Comparison and evaluation of different techniques for measuring absorption efficiency in suspension feeders

We compared several methods for measuring absorption efficiency (AE) in marine invertebrates using the horse mussel Modiolus modiolus . All were based on the principle of comparing the digestion of organic material relative to an unabsorbed tracer substance but varied in the nature of the absorbable fraction and the inert, nonabsorbed marker. For mussels 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 instead of organic content (weight loss on ignition) as the absorbable component in Conover’s equation gave acceptable values for AE. Differences between mean values from the various techniques were significant, but small, owing to the high precision of the measurements. The Conover ratio procedure is convenient for determining the AE of suspension feeders when the food source is natural seston, and the method provides reproducible values, but variance is greater when the inorganic content of the food is low, as in cultured algae.