Meet the ultra-black fish, so light-absorbent that they look like silhouettes under the brightest of light, and are virtually invisible in near-darkness. A group of scientists theorized, found, and studied these fish, and here's what they discovered:
Ultra-black is the term given to the 16 known species of fish that have less than 0.5% reflectance. They reflect virtually no light, not even the bio-luminescent light that is the primary source of light found at the depths they inhabit. How far down? Well, these particular fish were collected by trawling 1000 feet below the surface at night, when they hunt high in the water, and a mile below during the day. They aren't very big, either. The majority of known ultra-black species of fish are medium in size, though the smallest one found was only 2 inches long.
The scientists were able to discover how these fish absorb so much light. They have densely-packed melanosomes (organs that contain melanin), which are optimized in size and shape for low reflectance. The fish are also lacking un-pigmented gaps between pigment cells, unlike other dark fish. However, the other dark fish have collagen in these gaps, which makes their skin less fragile than that of the ultra-black fish.
While most of the species studied had ultra-black coloring on their entire bodies, a few had only a bit of the light-absorbing coloration. In a couple of these cases, the ultra-black ran along the body in stripes, helping with camouflage. One of the species has only a single band of ultra-black, but the positioning of that band prevents it's bio-luminescent lure from reflecting off it's body.
Ultra-black coloring can also be found on certain species of spiders, butterflies, and birds, though the light-scattering from the land creatures is done via chitin or keratin. The ocean creatures use ultra-black to avoid being seen at all, whereas the land creatures use ultra-black as a contrast to highlight their brightest colors.