The effect of body size and food concentration on the in situ filtering rate of Sida crystallina 1

The effect of body length and of concentrations of suspended material and periphyton on the filtering rate of Sida crystallina was measured in situ. The filtering rate of Sida ( V F , ml·animal −1 ·d −1 ) was predictable from body length ( L , mm) and the concentration of small (<35 µm ) suspended particles (S n , mg·liter −1 dry wt) by the equation This relationship shows that the filtering rate increased with body length and that the relationship between filtering rate and body length changed with food concentration. The filtering rate decreased with increasing food concentration, and the rate of this decrease declined with decreasing body size. The validity of the assumption that Sida ingests all particulate matter <35 µ m at the same rate as radioactive yeast cells ( Rhodotorula ) was examined and feeding rates ( I F ) were calculated: I F = V F S n . I F increased with increasing food concentration. The data suggest that the incipient limiting food concentration increased with body length of Sida. These data collected on Sida indicate that the consequences of correcting filtering rates to standard body size and of predicting filtering rates of field animals but ignoring natural size distributions may be significant. The concentration of periphyton, a postulated alternate food source for Sida, was found to have a small but significant effect on the filtering rate of naturally occurring animals.