Did you know that crabs make unique homes? Some build deep burrows with many tunnels, while others prefer to make a single shallow pathway. A biogeochemist and crabber speak about the impact of crustaceans on gas fluxes and the importance of sustainable trapping in this episode.
Transcript
Jess: So, how did you get into coastal biogeochemistry, and specifically, the research that you’ve done on crabs?
Charles: I’ve been doing work on biogeochemistry and coastal systems since I started my Ph.D. a long time ago at the University of Georgia. I didn’t have anything to do with crabs until a lot later in my career. As a biogeochemist, I usually just think about, like, the single-celled organisms that - the many, many, many of them - that live in the soil and the water and all the things that they do to cycle gases and nutrients and things like that. It wasn't until I moved to Louisiana after I completed my PhD and my postdoc and I started working in wetlands with plants, and all of a sudden, I had to think about larger organisms that have more than one cell and all of the important things that they do in the environment.
I was working with an undergraduate student one summer on a project and she really wanted to work with animals and that’s just really something I had never done before. She didn’t care what the project was as long as there were animals involved. And, as a biogeochemist, things that move the soil around and turn the soil over - that's interesting biogeochemically. So, I…I said “So, okay, we can do a project with animals as long as the animal is a fiddler crab, because they bioturbate, they dig burrows and they…and they mix things around. We'll do a project on gas fluxes because that's the thing that I was measuring that summer.” My one stipulation was that I was not going to touch a crab. She had to do all of the crab wrangling, she had to find them, she had to catch them, she had to take care of them, she had to do the experiment with them. And I would help with the measurements, I would help with the biogeochemistry. So, she did a great job with it and we finished the project and it was really, really interesting.
Jess: Why did the student want to work with you? Was it that, you know, with an REU there's only a certain amount of professors so she kind of had to finesse you into doing what she wanted?
Charles: Yeah, so this…this was a…a research experience for undergraduates (REU) summer program. So, there was a cohort of 10 or so undergraduate students working in other institutions that summer. I was one of the potential mentors. I can't remember exactly how we worked out the matching process. There were a lot of unexpected challenges. As a biogeochemist, having not worked with animals very much, I thought that fiddler crabs were like…fiddler crabs. I thought they were all the same. I was really excited to talk to my buddy, who's an ecologist, like, “hey! I'm going to do an experiment with animals this summer.” He's like, “oh, what's that?” And I said, “fiddler crabs,” and he kind of paused and he said, “You know there's more than one kind, right?” And I didn’t know that at the time, but I wasn’t going to cop to it. So I had to go and look that up. I found out that there are something like 190 species of fiddler crab in the world, and I think 6 or so along the Gulf Coast, so, we had to go and figure out what species of fiddler crab it was that we happened to have caught.
Jess: I feel like, actually, biology is one of the harder things because you can't see these organisms and you've got to deduce from systems understanding and the processes that we do know about what's happening, and sometimes, the processes we don’t even fully understand yet.
Charles: That's one of the challenges of working in biogeochemistry, I think, is that a lot of the things we do or are interested in and do want to observe, aren't things that we can observe directly. It's a thing that can be a challenge to get started in as an undergraduate student. I work primarily with undergraduate students in research at Rowan University. I don’t have graduate students here. So, it’s a thing I’m kind of used to, bringing undergraduate students into the program and helping them get started with research. One of the ways I kind of describe that process is that I ask questions about biology, but the tools that I use to answer those questions are chemistry. So, I’ll put some soil into a flask and mix it up with some water, and there’s microorganisms in there, and they’re doing something. We can’t see them, it’s hard to count them. We trust that they're in there and that they're doing the process that we’re interested in studying. And then after some time has passed, we measure how the chemistry of those soils or the water has changed, and we can deduce that the microorganisms in there have facilitated that process and we can calculate what happened and how fast it happened. And that’s how we study their behavior and what they’re doing. They’re not ducks or something – we can’t sit at our field site with binoculars and watch them and observe them in that way. We have to use these tools in the laboratory to quantify what they’re eating, what they’re excreting, to figure out what they've been doing.
Jess: Can you tell us what species are typically coexisting with crabs in wetland habitats?
Charles: Yeah, so you're about to get my biochemist perspective on the plants and animals that live in this particular type of environment and…and what they’re doing there. I believe that the fiddler crabs are detritivores, so they’re just kind of eating the organic material that’s moving through the system. There’s lots of other things that live in this type of environment. The fiddler crabs that I worked with, and I think most fiddler crabs around the world live in coastal vegetated environments. The part of the Gulf Coast where I was working at the time, that’s primarily salt marshes. So, these are really, really important ecosystems regionally and globally. They provide habitat for a lot of important animals like crabs, but also, like, a lot of the fish that we like to eat spend some part of their life cycle as juveniles in small tidal channels and estuaries in and up around the salt marsh and the grasses in the salt marsh.
The things I like to think about – they bury a lot of carbon in their soil, so they’re helping to combat climate change in that way. They remove a lot of nitrogen through a process called denitrification that takes place in the soil. As humans, we’re adding a bunch of extra nitrogen to coastal ecosystems, and this is a way that some of that can get removed before it causes problems like fish kills or algal blooms. So, it's a really, really important environment. When I think about threats to crabs and kind of more broadly to this type of coastal ecosystem that they live in, the thing that I think is the most important is sea level rise. As earth's climate is warming up, sea levels are rising everywhere. The Gulf Coast is really interesting because sea level is rising there faster than it is most other places in the world. Now that I'm up here in the Northeast, the thing I tell my colleagues here is, we really need to pay attention to what is happening on the Gulf Coast because it is giving us a window into the future.
The rates of sea level rise in the Gulf Coast are about double what they are here, where I live now, in New Jersey. But if we look ahead 50 years into the future, sea level is going to be rising the same rate here that it is on the Gulf Coast now. So, all the problems that are happening because of sea level rise in Texas, Louisiana, we're going to see that in New Jersey in my lifetime, in my kids’ lifetime. So, we should be paying attention. These plants, the plants that make the salt marsh the salt marsh, are intertidal. They like to live where they're inundated by the high tide and exposed by the low tide. So, it is a very particular window of elevation that they can survive in. And as sea level rises, it's effectively pushing those plants down that elevation window. They’re spending more time inundated by the tide than they did a decade or two decades ago. That’s not stopping, it’s only accelerating. At some point, every marsh will reach a point where it can't tolerate that anymore and it'll drown. That’s a problem for the grasses, but it’s also a problem for everything else that lives in the salt marsh too. The crabs need that soil that’s held in place by the grasses to burrow into. When the marsh drowns and the grasses die, then the crabs will go elsewhere.
Jess: Is the sea level rising or is Louisiana sinking?
Charles: Yes. I should have said relative sea level rise, right? The rate of relative sea level rise is higher in Louisiana than it is in most other places in the world. On average, sea level is rising much more similarly everywhere else in the world, but land is also moving around. We think of it as static but it’s not. So yeah. Subsidence is a big problem also, on a lot of places on the Gulf Coast, and that’s adding to sea level rising because the ocean is getting warmer and glaciers are melting.
Jess: What about the impact of crabs on the environment? You co-authored a study titled, “Fiddler crab burrowing increases salt marsh greenhouse gas emissions”, published in the journal Biogeochemistry in 2022. Results from a portable gas analyzer, which was attached to a PVC cap which represented nighttime emissions showed that CO2 fluxes were higher from the deeper crab burrows than from the shallow crab burrows, but there was no such pattern for methane. What do the results of the study mean for wetlands in the context of the potential impact of removing crabs through say, crab trapping? Are crabs bad for the climate, or are they good for soil aeration and healthier soils?
Charles: Yeah, I mean, this was the kind of question that motivated the experiment that we ended up publishing this paper about. I wanted to understand what the effect of burrowing that was carried out by the fiddler crabs would be on emissions of greenhouse gases like carbon dioxide and methane. In the salt marsh soil near the surface, there's relatively little carbon dioxide and methane in the soil. Then the deeper you go into the soil, the higher the concentration of those gases becomes. What I thought would happen is that when a crab burrowed down deeper into the soil, it would create a pathway through which those gases could leave the soil and make it into the atmosphere. So, what I expected to see was that if we had more burrows or bigger burrows, there would be higher emissions of carbon dioxide and methane. It was a lot more complicated than that in the end. We definitely saw higher carbon dioxide fluxes, but I never really saw an increase in methane fluxes. I tried a few different experiments. I kept trying different things because I thought, “Well, we must have done something wrong.” Like, “There…there has to be more methane coming out of this hole in the ground than there was the bare soil that was there without a burrow in it.” Like, no matter how I did it with crab burrows, I couldn’t ever show that. I got frustrated when working with the crabs, and I decided to dig my own burrows.
So, I had complete control over the shape of the burrow and the dimension of the burrow and the timing of the burrow digging, and all these aspects of how a crab burrow gets dug. So, I took, like, the tiniest little spoon I could find, and I painstakingly scooped out like, one little, tiny scoop at a time, trying mimic the way a crab would burrow. I dug and then, I measured the gas fluxes. When I immediately put a cap on the chamber and measured methane emissions right after I finished taking the burrow, I saw a pretty big increase in methane emissions, which was different than what I'd seen with all the crab burrows. So I took the cap back off of the experimental chamber, and I dug another burrow and I measured that one. I saw the same thing and I went back and put the chamber back on the first one that I dug and measured again, and the flux was a lot lower an hour later. Well, that was kind of interesting. So then I did a whole time series where I just kept coming back and measuring the same spot over and over again. And what that showed is that over the course of, like, a few hours - so there's like this sort of initial pulse of methane that gets released to the atmosphere. Over the course a few hours, it goes away completely until you’re back to background. There's like the kind of the small initial release of methane, but for the most part, the story we ended up telling in the paper is that there are microorganisms in the soil that are producing methane. That’s why there's methane there. But there's this whole other group of microorganisms in the soil that are consuming methane, that like to eat the methane. When the crabs dig their burrows, they're creating an environment where those methane consuming microorganisms like to live.
So, they're digging a burrow down to a place where there's more methane, but then they're creating the space for the methane eating microorganisms to live. And that’s why we don’t see this big pulse of methane. That's great, from a, from a climate change perspective. That's wonderful. Because methane is, depending on how you calculate it, like 45 times more potent as a greenhouse gas than carbon dioxide. So, if that methane is not being released, that's a really good thing. CO2 emissions increase because there's…there's more organic material in the soil that’s being consumed when that bioturbation happens. But there’s also more nitrogen removal that’s happening, which is a positive thing. I guess I don’t think of the crabs as good or bad. I think of them as an integral part of this ecosystem, and this ecosystem is really valuable and does a lot of things that are beneficial for people.
Jess: Are they burrowing for life or is this something that they have to do regularly as the tide comes in?
Charles: That's a great question. I mean, this is why we had so much trouble figuring this out when we were using crab burrows and why I had to dig my own burrows. We were putting a crab in its little home with soil in it and we were letting it be there for five days. And so, most of those crabs would, real quick, dig a burrow in, like, the first day and then they just kind of hung out in it for four days, until we came back and made our measurements. And so, in that amount of time, all the methane was gone already. That’s why I had to go and take my own burrow before I figured out that this is what was happening. It was so interesting to watch. We tried to replicate our way out of this problem initially. The crabs were so variable and their behavior and how they burrowed, I thought maybe we just need more of them to be able to see a pattern.
At one point we had like 30 of these set up, these little crab houses with a little crab in each one of them, and we were measuring the burrows. We were, like, slicing these tubes up at the end, seeing how deep did they burrow, how wide did they burrow, what was the mass of the material that they excavated. Some of them, if we put them in a tube that was a foot and a half deep, they would dig down to the very bottom of it. Some of them would make, like, three tunnels, but only in the top, like, few inches of the soil. Even though we were trying to make conditions very controlled for them, they were still each their own unique little snowflake of a crab.
Jess: In what ways do you want to see your work impact the local environment or community?
Charles: A lot of what I am doing is focused on assessing what the impact of disturbances are on salt marshes and the way that salt marshes function. I include efforts to restore salt marshes. So, as sea level is rising, one of the things that we're trying to do to combat that to…to try to help keep our salt marshes where they are is to find ways to increase their elevation so that they stay where they're supposed to be relative to the elevation of the ocean. So one of the ideas to do that is when like a shipping channel or something gets dredged, that dredged sediment can get put back on top of the marsh and increase its elevation. It's called thin layer placement of dredged material. I think it's a great idea, but sometimes the way it's implemented is still a disturbance. It…it affects the salt marsh.
If you look photos of these places after its happened, it was a salt marsh and now, it's like a bare area of mud. And the idea is that the grasses will grow back in again over time and you’ll have a salt marsh there over a longer time period than you would have otherwise and maybe it will be healthier than it was before, but it takes time. I'm really interested in measuring, how long does it take for us to go from a salt marsh to a mud flat back to a salt marsh again, after something like this happens? Does that recovery happen more rapidly than if we do it some other way? I really hope that by trying to answer some of these questions, that that can then be implemented as we do more of these projects in the future.
Jess: Daniel, when did you get into crab trapping? When did you start?
Daniel: I started as a young kid. My family would go into an area called Pass Manchac, and they had an old highway along there that was derelict and we used to drop nets off the side. As a family, you go out and catch the crabs. When I grew up, got married, and had a kid, we started making crab nets. We would go out into different canals off of Lake Pontchartrain, and we would crab like that. So, I've been crabbing somewhat all of my life.
Jess: Who taught you how to create the nets?
Daniel: I ended up seeing some that were made, got my hands on one and took it apart and realized that I could buy the netting, I could buy all of the materials. I didn't have to sit there and weave the netting on the trap. I figured out how to make ‘em, and started making ‘em.
Jess: What about crawfishing?
Daniel: We lived really close to one of the spillways off of the Mississippi River, called Bonnet Carré Spillway.
Jess: Have you noticed a difference in the catch, or have you even been back to those early sites?
Daniel: Yeah, they still catch crab. Some of the diversions that they have and some of the things that are going on with the Mississippi have kind of pushed the crabs to different locations. But, when that river goes back down, starts subsiding, those crabs come back to these areas in full force.
Jess: They're pretty resilient.
Daniel: Oh yeah, they come back to these areas because along with all the silt and the mud and everything, it builds nutrients.
Jess: People enjoy crab trapping, they love local cuisine, dishes that people can make with crabs. Clearly people love to engage in the sport. But what causes so many crab traps to be left out in the waterways? How do they get there?
Daniel: You say crab trapping – it’s basically crabbing.
Jess: Crabbing!
Daniel: It costs a lot of money for the traps, the fuel to get out there, the bait you get out there. When you have storms that come through, sometimes they can get out there to get them out of the way, sometimes they can’t. It might not even be a tropical storm. It could be a winter storm, or a thunderstorm that comes through in south Louisiana during the middle of summer, it'll move these traps. They put a certain amount of weight in the traps to hold it on the bottom, but they move. If you let them sit out there too long, you get a storm that comes through and off they go. There's areas that are more tidal than others, and when those guys lose their traps, that’s where they’re headed to find them - those narrow embankments where the tide gets real strong, it'll gather them up and just put them places, and they’ll go get them untangled. But some of them, it's not worth it for them to go and look for the trap. At some point they’ve got to say, “hey look, it’s cheaper for me to go get another trap.” The problem with that is, you get a lot of bycatch up in those traps. You’ll get small fish caught up in those traps, and that’s just bait, like, they re-bait themselves. So, the crabs come in, they can't get out, and they end up dying.
When we go do the crab trap rodeo to pick up these old derelict crab traps, we are identifying what is in these traps – whether it be male crabs, female crabs, some kind of fish. At that point, all of it goes back into the water, and we've done a lot of good things with this crab trap rodeo. Part of the problem is, we're just scratching the surface, so-to-speak. We're only getting the traps that we can see from the surface. There’s a lot of times when other trappers, other crabbers, will cut lines off of some of the other guys’ crab traps that they feel like are infringing on their territory, things like that. These traps are down there, and you have no way to get to it. A few years ago, we were in one of the crab trap zones and we spent one day covering one lake, just to make sure we got everything up. Then as the next day came, the crabbers went out to other areas.
Me and my son went back out there and started doing some side scanning. We made one transect. If you are looking at the bottom with the side scanner, it gives you a 180-degree view. We were riding on top of these traps and just looking out behind the boat, 180 degrees in each direction. So, we made one pass - a third of the way around this lake - and we identified 35 potential traps that were in this area that could be crab traps. After we did that, we spent 1 hour - every time we saw a trap, we marked the GPS. We had a grapple that we went back, and we started fishing for these traps. In one hour, we picked up 8 traps. And in those traps, some of them, they had no cord. Some of them had the cord but it was still inside the trap, meaning it fell off of one of the crabber’s boats. That’s the area that we would really like to dive into because if you go into these closures, we picked up as many as 4,000 traps in a closure. You’re barely scratching the surface.
Jess: How many companies like Coastal Mapping and Sciences are working on this issue? How many are able to use that side-scanning technology to clean the waterways?
Daniel: We are working with Wildlife & Fisheries to do this, and we try to get the crabbers involved. When we were out doing the side scan, those crabbers were with other scientists on the boat and they were picking up the derelict crab traps. It’s not a matter of how many companies do it, it's how much funding is there, and how many crabbers can we get to help do this work. Side scan technology is out there, and once you get the hang of it, then off you go, so…
Jess: And it's some kind of sonar?
Daniel: It’s basically a depth finder, which you are seeing here. [points to device on boat]
Jess: So, you have to take your time.
Daniel: You’re going at a snail’s pace. If you think of a body of water, we’re going to run transects. You’re going to have a waypoint in the beginning transect, then you’re going to go all the way across. Then, we're going to come back in a different direction 180 feet away. You’re just going to sit there and run transects over the whole area. Then you can sit there and map out where these things are at.
Jess: What kind of species do you find in traps that have been left out for a long time or lost?
Daniel: They’ll have turtles in them. The turtles are deceased, and you’ve got a shell. You’ll have fish, you’ll have redfish, you’ll have speckled trout, you’ll have any of the native species out there.
Jess: For the crabbers who are listening to this who want to do something to ensure that they're cleaning up after themselves, that they’re not contributing to this issue, what can they do to make sure they are having less of a negative impact on the environment?
Daniel: These guys really want to do the right thing there. But in some cases, you have crabbers that are taking out other crabbers’ equipment. You have situations where guys will go out there to pick these things up, but they don't have the resources to go do it. The big problem is the ones that you can’t see. It would be nice if, you know, they would say, “Hey, I have crab traps in this area and I think a boat may have come along and accidentally cut the cord, now it’s sitting down there.”
Jess: You were telling me that there was a project that successfully brought back the fisheries in Northeast using biodegradable crab traps.
Daniel: In the Chesapeake Bay, it is my understanding that they were crabbing when the crabs were dormant. If it gets too cold, the crabs are going to go down to the bottom where they can dig themselves into that sand. They’re just going to sit there and wait until it warms up. Well, these guys were going out there with rakes and raking the bottom. Once they realized what was going on there, they corrected themselves. You know, they didn’t allow that any more. It made the wildlife and fisheries up there say, “hey, look, they’re going to bring crabs from out of state, you’ll have to prove that they’re coming from a sustainable fishery.” And that’s what the state of Louisiana is doing with this program. This actually is helping prove that we have a sustainable fishery by going and getting these old derelict crab traps out.
Jess: Information about exactly the scale of the issue and what species are being affected, would that help?
Daniel: The main thing is, you know, the funding. It takes a lot of time, so once you get into a rhythm, we actually proved that we can be efficient at getting these things. Wildlife and fisheries, they are trying to go in and try different things, like paying some of the shrimpers to trawl the bottom for them. They’re going to wreck their nets, but wildlife and fisheries is going to pay for them. That would be another way that you could go and pick them up. I still think that the side scan is a pretty good way to drag an area. But like I said before, there are times when the weight that they put in it and the shifting is going to cause the trap to sink, and it’s going to sink so far that it’s going to start filling up with mud, which makes it really hard to pull up. And there have been times when as we were pulling them up, we ripped them apart. Once you rip it open, whatever was in it is going to get out.
Jess: When you use the nets, do you notice that there's less bycatch?
Daniel: Yeah, I mean, you don’t leave the nets unattended. You’re going to go somewhere where you’ve got moving water, throw the net out, and you know, at the end of the day, you’re going to pick the nets up and go home.
Jess: How many people does it take to do that?
Daniel: You can do it by yourself.
Jess: How many traps does the normal crabber have?
Daniel: It depends how serious he is. There is no limit to what he can have. A couple of guys that we used to work with, they were in the three to four hundreds. Some of them were higher, around 1500 traps.
Jess: And then they just sell those to local eateries?
Daniel: Local eateries, markets.
Jess: What is your favorite meal that involves crab?
Daniel: Fried. Fried soft shell crab.
Jess: Soft shell crab is a crab that has molted, so it's very unique. It can actually be fried and eaten with the shell on.
Daniel: There’s a certain amount of time where the shell is soft, very soft. If you catch them at that point, you can put them on ice and it will stop the molting process.
Jess: On top of working outside all day, you tend to also do a lot of outdoor recreation like fishing and camping with your family.
Daniel: Yes.
Jess: Do you ever think you'll stop doing that?
Daniel: No. No, but we are in the heat of summer right now and I am getting to the age where I’ve got to pay attention to it. That’s kind of a bummer.
Jess: You’ve told me stories about seeing alligators, hunting alligators with your family, or fishing, camping, and boating. You have a lot of knowledge just from personal experience of the environment here in Louisiana.
Daniel: Yeah. Crab Trap Rodeo is something that my son started doing when he was working for a group called Pontchartrain Conservancy. He called me up one day, he said “Dad, I found a scientist that can chart what’s coming up out of these traps. Can you come and help me?” I said, “Yeah. I’ll go.” I’m on the boat with, you know, a captain, and he’s on the front of the boat, pulling up these traps, and I’m sitting there on the computer. He pulls up one of the traps and it’s like 3 or 4 beautiful crabs. I’ve got my eye on the ice chest because I’m going to take them home. I’m like “Dude, we’ve got to take these home.” He said, “We got to throw everything back. We can’t take anything; it's got to go back in the water. We can't keep ‘em.” That about tore me up. The captain, he was howling in the back, because he was a crabber. He was laughing at me. He was like, “After the third or fourth trap, you’ll get used to it, you'll be all right.” Some of these crabs that get in there, they can't get out. They end up having things come through that trap that they can eat, so they just get bigger. On the traps themselves, there is a way for crabbers, when they flip it over and unhook the bottom of it, there is a shoot that allows them to shake that crab to the bottom of the trap so they can get them out of there. Some of these crabs were so big they wouldn’t fit through that shoot – we had to cut them out of the trap!
Jess: Wow. Thanks for telling me your story!