Absorption and gut passage time of microalgae in a suspension feeder: An evaluation of the super(51)Cr: super(14)C twin tracer technique

The bivalve Mercenaria mercenaria was pulse-fed labelled microalgae in experiments designed to test the applicability of the super(51)Cr: super(14)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 super(51)Cr ingested. An assumption of the radiotracer method is that super(51)Cr and super(14)C 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 super(14)C was greater than that of super(51)Cr. Examination of the time-course of super(14)C 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 ineffeciently utilized by M. mercenaria . The study provides evidence that clams are able to sort different algal species in their passage through the gut.