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 and 400 metres (700 and 1,300 feet; 100 and 200 fathoms). 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, and 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 arms, and two tentacles. They are further classified into the order Oegopsida for possessing the characteristic traits of having no tentacle pockets in the head and no suckers on the buccal supports. 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 (3 in) 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.
The firefly squid inhabits the waters off the coast of Japan. The depth at which these squids can be found varies (300–400 m or 1,000–1,300 ft during the day, and 20–60 m or 70–200 ft 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(3–6 °C or 37–43 °F during the day and 5–15 °C or 41–59 °F 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.
Diet and predators
The diet of a firefly squid changes through its life stages. During its paralarval stage, its diet is primarily composed of calanoid copepods (zooplankton). Subadult and adult stages see an increase in dietary diversity to include planktonic crustaceans, fishes, and squid.
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. As a participant in diel vertical migration, firefly squid primarily feed during the night. This feeding strategy is reflected in the squid’s gut anatomy, which has a longer cecum that allows it to absorb nutrients during the day when its metabolic rate is lower.
The firefly squid is found in the Western Pacific Ocean at depths of 180 to 360 m (600 to 1,200 ft) and is bioluminescent. The mantle, head, arms and tentacles are dotted with tiny, light-producing organs called photophores. However, light is also produced from many other small organs that are scattered around the body. The firefly squid creates light by a chemical reaction. This process involves wrangling together the dynamic duo of bioluminescence, which are two substances called luciferin and luciferase. The way it works is luciferin sits around waiting for luciferase, an enzyme that triggers luciferin to make light. https://www.nytimes.com/2016/04/27/science/firefly-squid-toyama-japan.html#:~:text=The%20firefly%20squid%20creates%20light,way%20a%20lightning%20bug%20does.&text=Together%2C%20they%20form%20a%20bright,only%20live%20for%20a%20year.  When flashed, the light attracts small fish, which the squid can feed upon.
This squid has three visual pigments located in different parts of the retina which likely allows color discrimination, each having distinct spectral sensitivities.
The squid spends the day at depths of several hundred metres, returning to the surface when night falls. It uses its abilities to sense and to 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.
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, although 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. The firefly squid can also light up its whole body to attract a mate. Once the squid's eggs have been fertilized and laid, it dies, having reached the end of its one-year lifespan. Spawning, which involves large aggregations of the squid, takes place between February and July.
This squid is commercially fished in Japan, accounting for an annual catch of 4,804 to 6,822 tons from 1990 to 1999.
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Patel, K. and D. Pee 2011. "Watasenia scintillans" (On-line), Animal Diversity Web. Accessed October 9, 2016 at http://animaldiversity.org/accounts/Watasenia_scintillans/