab this? 

Miranda Bell Tilcock Lens should just be... 

Kat Kerlin And the lens I should just reach in and grab? 

Miranda Bell Tilcock Uh-huh 

Kat Kerlin So after a bit more whining. Oh Miranda, I might have to give up. And a lot of encouragement from Miranda. We prevailed. OK, I need you to show me again. Where can I find this thing? Oh it's right there. 

Miranda Bell Tilcock Here it is. 

Kat Kerlin So. Oh, 

Miranda Bell Tilcock There's your lens. 

Kat Kerlin You know, you're right. It does kind of have like a . . . this is totally gross. I don't even smell it. It just feels like tough and awkward and yucky looking. 

Miranda Bell Tilcock It is very rubbery feeling. 

Kat Kerlin And then you get the eye.

Miranda Bell Tilcock The actual lens part. It's like a little bit like pulling a little pearl out of oysters...

Kat Kerlin Perfect little white ball. So that was super fun. But I never did get to the core of the lens. Just when I felt like I was done, Miranda told me I had anywhere between like 25 to 30 layers of eye lens to peel. 

Amy Quinton Wow. That is a ton of layers. That's amazing. 

Kat Kerlin Yeah, no kidding. Luckily Miranda and Carson have it down to a science and that is what makes this research so appealing. 

Amy Quinton Oh and you thought I was so punny. 

Kat Kerlin You are. It kind of gives new meaning to the phrase keep your eyes peeled. 

Amy Quinton We could go on and on with bad puns, but that wraps up this week's episode of Unfold. Next week, it's 'Nature Tells Its Story Part 2: Caves and Really Old Water."

Kat Kerlin You can hear more unfold episodes at ucdavis.edu/unfold. Thanks for listening. 

Amy Quinton Unfold as a production of UC Davis. It's produced by Cody Drabble. Original music for Unfold comes from Damian Verrett and Curtis Jerome Haynes. Hey, if you like this podcast, check out UC Davis' other podcast, The Backdrop. It's a monthly interview program featuring conversations with UC Davis scholars and researchers working in the social sciences, humanities, arts and culture. Hosted by public radio veteran Soterios Johnson, the conversations feature new work and expertise on a trending topic in the news. Subscribe wherever you get your podcasts.