Walking through the marsh one can't help but notice the fiddler crabs and their abundant burrows. Fiddler crabs get their name from the male's large claw. They use the claw to defend their burrows and to attract mates. Males have a highly ritualized combat of arm-waving, bobbing of the body, and standing with an open, upright stance.  Combat rarely gets physical as there is too much risk of injury to either crab. 

The mud fiddler, U. pugnax lives in burrows in the muddy saltmarshes of the Eastern United States. The crabs can reach approximately 25-30 mm carapace width. They are generally considered detritovores feeding on the meiofauna living in the mud.  Note the characteristic blue "front" of the crab below.  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 lower left frame shows several burrow casts (made using fiberglass resin) with a quarter for reference.  The largest burrow measured 52 cm.  The lower right photo shows the top of a "chimney."  Male crabs presumably use chimneys for territorial displays and to attract females.

Three species of fiddler crabs may be found around Chesapeake Bay: Uca minax, the red-jointed fiddler;U. pugnax, the marsh fiddler; and U. pugilator, the sand fiddler. These little crabs play important roles in many marsh processes. Fiddlers are among the most abundant "large" animals in the marsh; as such, they are important regulators of primary production (Spartina-derived), and decomposition (bacteria and fungi). Fiddler crabs are also important in energy flow within the marsh. They are estimated to consume up to a third of the net production of an emergent marsh and may assimilate 10% of that amount. They are an important food item in the diets of large predators, such as, blue crabs, rails, egrets, herons, and racoons; hence, they are an important link in the food web. Fiddler crabs are also significant organizers of the tidal marsh community. Their burrows are filters for the deposition and retention of meiofauna, and infauna; the presence of crabs indicates a greater diversity of marsh organisms. Fiddler crabs are avid burrowers, and their burrowing activity can erode or undermine marsh banks. Crab bioturbation (burrowing and feeding) of the marsh substrate affects not only the aeration, hence, growth of Spartina spp., but it may also affect the turnover, processing and diagenesis of nutrients and other chemicals in the sediments.

Fiddler crabs can reach high densities in salt marshes (70-200 crabs/m2 are not uncommon). 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. This burrowing activity also results in a significant transport of carbon and phosphorus from buried sediments back to the marsh surface. Cycling of nitrogen, phosphorus, sulfur, iron and carbon are likely to be strongly affected by this bioturbation, but little work has addressed the role of fiddler crab bioturbation on nutrient-related processes.

More on fiddler crabs.

Photos taken at the Chesapeake Bay National Estuarine Research Reserve in Virginia (Goodwin Islands and Taskinas Creek Reserves).  Photos and text by Jeffrey Shields, 1999, All rights reserved, Copyright, 1999.