Adaptations at Altitude: The Biology of Mountain Snails

Given the name of our podcast, it is no surprise that we talk a lot about bugs, but in this episode, we are stretching our invertebrate muscles and looking at a group of invertebrates that we haven’t considered before—mountain snails! 

To help us learn more about these fascinating animals is Dr. Lusha Tronstad. Lusha is the Invertebrate Zoology Program Manager for the Wyoming Natural Diversity Database at the University of Wyoming. The WNDD program is responsible for developing and maintaining data on the distribution, natural history, conservation status, and habitat requirements of rare invertebrate species in the state. Lusha has a Ph.D. in Zoology and Physiology from University of Wyoming and is interested in food webs, invasive species, conservation, bioassessment and biogeochemistry, while working in aquatic ecosystems and with pollinators.

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Photo: Lusha Tronstad

Thank you for listening! For more information go to xerces.org/bugbanter.

Transcript

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Matthew: Welcome to Bug Banter with the Xerces Society, where we explore the world of invertebrates and discover how to help these extraordinary animals. If you want to support our work, go to xerces.org/donate.

Rachel: Hi, I’m Rachel Dunham in Missoula, Montana.

Matthew: I'm Matthew Shepherd in Portland, Oregon.

Rachel: Given the name of our podcast, it is no surprise that we talk a lot about bugs, but today we are stretching our invertebrate muscles and looking at a group of invertebrates that we haven’t considered before—mountain snails!

Rachel: To help us learn more about these fascinating animals is Dr. Lusha Tronstad. Lusha is the invertebrate zoology program manager for the Wyoming Natural Diversity Database at the University of Wyoming. The WNDD program is responsible for developing and maintaining data on the distribution, natural history, conservation status, and habitat requirements of rare invertebrate species in the state. Lusha has a PhD in Zoology and Physiology from University of Wyoming and is interested in food webs, invasive species, conservation, bioassessment, and biogeochemistry, while working in aquatic ecosystems and with pollinators.

Rachel: Welcome, Lusha! We are very excited to have you here today! And to talk about mountain snails.

Lusha: Oh, thank you very much for having me. I'm looking forward to talking about some of my favorite snails.
Matthew: Before we get into talking about mountain snails, I'd like to start with looking more broadly at snails. I mean, first, sort of thinking about some of the really basic stuff. How many different types of snails are there in North America?

Lusha: There's nearly 3,000 species of snails in North America, and there's land snails, and aquatic snails. So, that includes both of those.

Matthew: And I mean, are there any areas where we're like more likely to find snails? I'm thinking other regional differences, or I'm sure there are habitat differences, too.

Lusha: Oh yes. They live in a lot of different places. And I should have said marine, as well. There's, you know, snails almost everywhere.

Matthew: And In thinking about snails, I mean, how are snails classified? Like butterflies are the order Lepidoptera, and beetles are Coleoptera. And where would snails be within that kind of tree of life?

Lusha: Yes. Well, they're all animals—you know, insects, snails, mussels—but, snails are mollusks. And that includes both mussels, or other bivalves, and snails—Gastropods. Gastropods is a class, and like I said, that includes land snails, aquatic snails, marine snails, even slugs. And so a quite a variety of critters in that class.

Matthew: Yeah. And the Gastropods, do they have any kind of unifying characteristics? And again, I'm thinking like, well, Lepidoptera means scale wing, so we know that. And the Coleoptera are the shield wings. So I mean, Gastropod must have a meaning as well.

Lusha: Yes, I believe the Latin meaning is stomach foot. Haha. Which, you know, they have a single foot, and a lot of their organs are in their foot. So I believe that's where the Latin name comes from. But their unifying characteristics include, yeah, a single foot, a single shell that's often coiled, and they also have a radula, which is their mouth part, and that's how they scrape algae—say, the aquatic snails scrape algae off of rocks, or terrestrial snails scrape microbes off of leaves and things like that. But snails are very diverse. You know, some do have a coiled shell. Some are limpets that look more like a little volcano. And then, slugs, a lot of them do have shells—they're internal, so their shell has retracted. Because they're all related in the past, you know. And even some slugs don't have shells—they've lost that.

Matthew: I remember reading about the semi slugs, which do still have a residual shell on the outside. And there's the earshell slug, which is not native, but I found in my front garden here one day, so there are all sorts of odd creatures out there. And I thinking when you mentioned the radula—.

Lusha: Oh yes.

Matthew: —I remember, I was remembering when I used to do more education and docent work with a local state park here. And it was described as being like a file, and so that's how we used to try and explain it to people. “If you imagine going out like having to use a file to scrape at everything.”

Rachel: That's funny because I used to work at the Seattle Aquarium and we talked about moon snails, and their radula was like a drill—they'd like drill into other shells of animals, and slurp them up like a Slurpee. And the kids are always like, “Oh, gross!” Haha. But yeah, snails are so diverse and they're such cool animals. But your experience we're talking about today is with mountain snails. Are they a large group of snails?

Lusha: They're not. I mean, they're fairly large, I guess, relatively. So we're talking about mountain snails, like you said, they're in the family, Oreohelicidae. And there's two different genera in that family. And so we're gonna be talking about one, Oreohelix, and there's about 85 different species that I could find for the mountain snails. And there's a lot of taxonomic disagreement about this.

Lusha: The thing about snails is that their shells—the morphology, the shape of their shell—it really changes based on where they're at. Temperature, elevation, how dry it is—all these environmental characteristics can change that. And so a lot of times we base the species designation on how their shell looks. And so we know the taxonomy of Rocky Mountain snails is wrong, because a lot of it is based on these shell characteristics. And so because we've used shell, we have a lot of species—like one species will live in this canyon, another one's only on this mountain—and so we have a lot of species that live in very, very small areas. And so they're considered rare. But we don't know if that's true. So, you know, we need to look further to see how many species there really are.

Rachel: Like a lot of other invertebrates, there's still some that we are discovering, which seems pretty wild. But for mountain snails, you mentioned the Rocky Mountain snails. Are there snails in every mountain range across the U.S.?

Lusha: Oh, excellent. So mountain snails live in the West United States. They'll range from Southern Canada down to Mexico. They're very small, and they're usually hiding, so we have to look for them to find them. Sometimes I won't find snails in really dry habitats, but there—there are snails, a lot of places.

Rachel: And you mentioned that their shells are different, and that's how you classify them. And you said there's a lot of factors that determine the shell differences. Do mountain snails have a unique shell? Is it—I mean, it seems like every species has a different type of shell, but are they collectively different from non-mountain snail shells?

Lusha: Oh yeah. So I had—I brought a shell. So mountain snails have a very compressed shell. So it looks like it's squished top to bottom. And you can tell it's much rounder on the top. They even have a belly button, if you look on the bottom. A lot of times they'll have two stripes—brown, dark brown—stripes that run around the shell.

Rachel: Is there any evolutionary advantage to sort of that smashed shape? Because when I think of a snail, I imagine like the really tall, almost like a top of an ice cream cone, like a soft serve. Haha. Where it's like swirly, and large, and like a little hat.

Lusha: Yeah, no, that's a really good question. You know, snails come in all shapes and sizes, but generally, they're either compressed top to bottom, or more compressed side to side. And you're right, I bet most people probably do think of the, you know, the elongate ones. But a lot of snails are compressed top to bottom, and I'm not sure—I've never read anything about evolutionary advantages of that. I can tell you that they have some mechanisms to survive. Mountain snails do live in drier habitats a lot of times, and they will make a like a plastic wrap on the outside of their shell. So the aperture, the opening there, they'll put plastic wrap, kind of—it's made of mucus, but it reminds me of that—and they'll put that over their shell opening during drier periods. And that helps them retain the moisture in their bodies so they can survive.

Matthew: It's the perfect segue, 'cause I was wanting to talk more about the kind of habitat and the natural history. But with the mountain snails, do they have any particular habitats that they prefer? I mean, you mentioned drier conditions, you've mentioned forests so far. Is it dry forests? They must have some preference for themselves.

Lusha: Well, I mean. Yes, but then we can find exceptions to every rule, right? So usually they're in mountains. They are usually in forests, but, you know, there's some species that live in sagebrush in really dry habitats. So they are a lot of different places in the Western United States. What I would say maybe is a more unifying characteristic of their habitat would be these micro habitats they find. They tend to find places that—just little places they can hide—that are wetter. Might be a north-facing slope on a mountain, that has, you know, just cooler and more moist. It could be under bushes, that understory. It's really important that they can find that wetter, cooler spot. So they kind of find those little spots in the environment that they can survive in.

Matthew: I used to live in Britain and I know that there I would see more snails in areas with chalk or limestone. Because the plants had higher calcium content. Do you find that same kind of relationship with the snails here?

Lusha: Definitely, yes. They need calcium in order to make a shell. And so when I go out to do surveys, one of the first things I'll do is get a geology map for the area and look at where the limestone pockets are. And it could be other types of rock—like dolomite or whatnot, that have a calcium in them—but that's where they are. For example, right now I'm in the Tetons. And the Tetons are largely granitic, which has no calcium. Haha. But if you look at a geologic map, there's little pockets of dolomite, or limestone out there. And if you go to those places, I found them.

Matthew: How long does a mountain snail live for?

Lusha: That's a really good question. So we did a study a few years ago where we went out and we marked snails every year, once a summer. And we come back and we try to find individuals—so it's called a mark-recapture study. And the longest-lived individual we found was seven years. But they might live longer, yeah. We also found that they moved 80 meters in a year, in one snail.

Matthew: 80 meters. I'm like, that's not very far per hour, is it?

Lusha: I mean, they're an inch, and they have one foot, so. Haha. And I was measuring straight line distance, I don't know how many—.

Matthew: Up and down. Yeah, totally. And I'm like, “Wow, they live for seven years and they have to survive—.”

Lusha: At least.

Matthew: “—they have to survive the winters.” I mean, are there times of the year, like in Wyoming or any of the mountains, are they more likely to be active at any particular time of year?

Lusha: Oh, that's an excellent question. Yeah, so they have to survive these harsh periods, and they have adaptations to do that. So in the wintertime, they hibernate. They make that, you know, mucus lining on the outside of the shell, again to cover up the outside. And then they reduce their metabolic rate to, you know, survive these cold times with snow cover. And so that's how they do that.

Lusha: But then in the summertime it gets really hot, and the biggest factor for snails is desiccating, you know, drying out, becoming a raisin. And so, what they do is, again, put that mucus membrane over the outside of the shell—we call it estivation when it's hot. And so they do that. But what I've noticed is that they can wake up really rapidly. If it starts raining, and conditions get a little wetter, they can respond really fast. And they start feeding and drinking. And then when it dries out again, they'll put that mucus membrane up on their shell again. They definitely have a lot to deal with.

Matthew: Yeah. You mentioned that they can react very quickly to changes in moisture. Does that mean that in the middle of the summer a thunderstorm goes over and makes it wet—is that enough to trigger that activity?

Lusha: Yes, I've seen them arouse after a thunderstorm and start becoming active. You'll see them crawling around on the rocks, or the leaf litter, and, yeah, get a drink, eat.

Matthew: I admit, I don't know anything much about mountain snails, but I have discovered that their egg laying is different from other snails that I've encountered. Can you tell us about that?

Lusha: Oh, yes. So most land snails are hermaphrodites—so they possess both female and male organs. Whereas most aquatic snails are separate—they have separate females and males. So that's kind of unique about land snails. And then within that, most snails lay eggs. Think of a fish laying eggs outside their body and letting them be—develop. Whereas most snails also do that—they lay eggs, and they develop without any parental care. They're on their own.

Lusha: But Oreohelix, they have internal fertilization of the eggs. And then they have an internal brood chamber that they rear the young, and they actually give live birth to baby snails. And they're about two and a half whirls. And the way we—that's the way we measure age in snails, is how many times the shell goes around. And so two and a half whirls—the shell goes around two and a half times—that's about the size when they're born. So this is a lot of parental care. The mountain snails can protect their young against both environmental harshness, and protect them from predation, a lot of other things by internally taking care of them.

Matthew: That's amazing. And I'm guessing this is an adaptation to living in extreme environments, is it?

Lusha: I think so, yeah. That would be my guess, as well. You know, they have a lot of parental care. Instead of laying a thousand eggs and hoping for the best, they're actually taking care of them. They produce far fewer young—they might produce five or 10, instead of a thousand—but the chance of survival for those five or 10 is much, much higher.

Matthew: Yeah. And if the eggs are fertilized internally, and there's a brood chamber, are the baby snails developing whilst the mother is kind of in estivation or hibernation?

Lusha: Well, I mean, I believe I've picked up estivating snails that are, if you wanna say pregnant, so I believe that they can.

Matthew: Yeah, I guess it just makes it one advantage is that the very short active season, the young also have the best opportunity to maximize the time.

Lusha: Yes, I think so.

Matthew: Yeah. Wow.

Rachel: That's so cool. Tiny little baby snails. Haha. In your work with the Wyoming Natural Diversity Database you develop and maintain data on rare invertebrate species in the state. Are mountain snails, I'm assuming, are part of these studies?

Lusha: Yes. So I'm working with some colleagues and we're trying to raise funds to do a project to revise the taxonomy. So figure out what the species are—Haha—and how many of them are rare, versus how many of them are widespread. And so I started with Oreohelix about 17 years ago when I got my job at—we call it “Wind” (WYNDD)—and I, you know, I started learning about them. And a lot of species across the West are considered rare. There's around 85 species out there, and over half of them are considered of management concern, or of conservation concern by state and federal agencies. Because they live in these really tiny little areas—like I said, that mountain, or on this scree slope—so because they live in such a small area and there's very few of them, that's a definition of rarity for conservation ranks.

Lusha: And so I realized pretty quickly in Wyoming that we couldn't really do much for these snails until we realized, is this a valid species or not? And so I started working with some colleagues to do this. And if you wanna do it, you have to do it across their entire range, which means the West United States. So that's what we're hoping to do if we can find the funds to do that. Haha. So it—. Yes, we have rare species across the West that are—of Oreohelix—very, very high percentage of them are considered rare.

Rachel: So obviously, there's more data that needs to be collected, but as you said, half of them are of concern. And being rare kind of already puts them in a vulnerable state. What are the threats that these mountain snails face?

Lusha: Yeah, I think a large one is—. Well, we have a couple things that that are probably harder on 'em. One is, you know, rising air temperatures and drying conditions. That's a big thing. When you live in a, on a mountain, you can only run up so far to get to cooler habitat. So, that's one thing. And then changing precipitation patterns, and things like that—they need the moisture. Like I said, one of the things that they worry about is becoming a raisin—and so, drying out. And so if we don't get as much moisture, if we have more and more of these prolonged droughts, that is going to make life tougher for a mountain snail.

Lusha: Another thing is they depend on those micro habitats, like we were saying. And so when we log, and remove understory and/or overstory, we're losing a lot of those micro habitats that keep them cool and wetter. And so, you know, understanding what the minimum amount of cover—both overstory and understory—is in a forest will be important moving forward to maintain—to make sure we maintain these populations.

Rachel: Yeah, it's talking about the inconsistency of moisture. I am obviously in Missoula, those of us who live in the mountains—the amount of snow pack we've had even in the past few years that I've lived here has been wildly different from year to year. And I imagine that's so stressful for a mountain snail to not be able to depend on that consistency, especially since they live up to seven years and maybe, and maybe longer.

Lusha: And the snow is also a very insulating factor for all of our invertebrate friends that live, you know, in the soil, or on the ground, because the more snow we have, the more they're insulated from those varying temperatures. And so there's a lot of, a lot of factors that come with varying snowpack.

Rachel: Is there anything that people can do to help protect mountain snails, or snails in general? Is there anything that folks can do to improve that habitat? I imagine like recreational users, when we're on the mountains, is there something we can be mindful of?

Lusha: Oh yeah, yeah. Again, these aren't in our yards, for the most part. Haha. So it is when we're out mainly using, and recreating, and anything that runs over the ground, like off-trail driving, four wheelers, and things like that could crush them. And if you look under a log to see who lives there, put it back because it's home to a lot of critters—I always think about that—or rocks. I think being advocates for more information about things like what is the minimum tree canopy or understory canopy that we need to leave in order for Oreohelix and other animals to survive?

Lusha: I think being an advocate is huge. Noticing them, and telling others about them. Because I wanna call these charismatic microfauna. Haha. Because they're big enough that people notice them. I see kids picking the shells up all the time in the forest. You know, the long dead, very white shells. And so I think it's an opportunity for people to notice invertebrates—animals that are in the forest. They are wildlife, they are animals, and they are part of what makes the ecosystem work. They're there breaking down organic matter, like leaves and things. They have a job, and they're an important part of the ecosystem. And so I think getting the word out that yes, invertebrates are wildlife, and they matter is important.

Matthew: Quite a few states, even within the legislative structure, they don't allow you to protect invertebrates. So Invertebrates are Wildlife has been one of our campaigns that we've been developing over the last couple of years to try and fill the gaps—at least at the legislative levels, state by state. But I think what you were saying, as well, about the individual actions that we can take to be more aware, more careful when we're out—because we're not always noticing the damage that we cause. And I know there's the leave no harm, leave no trace kind of attitude that they take, but that's not always thinking about what you're squishing underfoot. And those are the kind of the small details that will make so much difference to small animals.

Matthew: But the next question I have for you is: I don't know if there is an easy answer, but do you have a mountain snail that you are particularly fond of, of all of the mountain snails that are out there?

Lusha: Well, yeah, it's hard. But I'm gonna say the two that I've worked on the most. And the one is the Cooper’s mountain snail, which lives in the Black Hills. I've surveyed and monitored it since 2010—a few times over the years. And that's a good example of the taxonomic issues because there's somewhere between one and three species up there, but we don't really know. My other favorite snail is Oreohelix subrudis, and that snail lives in the mountains behind my home in the snowy range of Southeastern Wyoming. And those are the species that I worked on to measure how far they moved and how long they lived.

Rachel: Awesome. I love how many different types there are. Haha. So, thank you so much for all the wonderful information about mountain snails. I feel like I just want to keep asking you questions and learn more. I'm so grateful that we had this time with you. And we're gonna end on our two questions that we ask folks, and this one's my favorite, so I'm glad I get to ask you. What inspired you to pursue a career in invertebrate conservation?

Lusha: That's an excellent question. I think it all started with my family and my brother. So as a child, I grew up in Northwestern Montana. And we went fishing in the mountains every weekend, and on Thursdays with my dad. And when the fish weren't biting very well, I would get out of the rubber raft and wade in the ice-cold streams, and just pick up all the rocks and look at the bugs. My brother showed me all the aquatic insects that lived on the rocks. And so I would just spend hours picking the rocks up and looking at the invertebrates in the stream. And I think that's what got me started and, you know, eventually I went to college thinking I could be a biologist, like a fisheries biologist. And I got there and I found out that you could study invertebrates for a living. Haha.

Lusha: And right away I got a job in a lab, and I learned to identify benthic invertebrates—so invertebrates that live on the stream bottoms. And went to grad school—I mean, the rest was history. I fell in love with invertebrates, and when I got my job at WYNDD, my scope opened a bit because I am—I am still—responsible for all the invertebrates in the state of Wyoming. And so that includes pollinators, and mussels, and snails, and all the insects, and all the invertebrates in the state. I think one thing that I love about that is I love learning about different animals, and where they live and how they live. And I think that's one of my favorite parts about my job.

Matthew: I think it's symbolic somehow of the approach and the attitude people have to insects and invertebrates that you are the one person responsible for the biggest group. And they may well have multiple botanists, and multiple zoologists, and multiple ornithologists, but just you dealing with way more than everybody else.

Lusha: Oh, yeah. It keeps me very busy and there could be tens of thousands of species. We don't know, we're just breaking the tip of the iceberg here.

Matthew: Last question. If you could see any bug, any insect, invertebrate in the wild, what would it be?

Lusha: Oh, that is such a tough question because I, like I said, I love diversity, and I love learning about so many different invertebrates. I would go and see any of them, but since we're on a mountain snail theme today, I'm gonna say Oreohelix pilsbryi. And that is a mountain snail that lives in New Mexico. And it is currently being considered for federal protection. And so I would like to go see that snail. And it looks like it's a bit of an adventure to get there, so I like adventure—hiking and whatnot—so I think I would choose that one.

Rachel: I love that. Well, thank you so much for joining us today and teaching us about mountain snails. And thank you for the work that you do, and trying to protect these important animals, and just bringing awareness to our audiences about them. So next time I'm out hiking, I'm gonna look for some mountain snails and see if I can find some. And yeah, thank you so much for joining us today. It's been a pleasure.

Matthew: Thank you.

Rachel: Bug Banter is brought to you by the Xerces Society, a donor-based nonprofit that is working to protect insects and other invertebrates—the life that sustains us.

Rachel: If you are already a donor, thank you so much. If you want to support our work, go to xerces.org/donate. For information about this podcast or for show notes, go to xerces.org/bugbanter.