Pigment Cells

Abstract

The patterns of coloration of fishes, and the rapid changes of these color patterns in some species, are due to the presence and cellular activity of pigment-containing cells located in the integument. These highly specialized cells, collectively referred to as chromatophores, are capable of changing body coloration of the animal by virtue of two different mechanisms. In the first, chromatophores may actively redistribute pigment-containing organelles within the cell boundary in such a way that more or less of the pigment organelles are exposed to view. These changes can be rather prompt, being mediated, in many cases, through the activity of nerve endings that directly stimulate the chromatophore. If the activities of many cells are spatially and temporally coordinated, the macroscopic effect can be rather dramatic in that it produces changes in overall body coloration or pattern. Examples include the adaptive coloration of fishes in response to background or changing light conditions, or the display of color patterns reflecting and signaling mood or physiological state of the animal. Thus flounders placed in an aquarium with a checkerboard bottom will attempt to mimic this pattern in their dorsal body surface, and rivaling angelfish fighting for a mate will develop pitch-black stripes running down their flanks. These phenomena reflect an alteration in the physiological state of their pigment cells and therefore are referred to as “physiological color change”. A different mechanism of changes in body coloration, termed “morphological color change”, is based on a much slower, long-lasting process that involves either a decrease or an increase in the total number of pigment cells or the amount of pigment they contain. Such changes are gradual and not easily perceived, but the result may be quite dramatic once the transition from one state to the other, which takes several days or weeks, is completed. As a rule, both morphological and physiological color change occur simultaneously, with the former providing the baseline coloration upon which fine-tuned responses of the animal to internal or external stimuli are superimposed.