Experimental set-ups. Experiment I was executed with wild-type (wt) zebrafish with a control group sampled at the start of the experiment, a non-exercised group and an exercised group which was subjected to forced sustained swimming at the optimal swimming speed (Uopt) of 0.4 m s-1 for a period of 4 weeks in a swim tunnel. Experiment II had a similar set-up but was executed with wild-type zebrafish and Gr mutants. Now there were three exercise groups swimming at the sub optimal (<Uopt), optimal, and super optimal (>Uopt) swimming speeds of 0.3, 0.4, and 0.5 m s-1, respectively.

Acknowledgments
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