Characterization of Small-size Egg-bearing Thai ss-strain Rotifers Brachionus rotundiformis and Their First Offspring

This study selected small (100–120 μm) egg-bearing females from a local Thai ss population and characterized their first offspring's life history in an attempt to produce small-size rotifers. Mean lorica length of selected small adult females was 120 ± 6 μm at the time of collection, and this mean length did not increase over their lifetime. While their first offspring life span was only 5.11 ± 0.35 d, the reference population longevity was higher, 9.65 ± 0.4 d. The length of reproductive period of offspring from the small-size egg-bearing female was 2.58 ± 0.38 d, shorter than that of reference population 6.83 ± 0.58 d. Although the rate of egg production was not different between small egg-bearing female and reference population, the total number of eggs produced by the small egg-bearing female's first offspring was lower (4.78 ± 0.60) compared to the reference population (11.83 ± 1.26). The small egg-bearing female's first offspring had a higher egg to lorica length ratio (76.1 ± 1.64%) than the reference population (56.6 ± 1.3%), indicating a relatively high investment in reproduction. The small egg-bearing female's first offspring reached only 125 ± 6 μm in 36 h, and further culture of their offspring over 35 d resulted in a mean offspring size, 163 ± 11 μm, similar to the reference population (159 ± 16 μm). This shows that population mean size reduction is not likely to result from selecting small egg-bearing females within a single generation. The unique reproductive characteristics of small egg-bearing female place it in its own category.