Fig. 3. The pattern of heart rate variability (HRV) within 250 consecutive heartbeat intervals (R–R interval) of a single H. unitaeniatus in (A) normoxia, (B) hypoxia, (C) hypoxia and blockade of adrenergic receptors, and (D) hypoxia and total (adrenergic and cholinergic receptor) blockade. The 250 consecutive R–R intervals are displayed as (i) a tachogram plot, (ii) a frequency histogram in 5 ms bins, and (iii) a power spectrum of the Fourier transform that shows the frequency and power of oscillatory components of the R–R interval tachogram.