COMPARISON AND EVALUATION OF DIFFERENT TECHNIQUES FOR MEASURING ABSORPTION EFFICIENCY IN SUSPENSION FEEDERS

We compared several methods for measuring absorption efficiency (AE) i n marine invertebrates using the horse mussel Modiolus modiolus. All w ere based on the principle of comparing the digestion of organic mater ial relative to an unabsorbed tracer substance but varied in the natur e of the absorbable fraction and the inert, nonabsorbed marker. For mu ssels feeding on natural seston, estimates of AE were slightly higher when silicate was used instead of ash (the marker commonly used in the Conover ratio method), except when the proportion of silicate in the food was very low. When mussels were fed cultured algae, estimates of AE were lower and less reproducible with either ash or silicate as the nonabsorbable fraction. The use of carbon or total chloropigments ins tead of organic content (weight loss on ignition) as the absorbable co mponent in Conover's equation gave acceptable values for AE. Differenc es between mean values from the various techniques were significant, b ut small, owing to the high precision of the measurements. The Conover ratio procedure is convenient for determining the AE of suspension fe eders when the food source is natural seston, and the method provides reproducible values, but variance is greater when the inorganic conten t of the food is low, as in cultured algae.