Regulation of opening state and filtration rate in filter-feeding bivalves ( Cardium edule, Mytilus edulis, Mya arenaria) in response to low algal concentration

When filter-feeding bivalves experience very low algal concentrations, this eventually lead to reduced siphon-opening and valve-gape, or complete closure, along with cessation of the filtration activity, which lasts until the algal concentration is elevated above a lower threshold level. The opening–closing phenomenon and response times in three species of bivalves ( Cardium edule, Mytilus edulis, Mya arenaria) in the presence and absence of algal cells were studied in controlled laboratory experiments. Opening state and correlated filtration rate at varying or maintained levels of algal concentration were quantified in clearance experiments combined with simultaneous video observation. When initially unfed bivalves were offered algal cells, the animals soon opened their siphons/valves simultaneously with a pronounced increase of the filtration rate. On the other hand, when open and actively filtering bivalves experience decreasing algal concentrations below a certain level, this lead within a few hours to a reduced opening state and cessation of filtration activity. However, new algal additions rapidly (within 5 to 20 min) restored maximum filtration activity. The demonstrated regulation of the opening state and filtration activity in response to the presence and absence of algal cells may represent a physiological adaptation to presumably frequent situations with transiently meagre phytoplankton suspensions in the near-bottom water.