Marlin skin and how they ‘light up’
Marlin skin, when free from mucus, feels much like shark skin – like a fine abrasive paper. Normally, however, it is covered by a thin layer of heavy mucopolysaccharides (lubrication) that comprises the cuticle. Mucus is commonly found on the skin and is released from goblet cells in the epidermis in response to stress.
On dead specimens, it appears as a thick, jelly-like layer and is most obvious after the fish has been out of the water for some time. The colors presented by live marlin are one of their most spectacular features. Iridescent blues that flash when they “light up” have to be observed firsthand to appreciate their brilliance. Most scientists, however, describe them in their dull post-mortem blue or slate gray with occasional dorso-ventral stripes along the side.
The outer layer of skin, the epidermis, is divisible into two parts. There is an outer fusiform (spindle-shaped) layer containing mucus-producing cells and an inner basal layer with more regularly arranged columnar epidermal cells.
Also within this compact dermis layer are a variety of cell types, including meleanophores, and stacks of 10-20 cells which contain crystalline deposits, probably purines.
The melanophores assuredly account for the gray black appearance of dead marlin. The iridescent blue colors shown when “lighting up” are probably derived from stacks of crystalline materials in cells called iridophores.
These crystals create interference patterns between their layers and reflect the blue wavelengths seen during the lighting up response. The reflected color is unmasked by sudden concentration of the dark pigment granules (melanosones) from the process to the bodies of the melanophores.
The wavelength reflected by the iridophores changes to a deeper blue in response to stimulation by beta adrenergic agonists and upon maximal beta stimulation to opaque, allowing the melanosones to become visible once again, but this time through the iridophores.
Skin thus stimulated appears a gray color rather than black. The advantage to the marlin of such a system is as yet unclear.