Effects of ammonia on the survival, life cycle, reproduction, and egg viability of the minute rotifer Proales similis

Abstract

The minute rotifer Proales similis, has been successfully used as initial live food for fish larvae with small mouth gapes due to its small body size and soft body lacking lorica. However, the morphological traits may render P. similis more sensitive to environmental stressors. One major inhibitor of aquaculture productivity is high levels of unionized ammonia (NH₃). Determining NH₃ tolerance thresholds and understanding how P. similis responds to NH₃ stress are crucial for its stable and large-scale production. This study conducted a comprehensive evaluation of the effects of NH₃ on the physiological characteristics of P. similis at both the individual and population levels, as well as on egg viability (hatching success for the next generation). The results showed that P. similis exhibits greater (over five times) NH₃ sensitivity than other rotifer species; however, it may possess specific strategies for coping with NH₃ stress. Specifically, NH₃ ≥1.6 mg/L suppressed maternal reproductive capacity, delayed the time to first reproduction (generation time), shortened the reproductive period and lifespan, and ultimately led to reduced fecundity and population growth rates. In contrast, low concentrations (0.2–0.4 mg/L) stimulated growth of P. similis. Egg hatching was significantly delayed/inhibited under NH₃ (4.0–21.3 mg/L) exposure, but re-hatching of previously unhatched eggs was observed after returning to control conditions (a recovery process). These phenomena were not observed in Brachionus rotifers, suggesting a unique adaptive strategy of P. similis to NH₃ stress that might be associated with its benthic features.