Regulation of heart size and shape is one of the least understood processes in developmental biology. We have for the first time analysed the hearts of Astyanax mexicanus and identified several differences in heart morphology between the surface (epigean morph) and cave-dwelling (troglomorph) morphs. Examination of the
adult revealed that the troglomorph possesses a smaller heart with a rounder ventricle in comparison to the epigean morph. The size differences identified appear to arise early in development, as early as 24 h postfertilisation (hpf), while shape differences begin to appear at 2 days post-fertilisation. The heart of the firstgeneration cross between the cave-dwelling and river-dwelling morph shows uncoupling of different phenotypes
observed in the parental populations and indicates that the cardiac differences have become embedded in the genome during evolution. The differences in heart morphology are accompanied by functional changes between the two morphs, with the cave-dwelling morph exhibiting a slower heart rate than the river-dwelling morph. The identification of morphological and functional differences in the A. mexicanus heart could allow us to gain more insight into how such parameters are regulated during cardiac development, with potential relevance to cardiac pathologies in humans.