Excerpts from Jim Conrad's
from the January 18, 2015 Newsletter issued from Río Lagartos, on the north-central coast of Yucatán, MÉXICO
SALTPAN FIDDLER CRAB
Up the estuary at the salt pans of Las Coloradas our flamingo-watching boat pulled up to a levee separating one of the ponds from the estuary, so guests could see the ponds' water, bright pink from algae, and marvel at the size of the operation. Stepping ashore onto sand composed of shell fragments, numerous tiny critters skittered from beneath my incoming foot, and I wondered what they might be. They ranged from pea-size to ant-size. A picture of a well camouflaged pea-size one with its big, white "fiddling" claw in the upper, right corner is shown above.
With such an oversize claw, clearly we had a fiddler crab, and all fiddler crabs belong to the genus Uca. However, about a hundred fiddler crab species are recognized worldwide, so how could I hope to identify this one? Happily, on the Internet, help came by way of a 1987 study by Carl Thurman II, published in Crustaceana Vol. 53, No. 1, entitled "Fiddler Crabs (Genus Uca) of Eastern Mexico (Decapoda, Brachyura, Ocypodidae)."
That study lists seven fiddler crab species on the coasts of the Yucatan Peninsula, of which only five occur here on the northern coast. Pictures of all five of those species were summoned on the Internet, and of those only one matched our image, and that was the appropriately named Saltpan or Burger's Fiddler Crab, UCA BURGERSI.
The main field marks separating the Saltpan Fiddler Crab from the other species -- apart from it being found among the salt pans -- are that its top shell, or carapace, bulges forward to between the eyes, while on other species that area is flattish. Also, the carapace's speckles, the banded legs, its small size (barely 20mm, or less than an inch), and remarks from various sources that Burger's Fiddler Crab is common in a variety of habitats in the Yucatan all support the ID.
Saltpan Fiddlers occur throughout the Caribbean from coastal Florida south to South America. Their habitat is described as estuarine beaches, usually near mangroves, living in burrows constructed in the intertidal zone. They are "detritivores," meaning that they feed on detritus, which is non-living particles of organic matter in the sand, mud and washed-up material at the water's edge.
James Lazell's 2005 book Island: Fact and Theory in Nature, describing the flora and fauna of Guana Island in the British Virgin Islands, describes Saltpan Fiddlers as fairly swarming on mud around a salt pond, just like those on the shore of salt ponds at Las Coloradas. He also writes that the larvae of these crabs are a mainstay of the flamingo flock" which probably was the case at Las Coloradas, too, where flamingos were foraging with their heads beneath the water not far away.