MORPHOLOGICAL DIFFERENCES BETWEEN 2 TYPES OF THE ROTIFER BRACHIONUS-PLICATILIS MULLER,O.F.

The rotifer Brachionus plicatilis O.F. Muller has been used extensively for aquaculture in Japan; two morphotypes, S (small) and L (large), are distinguished, based on morphological and physiological differences. Morphological differences between these two types were studied biometrically. 67 strains of the rotifer collected from all over the world were classified by eye into 37 S and 30 L type strains by differences in the shape of anterior spines of lorica. For each of the 67 strains, seven characters of the lorica were measured on each of 20 individuals derived parthenogenetically from one individual. Classifications based on both cluster and stepwise discriminant analyses yielded equivalent results. Based on cluster analysis using the seven measurements, the 67 strains were classified into two groups containing the predesignated 37 S and 30 L type strains. Lorica length, lorica shape and shape of anterior spine were considered as the distinguishing morphological characters between S and L types. Two linear discriminant functions associated with S-L types were also derived, which can be used to classify new strains that may be observed in the future into S or L type. Accordingly, the rotifer was clearly divided morphologically into the two types, S and L.