The use of gut fluorescence to estimate grazing by oceanic salps

The fluorometric measurement of chlorophyll and its degradationproducts in the guts of salps has been used as a means of calculatingin situ grazing rates. Salps are particularly suitable for thismethod because they are non-selective filter-feeders large enoughto be individually collected by divers. Analysis of the pigmentin the gut has shown that chlorophyll degradation products canbe used as tracers to estimate in situ ingestion. Filteringrates are calcuLated from the amount of pigment, the gut clearancetime and the concentration of pigment in the water. For Pegeaconfoederata and Salpa maxima, gut-pigment rates are comparedwith rates measured in laboratory particle-clearance experiments.Salpa cylindrica has proved impossible to maintain in the laboratoryand the gut-pigment results are the first feeding data for thisspecies. Filtering rates for all species examined are high,on the order of liters per hour. Variability in the amount ofpigment found in S. cylindrica suggests this species, unlikethe others, may feed sporadically. The gut-pigment method permitsmeasurements of filtration and ingestion by herbivores on analmost instantaneous basis and can be a useful tool for assessinggrazing impact in field situations.