Effects of size, form, and toxicity of food species (red tide dinoflagellates) on clearance rate and intake rate in juvenile purple clam, Saxidomus purpuratus

Abstract

The purpose of present study is to know the cause of differences in clearance rate (CR) and intake rate (IR) of juvenile purple clam, Saxidomus purpuratus among different food species of red tide dinoflagellates (RTDs). Laboratory experiments were performed to measure the CR and IR of juvenile purple clam, S. purpuratus as a function of algal concentration when feeding on unialgal diet of either the standard food, Isochrysis galbana or one of 9 RTDs (Akashiwo sanguinea, Alexandrium affine, Alexandrium catenella, Amphidinium carterae, Cochlodinium polykrikoides, Gymnodinium catenatum, Lingulodinium polyedrum, Prorocentrum minimum, Scrippsiella trochoidea). In general, patterns of CR with increasing algal concentration were quite similar among different RTDs. It increased rapidly at lower algal concentration, reached maximum, and then decreased gradually as algal concentration increased further. There was ca. 4-fold difference in the maximum value of CR (Cmax) among RTDs. The highest Cmax was observed when feeding on A. affine (18.7 L/g/hr), while the lowest on C. polykrikoides (4.9 L/g/hr). The patterns of IR with increasing algal concentration were also similar among different RTDs. It increased rapidly at lower algal concentration, and then was saturated as algal concentration increased further. However, there was a great difference (more than 40-fold) in the maximum value of IR (Imax) among different RTDs. The highest Imax was observed when feeding on a toxic RTD, A. carterae (64.8 mgC/g/hr), while the lowest on G. catenatum (1.6 mgC/g/hr). The cell size and toxicity did not affect the Cmax and Imax, e.g. A. affine and C. polykrikoides were similar in size (24.8 vs. 23.2 mm), but Cmax differed greatly (18.7 vs. 4.9 L/g/hr). On the other hand, P. minimum (12.9 mm), S. trochoidea (25.1 mm), and L. polyedrum (37.9 mm) were different in cell size but showed similar Imax (11.7-15.1 mgC/g/hr). There were no significant differences in Cmax and Imax between toxic and