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CHROMATOPHORES ON BABY FISH

Chromatophores are certain cells in the dermis and in internal cells associated with anatomical parts of baby fish; they also occur in other animals and plants that possess pigments (see: Chromatophore). Chromatophores include: melanophores possessing blackish or brownish colors; guanophores/iridiophores possessing white colors; and lipophores/xanthophores possessing yellow, red, green, blue and other colors. Melanin is a term for a group of chemically ill– defined pigments confined to melanophores and produced by vertebrates, invertebrates and plants. Melanin can be extracted from biological tissues with boiling alkali. The melanin in melanophores are the dark pigments described and illustrated in this baby fish Web site (see: Melanophores). Melanophores are easily visible, mostly resistant to formalin preservation, and grouped in distinctive patterns (melanophore maculae) on different species of baby fish. Melanophores, which show up as blackish or brownish micro- melanophores and/or macro- melanophores on baby fish, occur in an extremely wide variety of shapes and sizes (See: Fig. 1, below).

The virtual illustrations in the “Illustration Gallery” were specifically drawn to describe and illustrate these melanin– based maculae (groups of melanophores). Unfortunately, melanophore maculae are not permanent on preserved specimens found in museum collections because melanin gradually fades with time. The melanophore maculae on these virtual illustrations, as shown in this Web site, are permanent and quite accurate. So, for future biological studies of melanophores and/or melanophore maculae, these illustrations are valuable and better than most curled, wrinkled and long– time preserved specimens in alcohol– based collections.

FUNCTIONS OF MELANOPHORES

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.

FUNCTIONS OF MELANOPHORE MACULAE

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 diffuse the strong, visible and near– ultraviolet light rays from the sun before striking the developing brain tissues. The rectangular melanophore macula just caudal presumably diffuse the strong, visible and near– ultraviolet light from the sun before striking the developing heart tissues below. The long melanophore macula along the back presumably diffuse the strong, visible and near– ultraviolet light from the sun before striking the neural tube and developing spinal chord below. (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 almost 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) (Note the round macula over the head, the 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). See below similar results for some Amphibians and some leeches.

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CHROMATOPHORES AND PIGMENTATION ON AMPHIBIANS

Studies on amphibians can be compared with studies on baby fish. More work has been done on tadpoles than on baby fish because of the ease of working with them in labs. The following information was summarized from Duellman and Trueb, 1986 (Ref. 05a). Many of the results are similar to studies with fishes.

Chromatophores and Pigmentation
The chromatophores and pigments in amphibians have been studied extensively, particularly by J.T. Bagnara and his associates...”

Structure and Function
“Amphibian chromatophores are located in either the epidermis or dermis. Melanophores are the predominant type of epidermal chromtophores, although erythrophores have been observed in the epidermis of some amphibians..." "Epidermal melanophores are thin, elongate cells with long dendritic processes that extend between surrounding cells.” "Epidermal melanophores are characteristic of larval amphibians; (see: Fig. 3, below, right) upon metamorphosis these may be lost or
their numbers reduced as the dermis thickens and dermal chromatophores develop.”

Figure 3. Photomicrograph of amphibian chromatophores. Epidermal melanophore pattern in dorsal skin of a tadpole of Bombina orientalis. The melanophores form a highly organized, reticulate network within the epidermis. Black bar=25 mm. [Taken from Duellman and Trueb (1986) who obtained it from S.K. Frost]

“Developmentally, pigmentary changes may occur at various stages in the life cycle.” “In the dermis of most amphibians that have been studied there are three types of chromatophores that are arranged in what has been termed a dermal chromatophore unit”. (see: Fig. 4, below, left) “In this unit, xanthophores (or erythrophores) are the most superficial cells; they lie just below the basement membrane separating the epidermis and dermis.” "Iridophores underlie xanthophores; these cells (also called guanophores) are white or silvery in appearance and have the capacity to reflect light of specific wave- lenghts through the overlying xanthophores.” “Melanophores are the basal- most chromaophores; dendritic processes extend upward to terminate on the upper surfaces of iridophores, between these cells and the overlying xanthophores.”

            Color Change.  “Two kinds of color change can be affected by the chromatophore units of amphibians. Rapid color changes involving intracellular mobilization of pigment-containing organelles are referred to as physiological color changes; these changes may require only seconds to accomplish and commonly are of short duration (minutes to hours). Color changes that are evoked slowly and that involve the accumulation or reduction of the amount of pigment are referred to as morphological color changes. This is a slow process because it involves the synthesis or destruction of relatively large amounts of pigments as a result of either the persistence or continuous lack of stimulation of the chromatophores. Such changes are of long- term duration (days to months)."

Figure 4. Diagramatic representation of a dermal chromatophore unit in a dark phase. [Taken from Duellman and Trueb (1986) who obtained it from J.T. Bagnara].

"Epidermal melanophores
undergo
morphological changes. . . Morphological color changes usually are preceded by physiological color change, but morphological color change is not a necessary consequence of physiological color change. Morphological color change involving an increase in the amount of pigment contained in a chromatophore seems to be related to the dispersion of the pigment- containing organelles in the cell, just as a decrease in pigment content is accompanied by an aggregation of the organelles in the middle of the cell.” 

"Amphibian larvae become pale when subjected to prolonged darkness; the melanosomes aggregate in the middle of the melanophores.” “The hormone melatonin, secreted by the pineal gland, is a melanosome- aggregating agent. The release of melatonin is regulated by light receptors in the pineal body.”

“Observations on numerous kinds of amphibians indicate that color change can be affected by changes in illumination and also by temperature. Moreover, in at least some species color change is affected by background color; the animals become darker on dark backgrounds. However, this is not a generality.”  In conclusion: “The dilemma expressed by Bagnara and Hadley remains unresolved: “… chromatophores may be affected by either hormonal or neurohormonal agents as well as by direct environmental influences...  Certain hormones may be inhibited from being released under conditions of illumination, whereas others may be released under conditions of darkness, or visa versa. In either situation the chromatic responses may appear similar, although their regulatory basis may be quite different.” (Ref. 05a)

COLOR CHANGES AMONG SOME PISCICOLIDAE AND GLOSSIPHONIIDAE LEECHES

Studies on some invertebrates, leeches in this case, have shown similar results to those above for vertebrates. See: Elliott and Mann (1980) and their extensive list of related papers. A Key to the British Freshwater Leeches. Fresh. Biol. Assn. Sci. Publ. 40. They state:

"Although the colour changes shown by some species of Piscicolidae and Glossiphoniidae may appear to be related to the colour of the substratum or host, they are actually due to variations in light intensity rather than adaptations to different background colours."

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