The distribution and dynamics of Uca pugnax (Smith) burrows in a new England salt marsh

Abstract

The mud fiddler crab Uca pugnax Smith, builds and maintains below-ground burrow structures in salt marshes along the east coast of North America. The distribution of burrows across a marsh habitat was described and the influence of substratum characteristics on the distribution of U. pugnax burrows was examined. In a representative New England marsh, substratum hardness and root mat density increase with increasing tidal height with most burrows found at intermediate tidal heights. The depth, volume, and turnover rate of burrow structures decreases with increasing tidal height. Substratum preference tests show that U. pugnax preferentially burrows in substratum of intermediate root mat density in close association with hard structural elements. Without structural elements in the substratum, crabs prefer medium density root mat to both softer and harder substratum. Female and smaller male crabs are shown to be far superior to larger male crabs in burrowing ability. Burrowing in the soft substratum at lower intertidal levels of the marsh is limited since soft, fluid substratum will not support burrow structures. Consequently, almost all burrows at low levels on the marsh are closely associated with structural elements in the substratum (primarily grass stems) which given burrows support and increase burrow longevity. At high tidal levels, heavy root mat accumulation, resulting in a hard substratum, limits crab burrowing. The addition of artificial burrows and softer substratum to high intertidal levels resulted in rapid recruitment of crabs to that habitat. Patterns in the recruitment of crabs to artificial burrows suggests that U. pugnax individuals actively, compete for larger burrows.