Precocial parasympathetic-mediated cardiac regulation in embryonic Gulf killifish ( <i>Fundulus grandis</i> )

Embryonic fishes provide valuable models for understanding the ontogeny of vertebrate cardiovascular regulation, yet the timing of autonomic control onset varies widely among species. In most teleosts, it is generally accepted that parasympathetic (vagal) influence over heart rate arises only near hatching, but historical observations suggest that killifishes deviate from this paradigm. Here, we characterized the development of vagal control in Gulf killifish (Fundulus grandis), a precocial species with an extended embryonic period. Using both direct mechanical and indirect vibrational startle stimuli, combined with atropine treatment, we identified two distinct cardiac response phenotypes: (1) a transient atrial and ventricular arrest evident by 70% of embryonic development, and (2) a later-developing transient bradycardia beginning at 80%. Both responses were abolished by atropine, confirming parasympathetic mediation, and neither habituated to repeated stimulation. These findings demonstrate that F. grandis embryos acquire functional vagal tone well before hatching, contrasting with the delayed autonomic development typical of most vertebrates. Here, we revisit and expand upon early observations from nearly a century-old study, establishing the Gulf killifish as a tractable model for investigating the developmental physiology of early autonomic control in vertebrates.