Excessive luminosity fades the skin color of cardinal tetra

Abstract

In order to understand the morphological and physiological changes on the loss of coloration in the tegument of cardinal tetra under excessive luminosity, specimens of Paracheirodon axelrodi were conditioned to different light intensities (0; 250; 500; 1,200 and 2,700 lux) at different time intervals (0, 12, 24 and 72 hours). Types of chromatophores, dispersion of melanosomes and density of chromatophores were analyzed after the experiment. The dark stripe on the species consists of yellowish-brown (dorsally located) and darkish-brown (medially located) melanophores. In the iridescent blue stripe, darkish-brown melanophores were closely associated with iridophores. Erythrophores were found only in the red stripe. Loss of skin color was observed when cardinal tetra was exposed to intense light. The melanic and neon stripes became pale due to a reduction in melanophores densities. On the other hand, the color of the red stripe was intensified due to the proliferation of erythrophores. At low light levels (0 to 250 lux), the melanophores (with dispersed melanosomes) proliferate in the black and neon stripes resulting in a more vibrant skin color. We suggest that in nature, the paleness of the skin may represent a camouflage strategy during the hours of the day with greater luminosity in the black water of the Rio Negro. Fading the skin color can help this species to visually confuse potential predators.