Mega Mounders: Burrowing Talitrids (Megalorchestia spp.) Excavate Large Quantities of Sand on California Beaches

Abstract

High densities of burrowing talitrid amphipods inhabit beaches around the world. This includes California, where densities of more than 1,000 individuals m⁻² are regularly recorded. Here, we provide evidence of Megalorchestia spp. excavating up to 10.38 kg dry sand m⁻² d⁻¹ on a beach in southern California, one of the highest mounding rates ever recorded on a daily per unit area basis for a biogeomorphic agent. Extrapolated across 25 km of sedimentary coast, the daily amount of sand moved by Megalorchestia spp. is comparable to the suspended sediment flux of major rivers in the region. By combining our findings with an 8-year data set of talitrid biomass estimates at the study site, we estimate that Megalorchestia spp. excavate approximately 2.79 × 10⁶ kg ha⁻¹ yr⁻¹, the highest annual excavation rate ever calculated for a terrestrial or coastal animal where long-term trends in population dynamics have been considered. These results have significant implications for the form and function of sandy beach ecosystems worldwide, and highlight the importance of beach fauna in sediment transport, beach morphodynamics, and landscape evolution.