Fiddler crabs produce more carbon dioxide than their marshy homes can handle

LOUISVILLE, KENTUCKY—Experts consider salt marshes foot soldiers in the battle against climate change: In the U.S. Northeast alone, they sop up enough carbon every year to counter the burning of 515 million liters of gasoline. But a tiny creature may be undoing some of those savings. Marsh-dwelling fiddler crabs (Minuca pugnax, formerly Uca pugnax) could be releasing enough carbon from the soil through their millions of burrows to seriously offset the carbon-storing powers of their environments, ecologists report here today at the annual meeting of the Ecological Society of America.

Fiddler crabs—named after their one large claw—are hard to study; they scoot into their burrows whenever approached. Yet they can reach densities of up to 700 per square meter. Their burrows, most the size of a hot dog, expose underground soil to air, fueling the decay of trapped organic matter and releasing carbon dioxide (CO2) into the atmosphere. Research has shown that burrowing crabs in mangrove forests in Brazil, Tanzania, and Hong Kong, China, can release quite a bit of CO2.

In search of accurate assessments for salt marshes, a graduate student dragged a suitcase containing a portable gas analyzer into a marsh in Cape Cod in Massachusetts and measured the CO2 released. The analyzer covers up to three burrows at a time, but she tested dozens of sites in different spots of the marsh. When fiddler crab burrows were tucked among the vegetation, little extra gas was released. But when burrows were in open patches of mud, the amount of emitted CO2 tripled compared with areas with no burrows, the researchers report.

To figure out what was going on, researchers measured the sizes of burrows from bare and vegetated sections of the marsh. Their findings: The burrows were much smaller among the salt grasses. That could mean, they say, that bigger burrows release more CO2 into the atmosphere—perhaps enough to make scientists recalculate the amount of carbon stored in marshes. It also means other burrowing animals, like shrimp and clams, may be adding in similar ways to our global carbon load.