That's a field marking for U. pugnax from the Chesapeake Bay region. The group of crabs on the right exhibit droving, a schooling behavior one can observe when fiddlers are away from their burrows. The largest burrow measured 52 cm. Male crabs presumably use chimneys for territorial displays and to attract females. These little crabs play important roles in many marsh processes.
Fiddler crabs are also important in energy flow within the marsh.
Fiddler crabs are also significant organizers of the tidal marsh community. Fiddler crabs are avid burrowers, and their burrowing activity can erode or undermine marsh banks. Fiddler crabs burrow into the marsh substrate for shelter. Their burrows provide protection from predation, and sites for mating. Their burrowing activity appears to improve the metabolic processes in marsh sediments, presumably through improved oxygenation, drainage, and organic enrichment of the surface via nutrient and sediment turnover.