The bivalve Mercenaria rnercenaria was pulse-fed labelled microalgae in experiments designed to test the applicability of the SICr: "C twin tracer technique to the study of food absorption in a suspension feeder. Absorption efficiencies of the chlorophyte Pseudoisochrysis paradoxa using Conover's ash-ratio method and the tracer technique were in good agreement. Clams absorbed approximately 14 % of the 51Cr ingested. An assumption of the radiotracer method is that 51Cr and 14C move along the gut at similar rates. The gut passage times of the 2 isotopes differed significantly, however, when clams were fed P. paradoxa, a 'good' food source: the gut residence time of "C was greater than that of 51Cr. Therefore, analysis of a single faecal subsample can cause significant error in the calculated absorption efficiency. The value will be overestimated or underestimated depending on whether the faeces are subsampled early or late, respectively, after transfer of the animals to unlabelled food. Therefore, pulse-chasing, or recovery of faeces over a fairly extended period of time (to be determined for any given experimental conditions) is strongly recommended. Examination of the timecourse of 14C egestion revealed that the gut passage time of P. paradoxa, which was absorbed with high (82 % ) efficiency, was significantly greater than that of 2 chlorophytes (Nannochloris atomus and Stichococcus sp.) and 2 cyanobacteria of the genus Synechococcus, which are inefficiently utilized by M. mercenan'a. The study provides evidence that clams are able to sort different algal species in their passage through the gut. Control of gut clearance rates, through more rapid elimination of those algal species which are also poorly utilized, may contribute to the species' adaptive strategy.

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