The flute crabs are migrating north to cooler waters

And I thought, what is this crab doing here? It is not supposed to be north of Cape Cod, Massachusetts. And I had an idea that I wasn't just holding 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 marine invertebrates. The Great Swamp - it's this living laboratory where I go to work and when I'm up here, I have to say, 'oh, this is my day at the office.' Even in my 22nd season, I'm still figuring things out.

And that makes me excited to be here.

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

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

Giblin: They are fish nurseries, so many of the commercial fish, such as striped bass, start their lives here. It is also very good for shell fishing and bird walking to just enjoy the beauty of the place. Water is warming faster in the Gulf of Maine ocean than almost anywhere else on the planet. So as the water warms, we will see an increase in warm water species.

Johnson: And the next phase of the research is to look at their impacts on the Great Swamp as they move.

So after we found out we had a new crab, I started surveying 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 swamp because the males have this giant claw, which is basically the biggest weapon in proportion 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 her.

So basically you go out and take out quadrants in different places every year. So it is an annual registration. So the first four years we saw the population was growing. We also found females with eggs. This tells 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 past half century that when they create these burrows, they release nutrients and help plants grow. But the grass population that is here in the Great Swamp has never seen fiddler crabs before. And there are no other crabs digging in this salt marsh, so far. So we wanted to test this idea.

And what we found was that when the 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 that the plants south of Cape Cod have somehow adapted to the presence of these crabs digging around their roots and compensated for it.

But size can also be a factor when measuring crabs up here versus down there. They are 25 percent larger and a larger crab can impact them by having mechanical damage as a result of burrowing. But there is absolutely a direct correlation 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 the water down so the mud can settle and that layer of mud builds up over time.

So if you have less biomass to capture that mud, that can affect 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 this material decomposes which builds up the marsh.

Johnson: In addition to the plants we're just studying, we want to see what impact these fiddler crabs have on the marsh's ability to store carbon?

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

Johnson: So as you start digging holes in the swamp, you introduce oxygen. And that carbon that was stored in the bog is then returned to the atmosphere.

Giblin: If this bog were to degrade and all the carbon that has been stored is lost, this would represent a real increase in carbon in the atmosphere.

Johnson: So this is a study we will have to do.

When we see a new species arriving in our ecosystem. We all have a knee jerk reaction to worry. And there will certainly be winners and losers in the rapid rate of climate migrations we see.

Giblin: So another species that seems to be on the move is the blue crab. We are seeing them more often, but their numbers are very, very low.

Johnson: The crab that bothers me to show up is called the Purple Swamp Crab. So I eat grass and roots.

Giblin: This is a crab that further south has become quite destructive to salt marshes.

Johnson: I have kept an eye on them. I have yet to see them north of Cape Cod, but fiddler crabs and this plant species have found a way to coexist for millennia.

They've made it south of Cape Cod. And so I don't foresee a total devastation that you might see with some of your invasive species, even though their population is growing at this point, are the impacts enough to affect their local economy and their local ecology? The answer is no. In the future, we do not know.

Giblin: The system is undergoing many different changes, so I think the jury is still out on the fiddler crab.

Johnson: This is why it is critical to continue to monitor the impacts of these fiddler crabs, or any other climate migrants that happen to move.