Plenty of Fish in the Sea
Most bioluminescent organisms are found in the oceans. Among the fishes, all luminescent species are marine. They use bioluminescence for different purposes, and they produce it in different ways.
A few groups, especially the 160 species of deep-sea anglerfishes, use bacterial symbionts to produce light. Flashlight fishes such as Anomalops and Photoblepharon also use bacteria. Others, including the majority of deep-sea fishes, produce their own light. Their photophores or light-emitting organs are often highly adapted and can control light intensity and angle. Photophores are found in different places on different animals and are used for various purposes.
Because red, long-wave frequencies of the visual spectrum are filtered in the first few hundred meters of the open ocean, most deep-sea fishes see and produce bioluminescence in the blue-green spectrum. A few exceptions to this rule exists, including several species of dragonfishes that can emit and detect longer wavelength red light.
Why do they do it? Haddock, Moline and Case have a table that summarizes the purposes of bioluminescence in marine organisms. Their paper is also an excellent summary of bioluminescence in the oceans.
Photo © Kenaley, Christopher
A Few Examples
Lanternfishes (Myctophidae) are one of the largest group of deep-sea luminous fishes, with around 246 species. They are abundant in midwater and some move up near the surface at night to hunt. Their photophores produce flashes of light, to either illuminate prey or to get the prey to fluoresce in response. In turn, this may make them visible to larger predators, but they can also flash to startle predators, allowing them to escape. Diaphus (left) is a lanternfish
Diaphus metopoclampus, the spothead lanternfish. Photo © President and Fellows of Harvard College.
Dragonfishes and hatchetfishes (Stomiiformes) are characterized by elaborate arrangements of photophores. They may sport glowing barbels, photophores under their eyes, and others along their ventral (underneath) sides. A few stomiatoid fishes such as Malacosteus niger have photophores that produce two radically different frequencies of light: one behind the eye that produces blue light and one under the eye that can produce red light. This is unusual because long wave red light dissipates more quickly as it passes deeper into the ocean, where short wave blue-green light is the norm. Using specialized pigments capable of perceiving red light, fishes like Malacosteus niger can communicate with one another via a private wavelength and also visualize prey.
Above: Echiostoma barbatum, the threadfin dragonfish, a stomiid. Photo © President and Fellows of Harvard College. Second photo, by Chris Kenaley, is of a freshly caught specimen; note especially the glowing barbel.
Himantolophus albinares is another luminous deep-sea fish. Popularly called the football fish because of its soccer-ball-like appearance, it is an anglerfish in the group Lophiiformes. It produces light at the end of its modified dorsal ray using bacteria.
Photo © President and Fellows of Harvard College.