Fig. 1. (A) Changes in contraction rhythm between a 2:1 and a 1:1 rhythm over a 20 min period in a single homozygous bre mutant zebrafish (7 d.p.f.) raised at 28°C. (B) A time window of 100 s at an expanded time scale to illustrate beat-to-beat activity. Any change in the rhythm of contraction was recorded and dots represent 1 s intervals.