The Red Light District and Its Effects on Zebrafish Reproduction.

Light-dark cycles mimicking natural settings in a zebrafish facility are crucial for maintaining fish with an entrained circadian clock making them an ideal vertebrate model to study such rhythms. However, failure to provide optimal conditions to include complete darkness can lead to a disturbed circadian pacemaker affecting physiology and behavior in zebrafish. To meet building code requirements, the aquatics facility in use was outfitted with EXIT signs emitting a constant light. To determine if light radiating from the EXIT sign has an effect on zebrafish embryo production, 100 fish (1:1 m/f ratio) were split and housed at 10 fish/L. Half were housed directly in front of the EXIT sign, whereas the other half (control) were housed under a true 14-h light-10-h dark cycle. Reproductive success was evaluated by recording fecundity and viability from 10 weekly matings under two light colors: red (640 nm) and green (560 nm). On average the control group spawned twice as many embryos compared to those housed in front of a red EXIT sign, whereas green EXIT sign showed no difference. This suggests the importance of providing a complete dark environment within the night cycle and a recommendation toward dim green EXIT signs to avoid a decline in reproductive performance.