Body size, ration level, and population growth in Asplanchna

1.) The daily ration required to maintain a population growth rate, r _m, of zero (threshold ration) increased with increasing Asplanchna body mass. This relationship is described by the equation T=0.342 W^0.797 where T=threshold ration (μg day^-1 dry mass) and W=Asplanchna body mass (μg adult^-1 dry mass). 2.) The threshold ration of large campanulate morphs of A. silvestrii was 3.7 times greater than that of conspecific saccate morphs suggesting that campanulates may be restricted to food-rich habitats. 3.) The daily ration required to maintain r _m that is half the maximal population growth rate increased with increasing Asplanchna body mass and is described by the equation H=1.107 W^1.103 where H=ration level and W=Asplanchna body mass. This population growth characteristic may reflect adaptations of rotifers to resource level. 4.) The relationships between ration level, food concentration, and Asplanchna body mass do not support the predictions of the size-efficiency hypothesis but are consistent with observed patterns of species distribution in nature.