Most luminescence is blue-green to match the deep sea’s weak sunlight. But the loosejaw group of deep-sea dragonfish (family Stomiidae) project a unique hue. Large and powerful photophores just beneath their eyes beam red light through the water. They accomplish this through a unique fluorescent protein in some species and a red-brown filter over the photophore in others.
Red is an unusual color in the deep sea. Seawater absorbs red light and more easily transmits blue, and so most of the bioluminescence in the sea is in a far-reaching blue-green hue. The predators and prey of loosejaws have eyes particularly sensitive to this blue and green light, having evolved beneath a mile of seawater.
The loosejaws are a rare exception, specially evolved to see the red light that they themselves produce.