Fiddler crabs are migrating north to cooler waters

And I thought, what is this crab doing here? It’s not supposed to be north of Cape Cod, Massachusetts. And I had an idea that I wasn’t just holding, you know, a crab. What I was looking at was climate change.

My name is David Johnson, and I study marine invertebrates and coastal ecosystems and look at the effect of climate change on those marine invertebrates. The Great Marsh — it’s this living laboratory I get to work in, and when I’m up here, I get to say, ‘oh, this is my day at the office.’ Even in my 22nd season, I’m still finding things.

And that gets me excited to be out here.

Anne Giblin: The Great Marsh is the name for a complex of marshes that actually goes into New Hampshire and down into Essex. And it’s the largest remaining swamp marsh left in New England. It’s important because it is such a large marsh, and it has so many services that it provides.

Johnson: A salt marsh is a grassland, so imagine a prairie that you might find in Kansas, but it’s flooded twice a day by the ocean.

Giblin: They’re nursery grounds for fish, so a lot of the commercial fish, such as striped bass, start their life here. It’s also very good for shell fishing and to go birding to enjoy the just the beauty of the place. Water is warming faster in the Gulf of Maine in the ocean than almost anywhere else on the planet. So as water’s warm, we’re going to see an increase in warm water species.

Johnson: And the next stage in the research is to look at their impacts in the Great Marsh as they move in.

So after we discovered that we had a new crab, I started doing surveys and sure enough, I found them as far north as central Maine.

As I found more crabs, I started monitoring them to find out, is this population growing? It’s easy to find them out there in the marsh because the males have this giant claw, which is basically the largest weapon proportional to its body in the animal kingdom. So when the tide goes out, you can find them trying to attract a female with that claw next to it.

So basically you go out and you put out quadrants in various places every year. So it’s an annual census. So the first four years we saw the population was rising. We also were finding females with eggs. That was telling us that they are here to stay. So the next question is do they have a different ecology, do they have a different biology than the crabs south of Cape Cod?

Johnson: They thought for the for the past half century is that when they create these burrows, they release nutrients and they help the plants grow. But the population of grass that’s here in the Great Marsh has never seen fiddler crabs before. And there are no other burrowing crabs in this salt marsh, until now. So we wanted to test this idea.

And what we found was that when fiddler crabs were present, they reduced the amount of grass by 40 percent. So the opposite of what we find south of Cape Cod. So a big question is why do we see a difference here? We think plants south of Cape Cod have somehow adapted to the presence of these crabs digging around their roots and compensated for that.

But size can be a factor also influencing when you measure the crabs up here versus down there. There are 25 percent bigger, and a larger crab could impact them by having mechanical damage as a result of burrowing. But there is absolutely a direct relationship between the amount of cord grass and a marsh’s ability to keep up with sea level rise. As the tide comes in, those stems slow down the water so that mud can settle out, and that mud layer stacks up over time.

So if you have less biomass to trap that mud, that could impact its ability to build up and keep up with sea level rise. The other way is by adding roots. 

Giblin: So it’s producing as much material in the sediments as it is above ground. Some of that material decomposes that builds the marsh up.

Johnson: In addition to the plants that we just study, we want to look at what is the impact of these fiddler crabs on the ability for the marsh to store carbon?

Giblin: Salt marshes store more carbon than almost any other ecosystems on Earth.

Johnson: So as you start punching holes into the marsh, you introduce oxygen. And that carbon that was stored in the marsh then goes back into the atmosphere.

Giblin: If this marsh were to be degraded and all the carbon that’s been stored is lost, that would represent a real increase in carbon to the atmosphere.

Johnson: So that’s a study that we will need to do.

When we see a new species arrive in our ecosystem. We all have a knee jerk reaction to be worried. And there are certainly going to be winners and losers at the rapid rate of climate migrations that we see.

Giblin: So another species that appears to be moving in is the blue crab. We’re seeing them more frequently but their numbers are very, very low.

Johnson: The crab that I’m concerned about showing up is called the purple marsh crab. So it eats the grass and the roots.

Giblin: That is a crab that further south, it’s become quite destructive to salt marshes.

Johnson:  I’ve been keeping an eye on them. I haven’t seen them north of Cape Cod yet, but fiddler crabs and this species of plant have figured out a way to co-exist for millennia.

They’ve done it south of Cape Cod. And so I don’t predict total devastation that you might see with some of your invasive species, even though their population is rising at this point, are the impacts enough to influence the local economy and their local ecology? The answer is no. In the future, we don’t know.

Giblin: The system is being subjected to a lot of different changes, so I think the jury’s still out on the fiddler crab.

Johnson: That’s why it’s critical to continue monitoring the impacts of these fiddler crabs, or any other climate migrants to happen to move in.