Color Changes in Fundulus with Special Reference to the Color Changes of the Iridosomes

Abstract

The fish F. heteroclitus when placed over a blue background becomes steel-blue in color. The xanthophores by their maximum contraction reveal the blue of the guanin crystals in the iridosomes and the iridocytes of the stratum argenteum. The partial expansion of the melanophores contributes the proper shade to the skin. The iridosomes respond to various stimuli by changing color, especially the intense light either from the sun, the Silverman illuminator, or the lamp of the Ultropak microscope. This reaction is local and independent of the nervous system, showing reaction, recovery, and refractory phases. This color change of the iridosomes progresses rapidly from colors near the blue end of the spectrum through intervening hues to colors at the red end of the spectrum, and in the recovery phase in the opposite direction. This progressive reversible change may be explained if we assume that guanin crystals or their lamina grow slightly during the reaction and decrease in thickness during the recovery phase.