Does neutral red suit the requirements of being a reliable indicator in clearance rate measurements of suspension-feeding bivalves? An empirical refutation

Abstract

One basic requirement in the measurements of feeding rates in suspension-feeding bivalves when using clearance methods is that the suspended particles that are used in the determination of these rates must reach size compatibility with full retention by the gill. In spite of this requirement, clearance of neutral red (NR) -a soluble vital dye- has long been used for this purpose and recourse to this method has even increased over the last years for determining the filtration rate as a specific physiological response to stress in ecotoxicological and biomonitoring studies. The aim of this study was to produce empirical evidence that would dismiss the suitability of the NR method for providing reliable measurements for feeding rates in suspension feeders. Experiments that were designed to this end in the blue mussel Mytilus galloprovincialis led to three main conclusions: 1) Temporal dynamics of NR retention by the mussels point to a diffusion mechanism whereby the marker would be up-taken by gill tissue rather than filtered by the muco-ciliary action of this organ. 2) The NR method largely underestimates the clearance rate (by 90% on average) when compared with conventional methods that use microscopic particles in the size-range of gill retention. 3) The close similitude between rates of retention of NR by intact mussels and mussels that had been sacrificed by sectioning apart the valves adductor muscle proves outright the lack of a quantitative relationship between this retention process and the pumping activity.