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= Firefly Squid =

Lead
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. It is the sole species in the monotypic genus Watasenia. These tiny squid are found on the shores of Japan in springtime during spawning season, but spend most of their life in deeper waters between 200-400 meters. 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. The firefly squid is a predator and actively hunts its food, which includes copepods, small fish, and other squids. The lifespan of a firefly squid is about one year. At the end of their lives females return close to shore to release their eggs, then die shortly thereafter. This mass migration of firefly squid to the shore is a lucrative business for Japanese fishermen, and during spawning season many go out to the bays to collect the dying squid. Many more also visit Japan during spawning season to see the bright blue light created from the firefly squid's bioluminescence light up the bay, making their spawning season not only a fishing opportunity but also a tourist attraction.

Anatomy and Morphology
The firefly squid belongs to the Cephalopoda class and the superorder Decapodiformes, commonly known as squids. Their body is divided between a distinct head and a mantle, and the layout of the body is bilaterally symmetrical. They are soft-bodied organisms which contain a skeletal structure composed of chitin. They have relatively large eyes, eight tentacles, and two arms. They are further classified into the order Oegopsida for possessing the characteristic traits of a head with no tentacle pockets and buccal supports without suckers. They belong to the family of Enoploteuthidae, based on the hooks on their tentacles.

On average an adult firefly squid is approximately 7.5 cm or 3 inches in length. They are brown/red in color, but emit blue light by their photophores. These photophores can be found all over the squids body, but the brightest light is emitted from three largest photophores at the tips of the arms. There are five slightly smaller photophores surrounding each eye, and hundreds more of smaller size dotted along the rest of its body.

Distribution and Habitat
The firefly squid inhabits the waters off the coast of Japan. The depth at which these squids can be found varies (300-400m during the day, and 20-60m during the night) over the course of a day, as they are one of the several species of squid that participates in diel vertical migration. For this reason, they also experience a significant change in environmental temperature conditions throughout the course of a day(3-6ºC during the day and 5-15ºC during the night). The firefly squid is especially well known for its yearly migration to the coastal waters of Toyama Bay for the purpose of reproduction.

Bioluminescence
Given that firefly squid emits three distinct wavelengths of light, it is theorized that bioluminescence plays a significant role in reproduction. The high intensity of the light emitted from the bioluminescent organs found on the tentacles of the firefly squid has also led some researchers to believe that the squid may use its bioluminescence either to startle its prey while hunting or to startle predators as a defense mechanism.

Mating
Firefly squids make a yearly migration to the coastal waters of Toyama Bay each spring, during their mating season. The firefly squid is almost entirely monogamous in its mating behavior, this is extremely uncommon in cephalopods. 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.

Mechanism of Bioluminescence
The arm organs of the firefly squid contains the sulfated form of the compound coelenterazine, which is also known as luciferin, while the liver contains the unsulfated form of coelenterazine, which is referred to as “preluciferin”. Coelenterazine is used as a luciferase substrate in the luminescence reactions in the firefly squids and allows the squids to create their bioluminescence.

Interestingly, firefly squids are the second example in which ATP is essential for bioluminescence. Present evidence of the structure of firefly squid luciferin and oxyluciferin suggests that ATP is used to phosphorylate to give 3’-phosphoadenosine 5’-phosphosulfate, which then serves as a sulfuryl donor to form the putative sulfated coelenterazine by a sulfotransferase reaction. This stimulates strong light emissions from the homogenates of the arm organs.

Commercial Use
Once a year, billions of bioluminescent firefly squid swim up to the surface of Toyoma Bay, Japan to lay eggs. Commercial fishermen take advantage of this event and haul in as many firefly squids as they can while the squids are near the surface and furthest away from their homes. Annual yields of firefly squid in Toyoma Bay widely fluctuated from 473 to 3,895 tons with the average being 1,655 tons from 1982 to 2001. The fishing season for the firefly squid in Toyoma Bay starts in early March and ends middle July. The potential and optimal sea temperature for squid fishing seemed to be 9-15ºC and 11-13ºC respectively.

Consumption of raw firefly squids rapidly increased in Japan in the 1990’s. The increase in raw consumption of firefly squids has caused some health concerns due to the fact that “spiuroid type x” nematode larvae, a causative agent of creeping eruption and ileus in humans, can be found in firefly squids. Most cases in which humans are found to have ingested “type X” larvae are reported from March to July. This period is consistent with the fishing season for firefly squids.