Burrowing and mud-mound building life habits of fiddler crab Uca lactea in the Bay of Bengal Coast, India and their geological and geotechnical importance

Abstract

Burrowing and mud mound-building life-habits of quasiterrestrial fiddler crab Uca lactea from Bengal coast under specific substrate (soft aqueous mud deceptively occurring below rigid sand), hydrodynamic (low-energy and shallow inundation) and geomorphic (coastal mudflats around estuary mouth) conditions are addressed to evaluate geological and geotechnical importance. They construct mud-mounds at burrow (simple, I-shaped and un-branched) heads during tidal recession through oozing out of burrow-base mud-slur. Depth of tidal inundation and substrate undulations control mound population, height and types (dominance of short simple-mounds in high-grounds vs. tall compound-mounds in low-grounds allowing subaerial respiration). Loss of dwelling habitat through mound collapse during high tide is promptly compensated by new constructions. Microbial stabilization of mounds through seasonal algal bloom enhances their preservation under freshly deposited sand layers. They also produce mud-chimneys and feeding-pellets in association with Turritella spp. trails and worm burrows. Despite considerable preservation potentiality of Uca mud-mounds their exact ancient analogues are not yet known. The study provides firsthand criteria to recognize fossil burrows and mud-mounds of Uca and comparative analysis between U. marionis mud-volcanoes and U. lactea mud-mounds. Morphologically they are identical enough to constitute a unique ichnotaxon. The studied ichnocoenose corresponds in fossil records to Skolithos-Taphrhelminthopsis-Ophiomorpha-Planolites-Thalassinoides association within shallow marine Psilonichnus ichnofacies to which Uca mud-mounds are new entrants diagnostic of palaeoshoreline, especially shallow coastal-mudflats at high-tide level. Uca mud-mounds, by virtue of requirement of unique substrate conditions, are considered as ichnological indicators of erosion-prone coastal segments. Their global zoogeographic distribution allows wide application of this ichnological tool.