The mangrove crab may need to change its name. A crustacean known for living in the mangroves of Florida is now settling into salt marshes hundreds of kilometres away, according to a study published in the Journal of Crustacean Biology.
“You have the mangrove fiddler crab outrunning the mangroves,” says David S. Johnson from the Virginia Institute of Marine Science.
In days gone by, the mangrove fiddler crab (Leptuca thayeri) was only found amid the shaded roots of Florida’s muddy mangroves. The northern extent of its range was Jacksonville on the state’s Atlantic side and Cedar Key on the Gulf of Mexico side. Now, as waters warm, that range is expanding.
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With the help of citizen scientist platform iNaturalist, which allows people to record and share wildlife sightings, Johnson and team have been tracking the crabs’ movements. They have now spread up the east coast of Florida and can be found 307km further north, in Beaufort County, South Carolina.
Florida’s mangroves have been expanding their range on the east coast too, but the mangrove crab has outpaced them. The crustaceans eat detritus and algae, but because they’re not mangrove detritus specialists, they can also survive in temperate salty marshlands.
The crabs didn’t walk to their new habitat, however. Instead, their larvae drifted there on northward ocean currents. Mangrove crab larvae spend around a month in the water, growing and molting, before settling on land and maturing further. For this to happen, the water temperature needs to be within a particular range, but ocean temperatures are rising.
Around the world, warm-water species are being forced to move poleward into cooler regions. This is known as ‘tropicalisation.’ Along the southeastern US coast, sea surface temperatures have increased by more than 1°C over the past 20 years. This suggests that climate change is causing the mangrove crab to expand its range. “That’s what makes this so fascinating,” says Johnson. “It’s one of the clearest examples of climate change that you can see,” says Johnson.
The findings build on earlier research by Johnson, documenting similar climate-induced range shifts in other marine species, including the Atlantic marsh fiddler crab and the lady crab.
If the mangrove crab’s expansion continues, it could reach North Carolina in about five years’ time, where it will impact the local ecosystem in ways as yet unknown. Future studies are needed to help determine the impact of this.
Top image: a mangrove fiddler crab in St. Mary’s, Georgia. Credit: David S. Johnson
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