In the early development of fish, cells containing small melanoblasts (cells that develop into either melanocytes
or melanophores) develop into full-fledged melanin– containing cells, named melanophores. All movements of melanophores in baby fish have not been satisfactorily explained or fully studied. The exact mechanisms controlling the expansion and contraction of melanophores (melanin pigment cells) vary with the species, the stage of development, the environmental conditions and perhaps other stimulants.

Figure 1. Five different shapes or stages of individual melanophores.
Melanophores can change, owing to physiological and/or environmental conditions, from punctate to punctate- stellate to stellate to stellate- reticulate to reticulate (branched) and visa versa.

Biological studies in the past have shown that the causes of the expansion and contraction of melanophores are controlleded internally by chemical and hormonal factors and externally by environmental factors. The internal factors mainly affect older specimens with near– or well– developed nervous systems. The external environmental factors known to directly affect the behavior of melanophores on baby fish, include: 1) background color, 2) overhead illumination and 3) temperature.

With respect to background color, biological studies have shown that melanophores expand (change from punctate to reticulate) more over darker backgrounds than lighter backgrounds. One explanation is: "Sumner's Rule": ("when animals are experimentally subjected to a continuum of backgrounds, i.e., black, dark gray, medium gray, pale gray and white, under uniform illumination, the amount of melanin (and/or numbers of melanophores) produced varies inversely as the logarithm of the albedo (ratio of light reflected from the background to that received by the background) of the background"). Thus, babies caught in shallow water over dark muddy bottoms possess expanded melanophores while babies caught in open, deep water, away from the bottom, possess contracted melanophores.

With respect to overhead illumination, biological studies have shown that melanophores expand more under dim illumination (evening or night-time) than under bright illumination (day-time). Apparently, the exact intensity of illumination, i.e., bright or overcast sunlight, affects the change of shape of melanophores, but has very little or no effect upon the numbers of melanophores on the body. Thus, most baby fish caught at night possess expanded melanophores while most babies caught during the day possess contracted melanophores. These actions can be useful to baby fish by protecting delicate, internal, developing tissues and reactive compounds from photo– oxidative damage.

With respect to temperature, biological studies have shown that melanophores expand with cold temperatures and contract with warm temperatures. Thus, when babies swim into shallow, warm waters, the melanophores contract while when they swim offshore into deeper cold waters, they expand.

The main affect on the number and pattern of melanophores (maculae) is genetics. Biological studies have suggested that maculae may perform a biological role by protecting delicate, internal, developing tissues and reactive compounds from photo– oxidative damage, i.e., the Photodynamic Effect (The theory that natural sun light, ultraviolet light and/or artificial light cause photosensitized oxidations which can be lethal to organisms.) On baby lake whitefish (see: Fig. 2, below) the circular melanophore macula over the head presumably provides protection to the developing brain tissues from damaging strong, visible and near– ultraviolet light from the sun. The rectangular melanophore macula just caudal presumably provides protection to developing heart tissues from damaging strong, visible and near– ultraviolet light from the sun. The long melanophore macula along the back presumably provides protection to the neural tube and developing nerve chord from damaging strong, visible and near– ultraviolet light from the sun. (I’ve observed these baby lake whitefish in early spring swimming in clear, surface waters (top 10 cm) along steep shores; they were exposed to bright sunlight in plankton– free water.) Unfortunately, it is not certain which of the maculae shown in the gallery are in the epidermis or in the lower dermis because histological studies were not performed; we were overwhelmed by the number of species and the many subtle changes during growth. Probably, some maculae are in the epidermis and some are in the dermis; further studies need to be performed.

Figure 2. An example of a baby fish (Lake whitefish, Coregonus clupeaformis) with several dorsal melanophore maculae. (drawn by Sally Gadd)
Included is a round macula over the head, a rectangular one over the heart area between the pectoral fins, and a third over the back extending from the pectoral fins caudally to the caudal fin.

The main affect on the number and pattern of melanophore maculae on any species is genetic, i.e., the DNA contents, and, from experience, the many different kinds of melanophore maculae appear to be species– specific. Below are shown numerous melanophore maculae from different parts of different baby fish living in Canadian waters.

1) Dorsal patterns: On certain species, linear rows or lines of melanophores are predominant on dorsal, lateral and/or ventral parts of bodies.
See examples of Back Maculae on Canadian baby fish.

2) Occipital patterns: Melanophores are quite common on the heads of certain species.
See examples of Head Maculae on Canadian baby fish.

3) Lateral patterns: Melanophores along the sides of baby fish are quite common and quite unique.
See examples of Lateral Maculae on Canadian baby fish.

4) Linear patterns: On certain species, linear rows or lines of melanophores are predominant on dorsal, lateral and/or ventral parts of bodies.
See examples of Linear Maculae on Canadian baby fish.

FUNCTIONS OF COLORS ON BABY FISH

The colored pigments, white (guanophores), yellow (xanthophores), etc., found on baby fish are not visible in these illustrations on this Web site or in fluid preserved specimens because they have been dissolved during formalin/alcohol preservation procedures. Presently, larval fish biologists around the world are photographing live baby fish to document the total patterns of melanophores and carotenoid pigments on them (see examples: Australian Museum).

Biological studies have concluded that carotenoids may perform a biological role by protecting delicate tissues and reactive compounds from oxidative damage, i.e., the Photodynamic Effect. Biologists have shown that carotenoids prevent oxidative damage from the sun. In particular, internal carotenoids in baby fish probably protect tissues from photosensitized oxidations (damage from visible and near-ultraviolet light). These carotenoids allow baby fish to remain swimming near the water’s surface and not be damaged by strong visible and near– ultraviolet light from the sun. [For example, I have observed the babies of banded killifish, Fundulus diaphanus, living in very shallow water in Lake Heney, Ontario, next to the shore in 3 - 5 cm of water.] Living banded killifish babies possess scattered white guanophores, so it can be presumed that these internal guanophores will protect sensitive tissues from the damaging light rays of the sun. [On other occasions, I have observed a blanket of blue xanthophores over the entire dorsum of living babies of fourbeard rocklings, Enchelyopus cimbrius; these babies were caught by nets in the neuston (near– surface waters) of the Gulf of St. Lawrence.] Thus, these blue xanthophores on the backs of baby fourbeard rockling will presumably protect their internal sensitive tissues from the damaging light rays of the sun. (Refs. 25, 33a, 35, 43)