cells were repeatedly added to the aquarium and filtration rate measurements were performed under simultaneous monitoring of the valve gape of the mussels during the opening phase (0% to 80% of maximum registered valve gape during the whole experiment).
In another series of experiments, the valve gape opening-closing response of mussels caused by presence/absence of algal cells was also recorded during periods with continuous algal supply followed by starvation periods without algal supply.
ceils [mL.sup.-1] (fully open mussels) to the time when the mean valve gape was reduced to 50% of the maximum registered during the preceding steady-state period.
Figure 2 shows an example of valve opening response (during the first 15 min) to an addition of algal cells to a group of starved mussels, and further, after a period with a maintained algal concentration between 1000 and 5000 cells [mL.sup.-1] to ensure maximum valve gape, the figure shows the closing response to a decreasing algal cell concentration.
The observed valve gape responses to presence/absence of algal cells in the ambient water shows that the valve opening/closure responses are strongly influenced by the preceding feeding conditions.
The duration of the opening response (defined as the time from readdition of algal cells after a short period of valve closure caused by stoppage of the dosing pump until the mean valve gape had increased to maximum) is shown on Figure 8.
cells [mL.sup.-1] responded to a slight reduction in the algal concentration by a nearly immediate reduction in valve gape (Fig.
The proposed causal relation between near-bed phytoplankton biomass (chl a) and mussel valve gape was investigated further in a shallow tidal lagoon close to Kerteminde Fjord.
11), but the present study suggests this effect to be an indirect one, because the mussels (located between dense macro-algae, Fucus sp.) were never exposed to such high current speeds reported to affect the valve gape of mussels (see e.g., Newell et al.
As stated by Riisgard (2001b) the valve-closing phenomenon, which has also been clearly demonstrated in the present work, represents a physiological adaptation to filter feeding in extremely meager situations, when a reduced valve gape reduces the metabolism of the mussel (Jorgensen et al.
Tidally induced variation in bivalve filtration activity (valve gape and/or siphon opening degree) has earlier been observed in situ in Mya arenaria (Roegner 1998, Thorin et al.
Field studies have revealed that phytoplankton concentrations higher than about 1 [micro]g chl a [L.sup.-1] stimulate the mussels to keep their valves wide open, whereas lower near-bed concentrations cause valve closure, or reduced valve gape. The in situ valve opening and closing times in response to variation in near-bed chl a concentration are considerably faster than in unfed mussels in the laboratory.