With climate change consistently bringing warmer temperatures to the Gulf of Maine, there is another unfamiliar species making its way north along the coast, and it might not bode well for Maine’s salt marshes.
Fiddler crabs, small crabs that get their name by having one claw that is noticeably bigger than the other, have spread up the coast as far as Maine’s midcoast, according to a recent scientific study. And they appear to be harming the growth of grasses in the marshes where they can be found.
David Johnson, an associate professor at Virginia Institute of Marine Science, co-authored a study published last month in the scientific journal Ecology that examined the impact of fiddler crabs on salt marshes north of Cape Cod, which is beyond their traditional range. He said he studied their impact on Great Marsh along Massachusetts’s North Shore, but that he has found fiddler crabs as far north as Phippsburg.
The crabs, which burrow into the soil where salt marsh grasses take root, are harming the grass’ growth, according to the study’s findings. Where fiddler crabs are found, they have reduced the amount of grass by 40 percent and the amount of grass roots by 30 percent.
This was unexpected, Johnson said, because the crabs seem to boost the growth of salt marsh grass south of Cape Cod, in their traditional territory. He said he is not sure why the effect is different, because the types of grass he found in Great Marsh are the same type of spartina grasses that support fiddler crab populations south of Cape Cod.
“I was totally surprised,” Johnson said. “It’s all the same grass.”
Johnson said he and his co-author on the study, Kayla Martínez-Soto, are not sure why the crabs’ impact on salt marsh grass differs depending on where the marsh is located. He theorized that, even though the species of grasses are the same, the grasses north of Cape Cod haven’t adapted yet to the crabs’ behavior.
Until Johnson and Martinez-Soto studied the crabs in Great Marsh, the consensus had been that the crabs were good for overall marsh grass growth. The crabs, which are about an inch to an inch-and-a-half wide, burrow into the muddy soils under the grass, which helps to aerate the soils with oxygen and boost nutrient levels. They do not eat the grass, even though they do damage roots that they dig through where they burrow into the ground, he said.
Johnson said that it simply might take time for the broader benefits of the crabs’ digging to offset the harm they cause to a plant when they burrow through a plant’s roots.
“I do think it’s possible the plants can adapt,” he said, adding that it might take 10 years or more for the benefits to manifest. “We don’t know.”
With a decline in salt marsh grasses, there could be potential harm to Maine’s coastline, he said. The ability of marshes to absorb sea level rise or storm surge is diminished if they have less grass, he said, and it can result in less carbon absorption in the marsh and higher carbon levels in the atmosphere and ocean, which would continue to exacerbate climate change.
Marissa McMahan, a Maine-based marine scientist who serves as senior director of fisheries for Manomet, has not studied fiddler crabs but said the importance of salt marshes to lessening the impacts of climate change cannot be overstated.
“Anything that is degrading marshes is degrading the impact of carbon in the atmosphere,” which contributes to warming temperatures and increasing acidity levels in the ocean, she said.
Fiddler crabs are among a group of marine animals that traditionally have not been found in Maine but are either taking up residence or are becoming more frequent visitors. Invasive green crabs, black sea bass, and longfin squid are some of the species becoming more commonly seen along Maine’s coast.
Blue crabs are another, and are a species whose presence in the state Manomet is documenting, with the help of fisherman and other observers. McMahan said marine scientists at Manomet and other organizations in Maine are keenly interested in documenting any emergent species that might be getting a foothold in the state.
The overall impact of such species coming to Maine is unclear. Blue crabs and black sea bass have commercial potential, which might help offset the slow decline of Maine’s dominant lobster fishery, but others such as green crabs and longfin squid are known to have harmed other lucrative fisheries such as softshell clams and shrimp.
McMahan said that in-migration of new species is happening quickly, and it sometimes is difficult to keep up with where unfamiliar fish are turning up along Maine’s coast. Tautog, a commercial fish species not traditionally found in the Gulf of Maine, is becoming more common off the Massachusetts coast, she said, and likely will become more common off Maine, too.
“If it’s a new species that hasn’t been here before, there likely will be positive and negative impacts,” she said.
