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Critical swim speed and fast-start response in the African cichlid Pseudocrenilabrus multicolor victoriae: convergent performance in divergent oxygen regimes

Dissolved oxygen (DO) can be a strong predictor of intraspecific variation in morphology and physiology in fishes. In the African cichlid Pseudocrenilabrus multicolor victoriae Seegers, 1990, fish reared under low DO develop larger gills, deeper bodies, and larger, wider heads than full siblings reared under high DO, which could influence swim perform- ance. In this study, we compared critical swim speed (Ucrit) and fast-start swimming in F1-generation fish from two field populations (one high and one low DO) of P. m. victoriae reared under high or low DO. There was no difference in Ucrit be- tween populations or rearing treatments. However, females exhibited a lower Ucrit than males. In fast-start trials, low-DO- reared fish reacted faster (lower response latency) and used double bends more often than high-DO-reared fish, but there was no difference in maximum velocity or acceleration. Low-DO-reared fish might compensate for morphological differen- ces by using double bends to achieve similar performance as high-DO siblings. These results suggest that divergent morpho- types of P. m. victoriae are capable of achieving the same level of performance under their home DO condition and highlights the importance of developmental plasticity in facilitating adaptive response to alternative environments.