InEx - a direct in situ method to measure filtration rates, nutrition, and metabolism of active suspension feeders

Sponges, bivalves, and tunicates play an important role in the trophic dynamics of many benthic communities. However, direct in situ measurements of their diet composition, filtration, and excretion rates are lacking for most species, and knowledge of these rates is based mostly on indirect, in vitro measurements. This paper presents and evaluates an in situ, nonintrusive technique of direct measurement of the rate and efficiency by which an active suspension feeder removes (or discharges) substances from (to) the water it filters. The technique, termed InEx, is based on the simultaneous, pair-wise collection of the water inhaled and exhaled by the animal. It was specifically adapted to allow reliable sampling of common, small suspension feeders with excurrent aperture as small as 2 mm. The difference in concentrations of a certain substance between a pair of samples provides a measure of the retention (or production) of the substance by the animal. Calculations of feeding (or production) rates are obtained through multiplying the concentration difference by the pumping rate. The latter is concurrently measured by recording the movement of a dye front in a transparent tube positioned within the excurrent jet. An important quality of the InEx technique is that it does not manipulate the studied organisms and thus allows realistic estimates of the organism's performance under natural conditions. Preliminary results showing the diet composition, feeding rates, and removal efficiencies of some coral reef sponges, bivalves, and tunicates are presented and discussed.