Sediment hardness and water temperature affect the burrowing of Echinocardium cordatum: Implications for mass mortality during the 2011 earthquake–liquefaction–tsunami disaster

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The heart urchin Echinocardium cordatum is an important bioturbator in shallow marine environments throughout the world. Although hard sediment and low water temperature are known to decrease the burrowing ability of benthic invertebrates, the effect of these factors on E. cordatum, one of the most important bioturbators, has remained unknown. Here, we quantified the burrowing ability (i.e., burrowing time) of this species in an aquarium where sediment hardness and water temperature were artificially controlled. The aquarium experiment revealed that both sediment hardness and water temperature significantly affected the burrowing time of E. cordatum. No significant interactions were detected between the two factors. Under hard-sediment and low-temperature (10 °C) conditions, reburrowing times for E. cordatum were around 3.5 times longer than with soft sediment and high temperatures (20 °C). The burrowing time in the hardened sediment due to liquefaction at low temperatures was compared with the observed in-situ interval between the occurrence of an earthquake and the first arrival of a tsunami. The results provide useful insights into the mass mortality of the species that was observed on the Pacific coast after the 2011 Tohoku Earthquake and associated liquefaction and tsunami events on March 11, 2011. Rapid reburrowing would have been vital at the time to prevent individuals from being swept away by tsunami currents after being expelled from the seafloor sediments by the seabed liquefaction. However, our results indicated that E. cordatum would not have been able to easily reburrow into the seafloor sediment, thereby becoming vulnerable to tsunami currents. This would have resulted in the heart urchin's observed mass mortality and prolonged absence from the region after the disaster.