Why Angelfish Stripes can Fade

Much of the beauty of angelfish is credited to their spectacular finnage. Arguably though, part of the hobby's fascination with angels also lies in their impressive and varied array of patterns and colors. Iridescent blues and greens of halfblack blushing contrast with intense reddish-orange hues of koi. Some colors are due to pigmentation, while others are due to interactions of light with the structures and compounds of the skin and scales. Nevertheless, the combined effects are truly striking.

A pattern found on some angel strains consists of a series of dark, vertical stripes, characteristic of wild-type angelfish. Silvers and zebras are examples of strains displaying these bars. An interesting observation regarding this pattern is its ability to vary in intensity. Keepers of these strains will notice that there are times when the stripes are dark and distinct and times when they seem to fade away completely - an often almost instantaneous occurrence, sometimes taking place in a matter of a second or two.

Fading in and out of the pattern is possible due to the sub-cellular physiology of angelfish integumentary cells. The skin of fish contains two layers: a layer deep to the scales called the dermis and a layer superficial to the scales known as the epidermis. The dermis is the site of scale development and is where pigment-containing skin cells called chromatophores and light-reflective purine-containing cells called iridophores are found. Iridophores often contain guanine and produce iridescence on fish. Combinations of different types of chromatophores and iridophores contribute to the appearance of different colors. One of the most common types of pigment-containing cells is the melanophore. Melanophores contain blackish/brown pigments called melanins stored in organelles called melanosomes. The melanins are responsible for the dark color of the wild-type pattern. It is specifically the movement of the melanosomes throughout each pigment cell which is responsible for the stripes' ability to disappear.

Adjustments to the pattern intensity occur due to changes in the fish's stress level. A calm fish displays dark, distinct stripes. However, exposing the same fish to bright light, or causing stress in another way, triggers the stripes to fade to near-invisibility. This response is under the control of the sympathetic nervous system which is a division of the autonomic nervous system. In calm fish, the melanosomes are well distributed throughout the melanophores, causing the cells to appear dark - contributing to the visibility of the stripes. Activation of sympathetic nerves of stressed fish causes the melanosomes to move via microtubules to the center of the cell. Their aggregation reduces the visibility of the pigment and the intensity of the stripes decreases. The cells themselves do not change in either size or shape - it is the rearrangement of pigment-containing melanosomes within the cells which results in decreased perceptibility of the pattern.

Temporary changes in pattern intensity are a normal occurrence with most angelfish. However, prolonged weakness in pattern intensity may signal an unhappy or unhealthy fish. Fish that are uncomfortable in their environment or are fighting disease will appear faded. If left in this stressed state long enough, the faded pattern can become permanent.

Tanks with dark bottoms, non-intense light sources, plants, and decorations will allow the fish to feel hidden and secure - reducing their stress. Quality food, water maintenance, and proper use of medications will promote health, and in turn enhance pattern intensity. Therefore, the pattern of fish can be used as a tool to improve upon one's husbandry practices.