Only a few rotifer species belonging to the genus Brachionus are used in aquaculture. As outlined in the introduction the most widely used species is Brachionus plicatilis, a cosmopolitan inhabitant of inland saline and coastal brackish waters. It has a lorica length of 100 to 340 mm, with the lorica ending with 6 occipital spines (Fukusho, 1989).
However, for use in aquaculture, however, a simple classification is used which is based on two different morphotypes, namely Brachionus rotundiformis or small (S-type) rotifers and Brachionus plicatilis or large (L-type) rotifers. The differences among the two types can be clearly distinguished by their morphological characteristics: the lorica length of the L-type ranging from 130 to 340 mm (average 239 mm), and of the S-type ranging from 100 to 210 mm (average 160 mm). Moreover, the lorica of the S-type shows pointed spines, while of the L-type has obtuse angled spines (Fig. 3.3.).
Figure 3.3. Brachionus rotundiformis (S-type) and Brachionus plicatilis (L-type) (modified from Fu et al., 1991).
In tropical aquaculture the SS-type rotifers (Super small rotifers) are preferred for the first feeding of fish larvae with small mouth openings (rabbitfish, groupers, and other fish with mouth openings at start feeding of less than 100 mm). Those rotifers, however, are genetically not isolated from S-strains, but are smaller than common S-strains.
The S- and L-morphotypes also differ in their optimal growth temperature. The S-type has an optimal growth at 28-35°C, while the L-type reaches its optimal growth at 18-25°C. Since contamination with both types of rotifers occurs frequently, lowering or increasing culture temperatures can be used to obtain pure cultures: rotifers at their upper or lower tolerance limit do not multiply as fast and can in this way be out-competed in favour of the desired morphotype.
It should be emphasized that, besides intraspecific size variations, important interspecific variation in size can occur as a function of salinity level or dietary regime. This polymorphism can result in a difference of maximal 15% (Fukusho and Iwamoto, 1981). Rotifers fed on bakers yeast are usually larger than those fed on live algae.