Floating articulated bivalves, Texel, North Sea

Articulated, closed, empty bivalves were collected from a modern beach on the island of Texel, The Netherlands. They included many 2-year-old Spisula subtruncata (average length 25.5 mm). Experiments indicated that they could float. Normally, dead but still articulated bivalves on the beach gape; because the adductor muscles no longer pull the valves together, the ligament opens the two valves. These bivalves most probably had arrived alive on the beach after being dislodged during a storm. The animal inside desiccated on the beach, the remains of the animal glued the valves together, air was trapped inside and the next high tide brought the Spisula floating to a higher drift line on the beach. Such closed, empty bivalves may float for days and cover considerable distances as exemplified by a freshwater Dreissena polymorpha also found on the beach of Texel as a closed, floating bivalve, 20 km from the nearest place where it occurs alive. For the palaeoecologist this indicates that closed, articulated bivalves do not necessarily indicate an absence of transport.