The coloration of a nocturnal fish is a fascinating example of how biology adapts to the physics of low-light environments. Unlike their brightly colored daytime counterparts, fish that are active at night, or in the deep sea, employ colors and patterns that prioritize camouflage in the darkness. Understanding what color these fish are and why requires looking at the specialized way light behaves underwater, especially at night. This unique combination of environmental physics and biological adaptation determines the survival strategies of fish in a world without sun.
General Coloration and Camouflage Strategies
Nocturnal fish in shallow environments, like coral reefs, often display muted or mottled coloration, switching from bright daytime patterns to duller shades of brown, gray, or blotchy reds at night. This background matching allows them to blend into the shadows and crevices of the reef structure while sleeping or hunting. The blotchy patterns effectively break up the fish’s outline, making its shape difficult to discern against the complex background in dim light.
In the open water of the twilight zone, or among species that remain active in the water column at night, a different strategy is common: silvering and countershading. Many mesopelagic fish have highly reflective, silvery sides, which act like a mirror to reflect the faint downwelling light from the moon or stars. This makes the fish virtually invisible when viewed from the side, as their body surface perfectly mimics the surrounding water.
Deep-water nocturnal species, or those that spend all their time in the dark, frequently exhibit deep red or black coloration. This red color functions as perfect camouflage in the deep ocean because the water filters out all red light. Since there is no red light to reflect, a red fish absorbs the remaining blue-green light, effectively appearing black to any observer and making it almost impossible to see.
The Physics of Light in the Nocturnal Environment
The underwater environment fundamentally alters the quality and intensity of light available to nocturnal fish. Light rapidly diminishes as it penetrates water, a process known as attenuation, which is far more pronounced than in air. The longest wavelengths of light, specifically red and orange, are absorbed first, often disappearing within the first few meters of the surface.
This selective absorption means that even the faintest residual light from the moon or stars is largely restricted to the shorter, higher-energy wavelengths, primarily blue and green. Consequently, the concept of “color” as humans perceive it, which relies on a full spectrum of light, barely exists in a nocturnal or deep-water environment. Any color a fish possesses, other than blue or green, will appear as shades of gray or black.
This environmental filter dictates that camouflage must focus on eliminating a silhouette or matching the dominant blue-green background. A fish’s coloration is therefore less about vibrant display and more about absorbing the minimal light to avoid detection.
Specialized Vision: How Nocturnal Fish See
To navigate and hunt in this dim, blue-shifted environment, nocturnal fish have evolved specialized visual systems that prioritize sensitivity over color perception. Their eyes are typically much larger relative to their body size compared to diurnal fish, which allows them to capture the maximum amount of light available. The pupils are also often large and rounded, further optimizing the intake of faint photons.
The retinas of these fish are heavily dominated by rod cells, the photoreceptors responsible for vision in low-light conditions. Rods are extremely sensitive and facilitate achromatic, or black-and-white, vision, which is far more useful at night than the high-resolution, color-detecting cone cells. This high concentration of rods allows them to detect minute changes in light intensity, which is crucial for spotting prey or predators.
Many species of nocturnal fish, such as certain catfishes, also possess a structure called the tapetum lucidum, a reflective layer situated behind the retina. This layer acts like a mirror, reflecting any unabsorbed light back through the retina, giving the photoreceptor cells a second chance to capture the photons. This adaptation dramatically enhances light sensitivity, resulting in the characteristic “eyeshine” seen when a light source hits the fish’s eyes in the dark.