To see if the presence of UV signals mattered to
female swordtails or Mexican tetras, the investigators put the fish in aquariums with illumination that mimicked natural underwater light conditions and filters that either blocked or permitted UV light.
Unlike the majority of fish in which
females lay eggs that are later fertilized by males, live - bearers — which include species such as mollies, platies, guppies,
swordtails and mosquitofish — have internal fertilization.