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.