Tsunami-Riding Clams: Decoding the life history of the mysterious Glycymeris fauna of Israel

The Mediterranean beaches of Israel are distinguished by extensive monospecific deposits of Glycymeris nummaria of unexplained origin. Radiocarbon ages of these shells show that the vast majority of the assemblage dates to 5500 to 800 yr bp, with only a few living specimens found via dredging in the mid 20th century. This discrepancy between live and dead assemblages has yet to be fully resolved. It has been suggested that past populations were more extensive and that environmental change may have contributed to their local extirpation in the modern fauna. Curiously, these species are occasionally observed articulated within offshore sedimentary deposits interpreted as associated to past tsunami events. Resolving their habitat allow us to reconstruct their distance of transport and also to understand what environmental changes precipitated their local demise. We used oxygen isotope paleothermometry to constrain the complete temperature life history of seven shell specimens sampled from multiple sites along the Northern and Central Israeli coasts. We determined that the three individuals survived for 1.5-2 years each, growing around 2 cm/year as determined by the seasonal temperature oscillations recorded, and with all recording temperature ranges under 4.5°C. This implies that the bivalves likely grew at 50-70 m depth based on modern temperature observations. For an articulated individual found within a tsunami deposit off of Caesarea radiocarbon dated to ~4300 year bp, this means it was transported suddenly over three kilometers and buried within an shelly layer of imbricated single Glycymeris valves. We are currently undertaking research to understand the energies required to transport a live individual over that distance in order to constrain the energy of the wave event. Further investigation is needed to constrain changes in substrate regime and nutrient supply at its habitat which could explain their extreme proliferation and sudden disappearance.