Firefly squid

Species of cephalopod also known as the sparkling enope squid

The firefly squid (Watasenia scintillans), also commonly known as the sparkling enope squid or hotaru ika in Japan, is a species of squid in the family Enoploteuthidae. W. scintillans is the sole species in the monotypic genus Watasenia. They are bioluminescent organisms and emit blue light from photophores, which some scientists have hypothesized could be used for communication, camouflage, or attracting food, but it is still unclear in the scientific community exactly how this species uses their bioluminescence. There are multiple (800 1000) small photophores covering the ventral surface of its body, five larger photophores around the lower margins of each eye, and three very large photophores at the tip of each of the fourth pair of arms. The reactant luciferin and the necessary enzyme luciferase are located in a crystalline structure within rod like bodies in their photophores. Firefly squid are the only cephalopods to have this structural arrangement which increases the efficiency of its bioluminescence and allows the light to be directed downward in a cone like projection. This directed cone of bioluminescence is hypothesized to allow the Firefly squid to better detect its prey and predators from below and attract small fish to eat.

The firefly squid inhabits the waters off the coast of Japan. The depth at which these squids can be found varies ( during the day, and during the night) over the course of a day, For this reason, they also experience a significant change in environmental temperatures throughout the course of a day( during the day and during the night). Firefly squid face high predation rates and may serve as the primary food source for some predatory species including northern fur seals, particularly during their yearly migration. The squid spends the day at depths of several hundred meters, returning to the surface when night falls. It uses its abilities to sense and produce light for counter illumination camouflage: it matches the brightness and colour of its underside to the light coming from the surface, making it difficult for predators to detect it from below. As a participant in diel vertical migration, firefly squid primarily feed during the night.

Chemical and structural analysis of the firefly squid retina reveal the presence of three visually active pigments located in distinct regions of the squid's retina. This is unique among cephalopods and may allow these squid to have color discrimination vision. The presence of two or more visually active pigments have only been found in the eyes of other organisms capable of color discrimination. The three pigments found include retinal (A1) with maximal absorption at 482 nm, hydroxyretinal (A4) with maximal absorption at 470 nm, and dehydroretinal (A2) with maximal absorption at 500 nm. Scanning electron microscopy shows that each pigment is contained in individual retinal photoreceptor cells which allows segregation of each pigment to specific locations on the squid retina. Firefly squid show rare evidence of cephalopod monogamy in their reproductive cycle when they make a yearly migration to the coastal waters of Toyama Bay each spring during their mating season. For example, females store sperm for long periods in bilateral pouches under the neck collar, and are capable of egg spawning after the breeding season when males are no longer present. Males show specific sperm production and release patterning to augment their reproductive success. One proposed explanation for this unusual behavior is that although the males reach sexual maturity prior to the breeding season, females do not reach full maturity until later in the season. As a result of the shorter life span of males, most males are only able to copulate once and are largely gone by the time that females are able to use the sperm stored during copulation.

Fishers have long known that firefly squid congregate in Toyama Bay off the Japanese coast to spawn. They are often caught at night when they rise to the surface or in fishing nets that trawl mesopelagic depths during the day. Commercial consumption of the W. scintillans is largely driven by the flashing blue display of photophores that makes them considered a menu prized item at restaurants. This squid is commercially fished in Japan, accounting for an annual catch of 4,804 to 6,822 tons from 1990 to 1999. Storage of W. scintillans has been difficult due to their adaptation to a deep sea environment that is notably cold and dark. Researchers found that long term sedation (3+ days) of firefly squid can be accomplished using magnesium sulphate with relatively no harm being conferred to the organisms. W. scintillans quickly returned to its normal state only minutes after being transferred into fresh seawater at the final destination. The transported animals maintained their photophore flashing capabilities, a key focus for researchers.