Stranger than Fiction: The Secret Lives of Freshwater Mussels
Within the rivers, streams, and lakes of North America live over 200 species of freshwater mussels that share an amazing life history. To metamorphose from larvae to adult, the mussels must pass through a parasitic phase on the gills of freshwater fish. To trick the fish into accepting their larvae, female mussels have developed a complex array of lures and baits to attract and fool their unsuspecting hosts. This talk will explore the fascinating reproductive biology and ecological role of one of nature’s most sophisticated fishermen.
Stranger than Fiction: The Secret Lives of Freshwater Mussels
Anne Kinsinger: Good evening. I'm Anne Kinsinger, Associate Director for Ecosystem Science in U.S. Geological Survey, and I want to thank you for coming tonight and welcome to our evening lecture series, which showcases the work of USGS scientists both locally and around the world. The Science in Action Lecture Series is intended to give you a better understanding of the science-based issues that affect your daily lives.
Our speaker tonight is Heather Galbraith, Fisheries Research Biologist at the USGS Leetown Science Center in West Virginia. In her present role, Heather studies Aquatic Ecology and Conservation Biology with an emphasis on the reproduction biology of freshwater mussels. Heather's current research projects include studying the conservation genetics of eastern elliptio mussel, ecological flow requirements of the federally endangered dwarf wedgemussel, and American eel restoration in the Susquehanna River Basin.
Heather received her Master's and her Ph.D. in Zoology from the University of Oklahoma. She continued as a postdoctoral researcher at the Trent University and Ontario Ministry of Natural Resources, where her research focus was on conservation genetics of endangered freshwater mussels.
Ladies and gentlemen, Heather Galbraith, "Stranger than Fiction: The Secret Lives of Freshwater Mussels".
Heather Galbraith: Thank you so much. Can everybody hear me? No, you can't hear me. Now just a second, we're trying to get this up closer. Better or worse? Better? OK, so everybody can hear me now? Perfect. OK.
Thank you for that nice introduction and thank you to all of you for coming out in less-than-ideal weather conditions.
I'm going to be talking to you tonight about some work that I've been doing since I started work with the USGS about a year ago. I want to start out by actually acknowledging my co-author on the talk, Bill Lellis. Much of the work that I'm going to talk to you about tonight Bill's been working on for probably 20 years. So Bill's been doing this research for a really long time. He couldn't be here tonight, so he sent me in his place.
Like I said, I just recently started working for the USGS about a year ago, and Bill is moving on to bigger and better things and I've been fortunate enough to inherit a lot of Bill's research projects. I'm really excited to be continuing his work and starting to develop my own work.
With that, I'm going to go and get started. Just to give you a brief introduction about who we are and where we're located, we are under the Mission Area of ecosystems and our lab is actually in Wellsboro, Pennsylvania, which is way up here in Northcentral Pennsylvania.
We are part of the Leetown Science Center, but our lab is actually located up here in Northcentral Pennsylvania. We're a really small lab; there are only about 15 of us in the building. We're located right here at the foot of the Appalachian Mountains, so it's absolutely beautiful there.
For those of you who are unfamiliar with the research goals of the USGS under the Ecosystem Mission Area, one of our goals is to conduct research to support the DOI lands, which accounts for about a fifth of the United States. We are also interested in studying priority ecosystems, and one of the major ones of interest to us is the Chesapeake Bay and the Chesapeake Bay Watershed. We're also interested in Trust species, which includes endangered, migratory, interjurisdictional and invasive species as well as species of importance to Native Americans.
At NARL, which is our lab, Northern Appalachian Research Laboratory, we do have a whole suite of different research projects ranging from environmental nutrition, so we do projects on the role of vitamins in animal health, the effects of climate and land use changes on freshwater ecosystems, the effects of energy development on stream ecosystems, and mainly what I'm going to be talking to you about tonight is our work with endangered species, and in particular we've been recently interested in endangered freshwater mussel species.
What I'm going to do is I'm going to give you guys some introduction to what freshwater mussels are, for those of you who are unfamiliar, and then try and highlight a couple of the different research projects that we've been working on related to freshwater mussels. Depending on how fast or how slow I talk, I guess, dictates how many projects I get to talk to you about.
When I say 'mussels', the first thing that probably pops into your mind are actually saltwater bivalves, so these are the things that you're probably familiar with eating, 'bivalves' being something that has two valves or two shells. So these are the common blue mussel or clams that you would expect to see on your dinner plate.
Or if you are familiar with freshwater mussels, you probably are most familiar with the invasive species, which include the zebra mussel and the Asiatic clam. But related to those different species are another group of organisms that belong to the family Unionidae, and these are the freshwater mussels that I'm going to be focusing on this evening.
There are actually over 300 species of Unionids or freshwater mussels in North America. The hotspots of diversity are in the central part of the U.S. and down near the Gulf. We're not doing too shabby over here on the East Coast with about 56 different species.
But I would say that the majority of research to date has actually focused on the processes that are going on in mussel communities in the center of the United States, with actually limited research on the East Coast in the Atlantic slope. So that's one particular area that we're really interested in.
In Pennsylvania, where I work, there are actually 65 different species. The majority of the species richness is over here in the western part of the state. These are mainly the two drainages that we've been working in lately, the Susquehanna Drainage and the Delaware River Basin, which have substantially lower species richness, but like I said, we're still trying to get a handle on how processes that go on in freshwater mussel communities differ in this area.
Mussels are extremely diverse. As you can see from this photo, they range in size from very, very tiny, so about the size of the tip of your finger, up to, we've seen mussels that are the size of a dinner plate. So they vary in their size. They obviously vary in their shell morphology. Some are smooth, some have ridges, some have pimples, some have these big wings, so they look very unique.
Rhey can also exist in these really dense congregations called 'mussel beds'. You can get mussel beds comprised of one or multiple species, depending on where you are.
Freshwater mussels are actually of interest to us because they are highly imperiled. So this is a graph, and you can see it's the percentage of species on the X axis here that are either vulnerable, imperiled, critically imperiled, or presumed extinct, and freshwater mussels are up here on the top with almost 70% of all species listed as threatened, and this is compared to the things that we hear about most in the media, which I guess are probably mammals and birds and charismatic megafauna. It's actually freshwater mussels that are probably the most critically imperiled organisms, and they're also ones we know very little about.
Some suggested reasons as to why they are imperiled, there's a whole list of them that includes climate change, habitat alteration, pollution, invasive species such as the zebra mussel, water management practices.
Actually, this picture here is really interesting. There's been a historical mussel harvesting industry throughout the United States. It's declined a lot recently because of their imperiled status, but freshwater mussels used to be harvested. These people are actually sitting on a giant mound of freshwater mussel shells. They used to be harvested and their shells were used in making buttons or as seeds for the pearl industry.
The mussel harvesting industry, like I said, has declined recently, but it still does exist, particularly in the Midwestern U.S.
We're thinking that mussels are particularly susceptible to a lot of these threats for a variety of different reasons.
First of all, they are generally sessile. They don't move very far. They can move, they do have a foot, but in general, they can only move a couple of meters to, maybe in some really highly mobile species, 50 to 100 meters. But in general they can't move very far. They're not like a fish that just get up when things get difficult and swim away. So basically they have to sit there and tolerate any disturbances that come their way.
They have a complex life cycle, which I'm going to talk to you more about here in just a few minutes. It depends on their larvae-parasitizing host fish, so basically, anything that happens to their host fish affects freshwater mussel success.
They're also extremely long-lived. You can see from this photo here that mussels put down annual, just like trees do, so they lay down growth rings every single year. So we have the ability to thin-section their shells and figure out how old they are. Some mussels can actually live upwards of 100 years.
So one of the problems we have with mussels is because they live so long, you can go out to mussel beds and there are these relict populations that have been living there for a really long time, and you can't really tell that anything's wrong because you can't monitor their reproduction particularly easily. So it's really hard to tell when freshwater mussel communities are in trouble. You get these long-lasting relict populations.
One thing that's really interesting about freshwater mussels is their life cycle, so I wanted to spend a little bit of time talking to you folks about freshwater mussel life cycle and showing you some pretty cool pictures and some videos about how mussels reproduce and how they're really unique. I'm just going to start off and sort of work my way around how the mussel life cycle plays out.
Male mussels are broadcasters, so they broadcast their gametes out into the water column in these sperm balls. It's a ball that's covered in probably several thousand different sperm. They broadcast them out into the water column. The females, which are filter feeders, filter in the sperm. So actually fertilization in mussels is internal, unlike most marine bivalves.
Here's a close-up picture of one of those sperm balls that I was just telling you about. Sorry, I've got to get out of my PowerPoint presentation to show you this.
This is some video footage we have of a male mussel releasing its gametes. You can see all of these teeny tiny little white dots are just millions and millions and millions of those sperm balls. So there's really a huge energetic investment on the part of the male and the female as well that goes into freshwater mussel reproduction. This particular individual mussel was released like this for probably 20 minutes to 45 minutes, so it's a really massive investment.
Like I said, the female mussels then filter in the sperm and fertilization is internal. And actually, the females brood their larvae inside their gills. Mussel larvae is called glochidia. This picture down here are the females' gills inflated, really filled up with all of these larval glochidia.
The females brood the glochidia for several weeks, and then the glochidia are released and they have to spend a portion of their life as a parasite on a particular host fish in order for them to successfully turn into adult mussels.
For the most part, glochidia attach to the gills of fish. There are some species, though, attached to the skin or to the fins, but for the most part, it usually happens on the gills.
What fish species they use varies according to what mussel species are these. Some mussel species are specialists and their glochidia can only attach to one species or one or two species. Other freshwater mussels are generalists and their glochidia can use a variety of different host fish species.
So you can see this picture right here is actually a picture of a fish's gill that's just loaded down with glochidia.
Now, mussels have evolved some really cool strategies for getting their glochidia attached to their host fish. Some mussels just basically release their glochidia out en masse. They broadcast their glochidia or release them out in these mucus strands, which you can see right here. I think this is an eastern elliptio mussel. The glochidia float through the water column until they come in contact with their particular host fish.
The viability of the glochidia, I guess, varies, but usually for a couple of days, they are viable. So if they don't come into contact with the fish, then they generally die.
There are other species that actually climb up onto the bank and spit their larvae onto the water surface. I guess that's still broadcasting; it's just a unique take on that strategy.
This is where it starts to get cool. Some mussels package their glochidia in these packets called 'conglutinates'. Now, some conglutinates are just sort of run-of-the-mill packets, but others have evolved to look like food items or invertebrates that live in the streams. Believe it or not, this is a package of larval mussels, not a bug.
I've got some close-ups here. This is a female mussel that has just released lots and lots of conglutinates. I say down here at the bottom that "a single female can produce between 200,000 and 17 million glochidia per breeding season," so, again, the energetic investment that goes into reproduction in mussels is just incredible.
Here is one of those conglutinates that looks like a little invertebrate with its eye spots that's broken open and you can see all of these individual glochidia that are spilling out of it. Again here's another close-up of all those individual glochidia, and you can actually see the hooks on the glochidia here for attaching onto the host fish.
This is my favorite. Some other mussels have evolved these mantle lures for attracting the host fish towards the mussel. This right here is not a fish. It's actually a piece of the freshwater mussel. It is its mantel, so it's the little flap of skin inside its shell that helps to create the shell. Oh, sorry about that. Wrong button.
This right here is a modified piece of its mantle, and the mussel will wave this in the water column to try and attract a fish towards it. And then when the fish strikes that, thinking that it's a prey item or a competitor, the mussel's gills are sticking out right in here and the fish ends up getting a big biteful of glochidia.
I've got a couple of videos here of some different mussel lures. This is the black sandshell and this is, I guess, maybe you would say a relatively primitive lure. Basically it's just the mussel waving its mantel trying to attract a fish towards it.
Those things that are waving, like I said, are a piece of the skin or the tissue that lines the inside of the shell, and those white things that look like teeth in the middle are each of the individual gill filaments that's just filled up with all of the larvae mussels.
This is an image of the rainbow mussel. I'll play this one a couple of times because it's relatively quick. Its lure actually looks like a crayfish moving. I think this one is really cool because it's not just the mussel waving its lure. There's an actual behavior there. I mean, that mussel is pulling itself forward with its foot to try and mimic a crayfish's behavior there.
This is a video of yellow lampmussel. Its mantel lure looks like a fish swimming in the water there. You can see, again, these right here are the mussels' gills, so anytime a fish comes in and tries to strike at that lure, it's going to get a mouthful of glochidia from that mussel's gills.
This is the orange nacre mussel, and this is actually a little bit different. This is a super conglutinate. Basically, what this mussel does is it sends out a big, long fishing line. At the end of that fishing line is a giant package of glochidia called a 'super conglutinate'. So it waves it out in the water. Again, it looks like a fish or maybe a worm. It's got eye spots there, so it's designed to attract a host fish.
And then the final mechanism that mussels have evolved for transferring their glochidia to a host fish is this host capture mechanism where the mussel actually physically grabs a hold of the fish, holds onto it, and off-loads its glochidia onto the fish.
And I've got a video of that. This is the northern riffleshell mussel. So there's the mussel just hanging out in the sediment there, and siphoning. There's its host fish, which I believe has a lot of hardship.
Heather Galbraith: And there we go. So it clamps down on the fish's head, and you can watch in slow motion as that mussel just starts pumping glochidia out onto the fish.
Now I can't imagine that the fish likes it, but they survive. I don't know that I want to say it doesn't hurt the fish necessarily, but the fish does survive.
Heather Galbraith: And that's done, the mussel lets go, and the fish is free to go.
Heather Galbraith: A little shell-shocked, but he's free to go, right?
Actually, if you did catch it... Let me go back. Check out the teeth on that guy. The mussel shell actually has these spikes on it to help hang on to the fish and hold it there while it's off-loading its glochidia.
So those are just some of the unique strategies that mussels have for infecting their host fish with glochidia.
Now, the glochidia spends several weeks to several months attached to their host fish. As of right now, we don't know of any necessarily negative effects that glochidia have on the host fish. When people think of parasites, they think, 'Oh, that's going to hurt the fish,' but to our knowledge right now, there are no necessarily detrimental effects to the host fish of having mussel glochidia attached to it.
After the glochidia have spent their requisite time attached to their host fish, they then drop off as these microscopic juvenile mussels, I don't know, maybe about 150 microns, so they're still really tiny at this stage. They drop off and they live in the sediments.
We know very little about what happens to juvenile mussels until they eventually grow up into this adult stage, so what habitat they prefer, whether or not they're actually living in and amongst adult mussel beds, we really know very little about this whole juvenile stage in general.
You're probably thinking at this point, 'OK, mussels, they are threatened, they do lots of cool things to have babies, but who really cares?' Well, mussels are actually considered by some to be ecosystem engineers, so they actually do really important things in freshwater environments.
First of all, they're very powerful filter feeds, which you can see up here in this picture. This tank has turbid water in it with no mussels. This tank had the same turbid water in it with mussels, and in about 55 minutes those mussels were able to filter all of the particulate matter out of that water. So they are extremely strong filter feeders.
They do move around, so they help oxygenate the sediment to prevent anoxia and hypoxic conditions. They excrete nutrients, which helps facilitate local algal communities. They biodeposit feces and pseudofeces, so they're contributing organic matter to the ecosystem, which actually serves as the basis for food webs.
Their shell also provides unique habitat to algal species and macroinvertebrates. And when I say 'unique', I mean a different form of rocks. If you compare the invertebrate and algal communities on a freshwater mussel, they actually differ from the algal and invertebrate communities on rocks. So freshwater mussels are doing something different in the ecosystem.
As far as what mussels do is concerned, we know two things. We know that mussel diversity is important and mussel biomass is important. So as you increase the number of mussel species in a community, which is species richness here on the X axis, the things they do in the ecosystem actually increases.
Here, this is algae on the mussel shells. This is bugs on the mussel shells. And you can see, as you increase the number of species, the algal biomass increases and the invertebrate biomass increases. So species diversity is important in mussel beds. But biomass is also important. As you increase mussel biomass, the nutrients that they contribute to the ecosystems also increases.
So this is just an example of the kind of biomass of freshwater mussels you can see in a community. This is one of the mussel beds that we worked in while I was doing my graduate work in Oklahoma, and in a 300-square-meter stretch of river, there was the equivalent of about four elephants' worth of mussels sitting there. So the biomass can be extremely high in these mussel beds, which makes you think that the effects that they're having on freshwater ecosystems can also be really large.
One of the mussel species that we've been doing a lot of our research with is the eastern elliptio mussel, Elliptio complanata. It's highly abundant. It has extremely high biomass. It's mainly found along the East Coast of the U.S. It's actually not considered imperiled, to my knowledge, along most of its range. It's fairly abundant.
But I want to tell you kind of a cool story that goes along with elliptio. In some freshwater mussel surveys that we did in the Delaware River about 10 years ago, we did snorkel surveys of about 120 miles of the entire Delaware River, which, I think, to my knowledge, is the largest freshwater mussel survey that's been done to date. I might just be making that up. But I'm pretty sure it is.
So we did this freshwater mussel survey in the Delaware River, and this is just to illustrate how abundant the eastern elliptio mussel can be. This is the list of species that we found in our survey. The eastern elliptio comprised about 98% of all the freshwater mussels in the Delaware River.
On average, we found about 165 eastern elliptios per hour of search time. What that translates to is if you were to just drive over the Delaware River right now, pick any random spot in the river and get in, it would take you about 22 seconds to find an eastern elliptio mussel. So they are that common.
Now compare that to the next most-common mussel, it would take you 24 minutes. So, I mean, really they are almost an order of magnitude, more abundant. And then if you look at the dwarf wedgemussel, which is one of the endangered species that we've been working with in our lab, it would take three weeks to find them. So there are elliptios all over the place.
What we did then is we translated our survey effort into an estimate of what elliptios are contributing to the ecosystem in the Delaware River. We figured that, based on our numbers, there are approximately two million eastern elliptio mussels per mile. Now if you translate that into biomass, that's 32 elephants per mile of river.
If you look at the filtration capacity, so you multiply all those mussels by the volume of water that the eastern elliptio can filter, we're looking at 16 to 48 million gallons per mile per day of water filtered by freshwater mussels. Basically, every drop of water in that Delaware River is passing through a mussel's gills at some point.
We translated that into sediment removal capacity. They have the potential to remove over a half of ton of sediment per mile per day. So this is a really phenomenal contribution to river in ecosystems. Like I said, elliptios are dominant and abundant in the Delaware River.
Now, Pine Creek is the creek that runs right outside our lab in Willsboro and it's part of the Susquehanna River Basin. We also have eastern elliptio in Pine Creek as well. While we were doing this survey, we started to notice that in the Delaware, we were finding a lot of baby eastern elliptio mussels, and it dawned on us that we spend a lot of time in Pine Creek and I don't think we have ever seen a baby eastern elliptio mussel.
So we went and looked at some survey data that we had from Pine Creek, and that's what this graph is here. This is mussel length along the X axis and the percentage of the population on the Y axis.
And you can see the Delaware River, which is this blue line here, you get a pretty wide distribution of size ranges in elliptio in the Delaware. There are small individuals, but most of them are larger, and then few are really, really big individuals.
If you look in Pine Creek, though, all of the mussels are big. You're missing all of these small mussels. So we started wondering, 'Is there a recruitment issue? Are mussels not reproducing in Pine Creek? Does this go farther than Pine Creek?'
So what we did is we did some mussel survey work in 2008, so about two years ago, and we picked 13 different sites throughout the Susquehanna Drainage that have historically had abundant elliptio populations. We went to each of those 13 sites and we collected mussels and we thin-sectioned them to age them and we did detailed quantitative survey work to try and determine if there were small elliptios in each of these beds.
And what we found out, we found the exact same thing as we did in Pine Creek. At each of these 13 historically abundant eliptio sites, there weren't any babies. So we started scratching our head and saying, "Why is this happening?" Elliptio is so abundant in the Delaware River. Why are there no babies in the Susquehanna Drainage?
So we started thinking that maybe it was a function of their host fish and maybe the host fish that elliptio is using is not in the Susquehanna Drainage anymore.
So what we did is we tested 38 different species of fish. We put the fish in buckets, dumped in a bunch of elliptio glochidia, stirred them all around, and then monitored them. Everyday we would siphon out the bottoms of the buckets looking for either glochidia that had sloughed off or juveniles that had transformed, trying to identify what the host fish that elliptio is using.
We tested all of these fish and had no successful transformation. The only fish that we tested where we actually successfully got juvenile mussels were these fish listed here, the American eel, lake trout, brook trout, and then two species of sculpin.
We know that lake trout are not native to the Susquehanna, so it doesn't make sense to us that the lake trout would be the natural host fish species for elliptio in the Susquehanna Drainage. So in our mind, we've kind of moved the lake trout out.
We also know that although brook trout were historically abundant and sculpin are still abundant, they're not found in locations where freshwater mussels are found. These are typically more headwater species, and their ranges don't seem to overlap with eastern elliptio ranges.
Which basically leaves us with the American eel, so right now our working hypothesis/theory is that eastern elliptio uses the American eel as its host fish or historically has used the American eel as its host fish.
For those of you who are unfamiliar with the American eel and its life cycle, eels used to be highly abundant along the entire Atlantic coast. They accounted for upwards of 25% of the fish biomass in a lot of rivers along the East Coast.
Eels are in trouble, mainly because of impoundments and their complicated life cycle. All eels reproduce in the Sargasso Sea, so the adults migrate from freshwater down to the Sargasso Sea. They have these leptocephalus larvae that floats at sea for probably about a year until they drift into the estuaries as these glass eels.
The glass eels then migrate up into freshwater systems. They develop into this yellow eel phase and they live in freshwater for about 15 to 20 years until they become reproductive silver eels again and make their migration back to the Sargasso Sea.
Unfortunately, a lot of the impoundments in some of the major rivers have decimated eel populations along the East Coast. The glass eels are unable to migrate out over these giant dams, so they can't get upstream, and any adults that are already upstream end up getting killed in the hydroelectric turbines on their way back into the Sargasso Sea. So basically eels have been essentially eliminated from the Susquehanna Drainage.
So our working hypothesis right now is that because the eels have been eliminated from the Susquehanna, there is no host fish for elliptio's glochidia to attach to, and therefore we are losing out on elliptio reproduction.
We have been working on an experimental eel reintroduction project. We've been working with the U.S. Fish and Wildlife Service and the Audubon Society, and we've been reintroducing American eels in two rivers in the Susquehanna Drainage.
We've been introducing all different life stages. We've been collecting adults. Fish and Wildlife Service is really good at this. They do most of our eel collection for us. They collect adults for us down in the lower Susquehanna Drainage. We bring them into the lab and we've been PIT-tagging them, so we put these little electronic tags under their skin. They each have a unique identifier in them, so if anybody catches an eel, we can scan that tag and figure out which eel it was, how much it used to weigh, how long it was, etcetera. So we're stocking tagged eels.
They're also collecting elvers at the base of the Conowingo Dam on the Susquehanna in these eel traps, and then they're trucking the elvers up into Buffalo Creek and Pine Creek and we're releasing elvers. And then they've also been collecting glass eels down near Ocean City and bringing them to us up in our lab, and we've been rearing them up to the elver stage and then stocking them in Buffalo Creek and Pine Creek. This past year, I think we raised about 100,000 glass eels in our lab and stocked those between the two different creeks.
We've had a lot of public involvements. We've had some press releases. The folks in and around these creeks are really excited about this. We get lots of stories about how they remember when they were kids they used to catch eels and they remember there being historical abundant mussel populations.
So the public's actually been really good and really fired up about our introduction efforts. We've been handing out these little cards with fishing licenses, telling fishermen, 'Please, if you catch an eel, give us a call. Let us know where you caught it, if it had a tag on it...' So it's been a lot of fun.
Our goal for this project was to introduce about 45,000 eels into Buffalo Creek, 50,000 eels into Pine Creek, and then to do this over the course of three years. Right now, I think we just completed our second year of eel-stocking, so we have one more year to go. I think we're fairly close to reaching our stocking goals.
So what we've been doing is we've been monitoring both the fish communities and the mussel communities, and this is a long-term project that's expected to go for 10 years. We're interested in how reintroducing the eels is going to change the native fish communities that are in these rivers, but we're also really interested in whether or not stocking the American eel is going to help facilitate freshwater mussel reproduction in these rivers.
Like I said, we've nearly reached our stocking goal and we're only in the second year. This past summer, we went into both of the rivers and did some electrical shocking, and we shocked up quite a few eels. Some of them, actually, their gills were infected with elliptio glochidia, which was promising to us, so we're really hopeful and really excited about that.
We had barely high eel recapture success, which was pleasantly surprising to us. I mean, these are migratory species, so we weren't really sure what was going to happen when we dumped a bunch of eels in the river, if they were all going to head north or if they were all going to head south. But it seems that they're actually staying put for the most part. We had shocked a 75-meter stretch of Buffalo Creek and actually came up with about 400 eels in 75 meters. So they're staying where we put them, which is right over the top of the mussel beds, which is exciting.
I guess what our big question is, can we restore successful elliptio reproduction in these experimental rivers? And then we need to start thinking about, OK, well, long term, this is not necessarily sustainable, hauling eels from below the Conowingo Dam up here. Is there a more sustainable way to reintroduce the eel back into its natural habitat to help facilitate freshwater mussel reproduction?
So we've really been thinking about this in relation to the Chesapeake Bay and water quality. This is a publication that came out on "6 Most Cost-Effective Ways to Reduce Nutrients" in the Bay, and that includes wastewater treatment plant upgrades, animal feed adjustments, traditional nutrient management and enhanced nutrient management, cover crops, conservation tillage.
And what we're really interested in is, can we add a seventh item to this list, which is just simply restoring natural riverine ecological services. We don't know the answer to that, and our lab is trying to start to peel off the layers and address this question.
Actually, almost all the work that I just presented to you was work that has been done in the last 10 years. So we've learned a lot of stuff in just 10 years, and so we're hoping that as we slowly start to figure out more about the host fish requirements of elliptio, more about nutrient dynamics in and around freshwater mussel beds, that we can start to make better management recommendations regarding freshwater mussel communities and American eels.
With that, I'm happy to take any questions.
Heather Galbraith: Yes. Do I get to pick who...sorry. Yeah.
Audience 1: You were saying that the dams are one of the impediments to the whole migration. So is there any talk about somehow modifying the dams to allow the eels to go back and forth?
Heather Galbraith: Yeah. There are fish ladders in place in a lot of these dams for other migratory fish like shad, and we don't know why, but eels don't seem to use those.
This is a hot research area right now is trying to figure out how to get eels up and how to get eels back down stream. A lot of the dams on the Susquehanna are up for their re-licensing soon, so there's talk about whether or not it should be mandatory that they have to install eel ladders on their dams for their re-licensing. It's sort of an up-in-the-air political issue right now. But that's a good question.
Yeah. Yeah, go ahead. Yes, you. Oh, sorry.
Audience 2: Mike Hamm. I have a two-tier question. You showed in your early slides harvesting of mussels, including the elliptios, and if so, what are the chances of a shore introduction...? The second question is, would it help clean up Chesapeake Bay by having a greatly enhanced mussel populations in the feeder river?
Heather Galbraith: I'm sorry, in what river?
Audience 2: In the feeder river. In other words, by cleaning up particulate matter in the tributary to Chesapeake, would that help reduce the... in the Bay?
Heather Galbraith: Let's start with your first question, which was mussel harvest. The mussels that have been typically harvested for the pearl and the button industry are thick, heavy-shelled mussels, which probably would include elliptio. A lot of the harvesting went on in the Mississippi Drainage if elliptio does not occur in the Mississippi Drainage.
What we do know, though, is that there was historical very heavy Native American use of elliptio on the East Coast. There are a lot of relict sites where you can go and just see these giant pits that are aligned with eastern elliptio shells, so Native Americans used to collect shells and line pits and then used them to drain their corn in these pits. Whether or not they actually killed the mussels to do this or they just collected already dead shells, I don't know if we know the answer to that. We really don't have a good estimate of what historical elliptio populations looked like.
Question about whether or not we can start restoring elliptio populations in the tributaries to help Chesapeake Bay, I don't have an answer to that. But that is what a lot of the research in our lab is looking at.
Freshwater mussels are long-lived and they have a very slow growth rate, so we have to try and figure out the best way that we could sustainably burrow and then stock elliptios. And then trying to figure out what their historical densities were that we would stock them back to is another question.
Like I said, we're still trying to figure out the intricacies in our lab of what it is that elliptio is actually going to do as far as ecosystem contributions is concerned by restoring these populations.
We're pretty certain that freshwater mussels are going to serve as a nutrient sink that we can use elliptios to help ameliorate some of the nutrient problems. But, I mean, they also excrete nutrients, too, like any other living organism does, so we're really trying to get at the nutrient dynamics question of what exactly is elliptio is doing to nutrients in riverine systems.
I'm not sure if that answered your question or not. Yeah.
Audience 3: Hello.
Heather Galbraith: Hi.
Audience 3: In the planet Earth, Heather, 10% of the planet Earth is freshwater and 90% is saltwater. In realization due to what we've seen tonight, a species we saw tonight, is the rapid growing of species that transition from saltwater to freshwater. The evolution of this species has gone a long way, actually more quicker than saltwater, to make that major change.
Can this species unite with water pollution, 40% of the lakes and rivers included in the United States, do you people see what any change in the size of how these species will change into a type to be able to live near deep?
Heather Galbraith: Yeah. That's a whole other line of work that we're doing in our lab that I didn't talk to you about tonight.
One thing that we're really interested in is the physiological requirements of freshwater mussels, how they tolerate stress, how they respond to stress, and then how we can manage our rivers to maximize human gain and minimize stress on aquatic organisms.
I don't know the answer to your question. I don't know if we've pushed mussels beyond the point of recovery. I hope not, because they're my favorite animal in the whole world. But they're resilient and they've lasted a long time in evolutionary history. Hopefully we'll be able to figure that out.
We're working with the federally endangered dwarf wedgemussel right now, which is in the Delaware River. That's a highly managed river. I mean, it can go from flood stage to absolutely bone-dry in a matter of 24 hours.
There is a federally endangered mussel there, so what we're trying to do is figure out how it responds to flow stress, how it responds to temperature stress, and then manipulate the way that the water is managed in that river and make predictions about how that is going to affect the dwarf wedgemussel's habitat. So we are taking these things into account as far as water management is concerned.
Audience 4: How long is a typical eel? And do they come in different colors?
Heather Galbraith: Yeah. As they grow, their color changes. As glass eels, they are clear. When they transform into the elver stage, they're kind of just a plain brown. And then they transform into the yellow stage, and they're actually yellow, and that's the stage that they spend most of their life in.
They vary in size, but they can get fairly large. I think three or four feet is probably maximum. I was in the Delaware River two weeks ago and had one that was maybe two-and-a-half-feet swim right underneath me. That was a little scary, but it was pretty cool. It was the first time that ever happened.
And then after the yellow phase, they transform into the silver eel stage and they develop this really silver hue as they migrate downstream. They vary in size. The glass eels are teeny, teeny tiny, maybe two inches, and then they've got a fairly fast growth rate. Like I said, they grow up to be about three feet.
Audience 5: Do you have any information about mussel populations in the Potomac Watershed or Roanoke, Virginia?
Heather Galbraith: Off the top of my head, I don't, but we've been working with Maryland's DNR and they've got some crazy, crazy data on mussels and vertebrates, algae, you name it, and they've got these long-term monitoring sites that they keep track of. Do you know how far back their data is? Ten or 20 years.
I think they've got this rotation where they would have all these different sites and every year they'd pick different sites to monitor. So I would check out their website, because I bet you can get a lot of good information on the freshwater mussels in the Potomac.
Audience 6: Why do the mussels, why do they need certain fish?
Audience 7: ...
Heather Galbraith: We don't. That's another part of the freshwater mussel life cycle that I think that's really understudied. We don't know what it is that makes particular species specialists and others generalists. We're assuming that there's some sort of recognition protein on the surface of the fish that has to match up with something on the glochidia. I don't think we know the answer to that. That's a really good question.
Audience 8: But my question is, more generally, what ... reaction do we get from the fish to ensure...?
Heather Galbraith: I don't even know if we know the answer to that question.
As far as I know, there is no literature saying that anything actually transfers from the fish to the glochidia. Now, that doesn't mean that it doesn't happen. I don't think we know the answer to that. We just know that they have to latch on, they have to spend a portion of their life there, and doing so somehow triggers some sort of developmental gene so that they make the transition from glochidia to juvenile.
Audience 9: Just curious as to how the mussels tolerate high-intensity rain events like we just had.
Heather Galbraith: Yeah. Some do and some don't. Our little endangered dwarf wedgemussels, which are maybe an inch long, they don't tolerate flow very well, so they get very easily blown downstream.
Now, other species have very heavy shells and can bury way down into the sediment. I mean, we're talking like two feet. Some of these mussels can bury down really, really far. So those species tend to fair out fairly well.
It all depends on microhabitat, too. I mean, if you're a little mussel and you're lucky enough to find a nice flow refuge tucked underneath a rock, you will probably be OK. If you're laying out in a pile of sand, you're probably headed downstream.
Anne Kinsinger: OK.
Heather Galbraith: All right. Thank you so much. This has been really enjoyable.