Physiological Regulation of Valve-Opening Degree Enables Mussels <i>Mytilus edulis</i> to Overcome Starvation Periods by Reducing the Oxygen Uptake

Abstract

During periods of starvation, the blue mussel Mytilus edulis reduces its valve gape and thus the filtration rate whereby the oxygen uptake becomes reduced. Considering the frequency with which M. edulis in the field experience shorter or longer periods with low phytoplankton concentrations it is of great importance to understand the bioenergetic implications the valve opening-closing mechanism. Here, we tested the hypothesis that M. edulis during starvation regulates the opening degree of its valves in such a way that the oxygen concentration in the mantle cavity is reduced in order to minimize the respiration and at the same time prevent anaerobic metabolism which is energetically expensive. This was experimentally done by measuring the oxygen-concentration changes in the mantle cavity of both starved and fed mussels using a fibre-optic oxygen meter with a small sensor inserted into the mantle cavity through a hole drilled in the valve. It was observed that when there were no algal cells in the ambient water, the mussels gradually closed their valves resulting in a decline of the filtration rate along with a simultaneous decrease in the oxygen concentration in the mantle cavity and subsequently a remarkable decrease in the respiration rate. Typically, a starved M. edulis closed its valves for a certain period of time followed by a short period when it re-opened and this resulted in an alternating fall and rise of the oxygen concentration in the mantle cavity. Therefore, the low oxygen consumption rate of M. edulis in phytoplankton depleted water can be interpreted as an efficient physiologically regulated mechanism that allows the mussel to save energy during a starvation period.