ROWLEY, Massachusetts — David Johnson was up to his ankles in the gooey mud of a salt marsh creek when he saw a tiny critter hop out of the mud and scuttle off.
That would have been the end of it, were it not for one important factor: Johnson is a biologist with the Woods Hole-based Marine Biological Laboratory who has spent over a decade exploring and documenting the Great Marsh. This was something different, something he hadn’t seen.
He caught the critter and brought it back to his lab for studying. Now, six months since that initial find and with numerous other pieces of evidence collected, Johnson is unveiling a discovery that has important implications for both the Great Marsh and the wider world.
The creature he found is a fiddler crab, a species that never had been documented north of Cape Cod. Ocean temperatures in the greater Newburyport area always have been too cold for them, but not anymore. There are colonies of fiddler crabs moving into the Great Marsh and beyond. It’s an indisputable sign of warming ocean temperatures and global warming, Johnson said.
“What I am seeing, and what others are seeing as well, is climate change,” he said, as he brought a Daily News reporter and photographer to the spot on the Club Head Creek where he made the discovery in May.
After discovering that fiddler crab and its small colony along the steep and mucky creek bank, Johnson spent the summer exploring the local coast, from the North Shore up through Maine, looking for more evidence of fiddler crabs. He documented several salt marsh colonies of fiddler crabs — including in Danvers, Manchester-by-the-Sea, Gloucester, Essex, Ipswich, Rowley, Salisbury, and as far north as the marshes of Hampton, New Hampshire, the latter being about 50 miles north of their previously documented range.
“In terms of climate science, the question used to be, is the climate changing? The answer is a resounding yes. The next question is, ‘How will organisms respond?’ We are now seeing that one way is by shifts in where they are found,” he said.
Fiddler crabs had historically not populated this area because the waters were too cold for their drifting larvae to survive, Johnson said. In the spring the larvae are released to drift on the ocean currents, often depositing in salt creeks and marshes.
The larvae can’t live in waters that are colder than about 65 degrees. But in 2012 and 2013, a noticeable increase in local water temperatures occurred. It brought with it reports of warm water fish being caught in local waters, such as black sea bass and bonito. Normally those fish aren’t found north of Cape Cod.
Johnson suspects the fiddler crab larvae drifted north during that time. The 2- to 4-degree increase in average temperatures was just enough for them to survive.
This year’s ocean temperatures were closer to the historic trends, but Johnson said the general trend over the past several years is warmer temperatures. Now, the fiddler crabs are deeply entrenched in their Great Marsh burrows, able to withstand the cold winters and able to release their larvae locally in the spring.
Discovery
The crabs are about an inch and a half long, with very small claws. Male crabs have an unusual feature, one enormous claw that they use for fighting and to attract a mate. Their habit of running their tiny claw over their large claw while feeding has something of the look of a fiddle being played, and thus they got their name.
Unlike green crabs, which have become a major problem for the local shellfish industry, Johnson said fiddler crabs are not known to be causing environmental damage to the marshes — at least not yet. They dig burrows into the mud, which in small quantities may have a beneficial impact for salt marsh hay. Johnson said there are relatively few fiddler crabs in the Great Marsh now, but if their numbers increase to their potential population levels, he speculated that the burrows might damage the salt marsh.
Right now, he said he has discovered about one burrow per area the size of a doormat. However, to the south where they are long established, they are known to have as many as 800 burrows in a doormat-sized area.
Burrows are the key to finding them. Johnson has become an expert at spotting the signs — a thumb-sized hole, and “ice cream sprinkles” that are the crab’s telltale feces pellets.
More study
Johnson said now that the fiddlers have been discovered they will be closely monitored by scientists. One of the great unknowns is how the crabs might change the salt marsh over the longterm.
“As the world’s oceans continue to warm, we will continue to see climate-driven range expansions,” he said. “The next question for scientists is, ‘What is the impact of these new species on the ecosystems they move into?’”
Some local salt marsh observers have witnessed in the past couple of years a noticeable increase in the collapse of the steep banks along the creeks and rivers in the Great Marsh. Small burrows have been found in them and are believed to be a contributing cause. Johnson said there is no evidence yet to connect fiddlers to this phenomenon. Some believe the burrows are being made by green crabs.
The local marshes are a place where fiddler crabs could thrive.
“For fiddler crabs this is a great spot,” he said. “There are fewer predators and less competition.”
Johnson said he loves working in the marsh. It’s not glamorous work, as his mud-caked pants and shoes attest. He wears “surfer booties” — tight-fitting waterproof socks — to pick his way through the deep and stinky muck of the saltmarsh creekbeds. He has learned over the years that boots are useless, because they get sucked into the thick gooey mud.
He has gotten used to the dreaded greenhead flies that plague the marshes in July, in fact he thinks they are no big deal compared to another marsh inhabitant, the “no-see-um.” The tiny biting insects swarm when the air is still, and Johnson said they always manage to find their way around mosquito netting.
As he looked across the vast expanse of the marsh Friday, he admired the ripples of sunlight cast against the yellowing tufts of saltgrass that stretched to bright blue waters that shimmered in the distance.
“This is my office,” he said.
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