Assimilation of bacteria by the dwarf surf clam Mulinia lateralis (Bivalvia: Mactridae)

Estimation of assimilation efficiency of bivalves, fed with radiolabeled bacteria, differs substantially depending on choice of tracer. We have investigated assimilation of bactena by the dwarf surf clam MuLinia lateralis (Say) using bacteria labeled w t h either ~ [ ~ ~ S ] m e t h i o n i n e or [ m e t h y l 3 ~ ] thymidme. Labeled bacteria were fed to M. lateralis 6.18 f 0.45 mm (mean +SD) in shell length in static pulse-chase experiments. After feeding and 4 consecutive 1 h chases in 0.2 pm flltered seawater, M. lateralis retained 93.34 f 6.45 % (n = 20) of incorporated 35S-methionine radioactivity. Altemahvely, when 3H-thymidine was used, only 51.40 f 16.54 % (n = 13) was retained. During our chase procedure, excretion patterns also differed conspicuously For excreted 3 5 ~ activity, 49.05 13.30 % was recovered m the particulate phase (> 0.2 ~ m ) and 50.94 f 13 30 % was dissolved. The corresponding values for excreted 3~ were 11.43 -t 4.70 '10 and 88.57 + 4.70 %. Ass ida t ion efficiency, estlrnated using 35Smethionine, is considerably hlgher than previously reported values using I4C, 3H and 15N tracers. We can explain assimilation and excretion differences between tracers in terms of metabolic pathways associated w t h the labeled moiebes. The assimilation of methlonine and high retention of 35S tracer demonstrates the role of bacteria as a source of proteins and mineral nutrients for M. lateralis.