'Variation ... a central attribute of living systems': patterns and implications of variability in the development of...

Diversity in the ontogeny of physiological systems is a central, too often overlooked tenet of biodiversity. Here, we use intensive screening of the development of heart function in embryos of the gastropod Radix balthica to investigate (i) variability in the physiological development of this system; (ii) how variability is associated with the hierarchical levels of population, egg mass and individual; and (iii) whether such variability links to hatch size. Despite considerable between-individual variability in heart function, a biphasic model was effective in modelling how heart activity changed through time. Heart rate variability analysis revealed marked reduced beat-to-beat variability in the period spanning the second slope of the biphasic pattern. The total number of heartbeats during development was positively correlated with hatch time, suggesting no detectable energetic trade-off between growth and heart function. Variability in the timing of first heart function was greatest at the egg mass level, whereas variation in the timing of the break in the biphasic pattern was greatest within egg masses at the individual level, with the least variability in both traits explained at the level of population, indicating that such physiological diversity is an intrinsic property apparent at every level of biological organization. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.