Composite: Firefly Squid (Watasenia scintillans), top is bioluminescence in this species, bottom is the squid in normal light

Firefly Squid (Watasenia scintillans) migrate from deep waters toward the shallower waters of coastal Japan to breed each year. A culturally important fishery exists in Japan around this species and their annual migration into shallower Japanese waters. The Japanese word for firefly squid is hotaruika. Firefly squid are a bioluminescent species, having photophores on their ventral surfaces for counterillumination (see explanation below). This squid was photographed in the Sea of Japan (2007) – Courtesy of the Uozo Aquarium. Bioluminescence in cephalopods can come from one of two sources: (1) The animals produce the light. What I mean by this is that the cephalopod itself generates an organic substance, or a substrate (luciferin), and an associated catalyzing enzyme (either luciferase or photoprotein - sometimes other molecules are in the mix)…and they make their own light by way of oxidation. There is very little heat produced with the light which is why bioluminescence is sometimes called "cold light." When an animal produces its own light it is called "intrinsic bioluminescence." (2) Some cephalopods have evolved a relationship with bioluminescent bacteria and accommodate said bacteria in special structures on or in their body. These cephalopods produce light by way of their symbiote species. This is called "bacterial bioluminescence." Over three dozen species of squids from two families have been found to have a special symbiotic relationship with bioluminescent bacteria (such as Vibrio fischeri and Photobacterium leiognathi). The same symbiotic relationships have been found in over 10 families of fishes! The tissues that accommodate the bacteria are specialized to support the symbiote and other squid tissues are used to direct the light produced by the symbiote or orient the illumination in specific ways. For example, some squid house bioluminescent bacteria in their mantle and reflective tissues there direct the light downward, toward the ventral surface of the animal, to provide counterillumination. Counterillumination is a character found in some marine organisms (fish, cephalopods, shrimp) wherein light is directed downward to help eliminate the silhouette of the animal from predators looking upward. Think of it as hiding in plain sight. Classic examples are species like deep sea hatchetfishes and Firefly Squid (see example images here). Bioluminescence in molluscs has evolved independently at least seven times…demonstrating the benefit to light in dark or low light environments. In squid alone, there are over 70 genera containing bioluminescent species. How else might these lights be used? Cephalopods undoubtedly use the arrangement and colors of light producing organs (photophores) to recognize same species individuals and opposite sex individuals. The light producing bulbs that exist at the ends of squid tentacles (see example above) might be used in reproductive displays or other associated prereproductive behaviors. The Vampire Squid (Vampyroteuthis) can release a glowing substance into the water to potentially distract potential predators. FYI, Vampire Squids aren’t squids at all – they are in their own order and are quite distinct from all other cephalopods. One octopus (Octopoteuthis) autonomizes its glowing arm tips when threatened by a potential predator…to distract the predator while it attempts escape. One deep water “Octopus Squid” (Taningia) might use flashes of light to confuse or disorient potential prey items. Deep sea cirrate octopods have glowing suckers on their arms that might attract prey items. Is it possible not to love bioluminescence in cephalopods?