Biophysical Model of Intertidal Beach Crab Burrowing:Application and Significance

Abstract

The biophysical principles and mechanisms involved in intertidal beach-crab burrowing have been studied. These answer some important and fundamental questions, such as how and what basic burrow patterns evolve in dynamic environmental conditions through a delicate balance struck between physical and biological (ethological) processes, and how does a burrow form faithfully record its environmental history in a sedimentary sequence? The study is primarily based on field and experimental data collected on the amphibious brachynuran crab family Ocypodidae (four species of Ocypodeand one species of Ilyoplax) from modern intertidal beach sectors on the Bay of Bengal coast, India. The model introduces a new concept of crab burrow cycle that includes three phases and involves three genetic categories of burrow forms—occupied, abandoned, and collapsed. The first phase (subaerial) begins with the excavation of simple (I or J types) occupied burrow populations and terminates with the development of plugged burrow apertures and sub-surface trapped air-column systems. Pressure equilibrium is reached between the burrow-base capillary water, air columns and sediment plugs. Formation of an air column is a mechanical means of supporting hollow burrow from collapse and creating an underwater quasi-terrestrial condition of living. The second phase (subaqueous) starts with tidal inundation and results in the final burrow architecture (Y, U, multibranched Y, and multiform U). The continued effects of external dynamic pressure fields on the plugs increases the internal air-column pressure causing pressure disequilibrium and lowering of capillary water levels unfavorable to the crabs for gill respiration. The burrower neutralizes the extra pressure to achieve previous pressure equilibrium and capillary water levels by excavating secondary branch(es) advantageously on the landward side of the main shaft and dumping the materials at the burrow-base. This mechanism, besides adding morphological complexities, ensures continued pressure equilibrium, the failure of which in any stage results in burrow collapse. The third phase (subaerial) creates a permanent pressure imbalance as the tidal water quickly recedes and protects the burrower inside as the air column bursts to set the crab free to start another burrow cycle afresh. The model has application potential in varied post-Jurassic coastal intertidal environmental settings occupied by quasiterrestrial crabs. It provides insight into various aspects of ichnology (taxonomy and recognition of ancient crab burrows; characterization and variability of the Psilonichnusichnofacies and intertidal ichnocoenoses; burrowing life habits of crabs and modalities of burrow preservation), environment (influence of bathymetry, tide and wave action, substrate properties, storm-induced episodic erosion and deposition on burrow architecture; environmental zonation of burrows; paleotidal range and marine transgression-regression events) and geomorphology (recognition of shoreline features). Being the first biophysical model of intertidal crab burrows, it contributes original data to the theoretical background of ichnology.