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. edulisclosed 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.
Highlights
The filter-feeding blue mussel Mytilus edulis is a widely distributed and locally abundant bivalve mollusc in the North and Mid-Atlantic Regions, and a valuable commercial species [1]
When there were no algal cells in the ambient water, the mussels gradually closed their valves (Figure 2(c) and Figure 2(d)) resulting in a decline of the filtration rate along with a simultaneous decrease in the oxygen concentration in the mantle cavity (Figure 2(a) and Figure 2(b), Figure 3(b)) and subsequently a remarkable decrease in the respiration rate (Table 2 and Figure 4)
The present demonstration of valve closure in Mytilus edulis during starvation periods is in agreement with Eriksen and Iversen (1997) who observed that when there were no food particles in the ambient water the valves of M. edulis were only slightly opened, but after addition of algal cells the valves opened within 1 to 2 h and a 50% increase in respiration rate was recorded [21]
Summary
The filter-feeding blue mussel Mytilus edulis is a widely distributed and locally abundant bivalve mollusc in the North and Mid-Atlantic Regions, and a valuable commercial species [1]. Because mussels are often living in dense beds, the actual phytoplankton biomass to which the mussels are exposed may frequently be strongly reduced [8]-[12], and further, starvation often occurs in winter periods with no primary production [13] During such starvation periods M. edulis reduces its valve gape, and the filtration rate, whereby the oxygen uptake becomes reduced because the oxygen uptake is governed by a diffusive boundary layer between the laminar ventilation current and the mantle cavity [14]-[16]. It was found that the actual weight loss was 10 to 12 times lower than the estimated respiratory weight loss [18], and this result supports the suggestion that physiological regulation of the valve-opening degree is an efficient mechanism that allows mussels to survive long periods of starvation
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