Non-destructive measurement of the time evolution of burrowing shrimp mound topography

Abstract

We applied a profiling laser scanner system, originally developed for quantitative soil erosion studies, to produce 3-dimensional maps of underwater mounds produced in aquaria by thalassinidean shrimp Trypaea australiensis. With appropriate calibration against a surface of known geometry installed within the aquaria, the technique is highly accurate (1 and 2 mm resolution in the 2 horizontal dimensions and 0.2 mm resolution in the vertical). The method is non-destructive, and the evolution of mounds can be measured so the net daily rate of movement of material from the burrow to the surface mound can be reliably estimated. The technique also detects and measures slumping of the mound material to produce ‘funnels’ — a process which cannot be characterised by conventional techniques based on direct entrapment of sediment. We used the technique to measure mound and funnel formation in 3 laboratory aquaria populated with thalassinidean shrimp over a period of 359 d. Significant changes in mound volume were observed to occur from one day to the next (mean rates of sediment rise ranged from 0.36 to 1.1 cm3 d–1 opening–1, and mean rates of sediment fall ranged from –0.32 to –1.3 cm3 d–1 opening–1). The rapid rate of movement of material, both into and out of the burrow, has important implications for sampling frequency if the rates of movement of sediment are not to be drastically underestimated. Our results suggest that daily sampling is required. This is rarely achieved in other studies, and, consequently, existing published values may be too low. The technique could be applied to quantify sediment transfer by other species that produce perturbations in the level of the sediment surface.