The firefly squid (''Watasenia scintillans''), also commonly known as the sparkling enope squid or hotaru-ika in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, is a species of

squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

in the family

Enoploteuthidae Enoploteuthidae is a family of squid comprising approximately 40 species in four genera. Most species have a mantle length ranging from . Hooks are present on all arms and tentacles. The family is best known for the large array of photopho ...

. ''W. scintillans'' is the sole species in the monotypic

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

''Watasenia''. These tiny squid are found on the shores of Japan in springtime during

spawning Spawn is the Egg cell, eggs and Spermatozoa, sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of freely releasing eggs and sperm into a body of water (fresh or marine); the physical act is ...

season, but spend most of their lives in deeper waters between . They are

bioluminescent Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some fungi, microorganisms inc ...

organisms and emit blue

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

from

photophores A photophore is a specialized anatomical structure found in a variety of organisms that emits light through the process of Bioluminescence, boluminescence. This light may be produced endogenously by the organism itself (symbiotic) or generat ...

, which some scientists have hypothesized could be used for communication,

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

, or attracting food, but it is still unclear in the scientific community exactly how this species uses their

bioluminescence Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some Fungus, fungi, microorgani ...

. The firefly squid is a

predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

and actively hunts its food, which includes

copepods Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (living in the water column), some are benthic (living on the sediments), several species have ...

, small

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, 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 An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

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 Fishing is the activity of trying to catch fish. Fish are often caught as wildlife from the natural environment (Freshwater ecosystem, freshwater or Marine ecosystem, marine), but may also be caught from Fish stocking, stocked Body of water, ...

opportunity but also a tourist attraction.

Anatomy and morphology

The firefly squid belongs to the

Cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

a class and the superorder

Decapodiformes Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one ...

, commonly known as squid. Their body consists of a distinct head and mantle, and has a

bilaterally symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symme ...

layout. They are soft-bodied organisms with a skeletal structure composed of

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

. They have relatively large eyes, eight arms, and two

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s. They are further classified into the order

Oegopsida Oegopsida is one of the four orders of squid in the superorder Decapodiformes, in the class Cephalopoda. Together with the Myopsina, it was formerly considered to be a suborder of the order Teuthida, in which case it was known as Oegopsina. Thi ...

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

, based on the hooks on their tentacles. On average, an adult firefly squid is approximately in length. They are brown/red in color, but emit blue and green light by their

photophore A photophore is a specialized anatomical structure found in a variety of organisms that emits light through the process of boluminescence. This light may be produced endogenously by the organism itself (symbiotic) or generated through a mut ...

s. Firefly squid possess three types of photophores. 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 photophores that dot the body of the squid produce two different wavelengths of light (both blue and green bioluminescence) while those around the eye and on the arms only produce blue light. 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 photophores on the tips of its fourth arm pair produce a very intense light that can be seen by the naked eye.

Distribution

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, as they are one of the several species of squid that participates in

diel vertical migration Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The adjective "diel" ( IPA: , ) comes from , and refers to a 24-hour ...

. 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). The firefly squid is especially well known for its yearly migration to the coastal waters of

Toyama Bay is a bay located on the northern shores of the Hokuriku region of Honshu, Japan on the Sea of Japan. The bay borders Toyama and Ishikawa prefectures. The bay is known for the mirages on the horizon during the winter months and for being a spaw ...

for the purpose of reproduction.

Diet and predators

The diet of a firefly squid changes throughout 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. 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 Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength. Marine animals of ...

: 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. 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.

Bioluminescence and vision

Background

The firefly squid resides in the deep waters of the Western Pacific Ocean where limited amounts of visual light penetrate from the surface and are

. The shorter wavelengths of

visible light Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...

are blue, green, and yellow. These shorter wavelengths have more energy and can penetrate deeper into the water column. The squid's visual system is adapted to capture the greatest amount of light at these depths. Each eye has a large pupil to allow more ambient light to enter the eye, no cornea to reduce or distort absorbed light, a spherical lens to greatly limit distortion (coma and astigmatism), and a predominant visual pigment,

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...

(A1) with a maximal absorption at 482 nm.

Research

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 A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

shows that each pigment is contained in individual retinal photoreceptor cells which allows segregation of each pigment to specific locations on the squid retina. Light of specific wavelengths need to reach the specific photoreceptive cells in the retina to avoid longitudinal

spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. This phenomenon commonly affects lenses and curved mirrors, as these components are often shaped in a spherical ...

(LSA). Cone cells of the vertebrate retina are clustered in the same retinal location and use multifocal lenses to refract the wavelengths to activate the specific photoreceptor cells. Firefly squid do not have multifocal lenses, but use a banked retina –specific photoreceptive cells are located at different distances from the lens – to compensate for LSA.

Mating

Background

Cephalopods species have historically been polyandrous, in which a female mates with multiple mates, through common reproductive traits and life history. 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. 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.

Research

Research was conducted in 2020 around the Oki Islands in the Sea of Japan, a prevalent mating ground for ''W. scintillans'', during the estimated mating period (EMP) of mid-February to mid-March to test the firefly squid monogamy hypothesis. Researchers found that mated females stored an equivalent amount of sperm in both pouches surrounding their seminal receptacles. They also observed a gradual decrease in the quantity of sperm during the reproductive season. This data indicates the preservation of sperm through the lifespan of the female firefly squid. Researchers found that 95% of females tested stored sperm from a single male. Further data collection confirmed that a single male's sperm fertilized all of the female's eggs. Both of these findings support monogamous reproduction of ''W. scintillans''. To test monogamy in male firefly squids, researchers measured the maturity and fecundity of individuals. Data show that average male sperm levels would allow for no more than 2-3 copulations. The evidence for a low sperm production capacity and limited mating opportunities for males based on biased operational sex ratio and a lack of female remating supports the monogamy hypothesis in males. Female monandry was established first and subsequently males followed suit to create mutual monogamy in ''W. scintillans''.

Commercial use

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.

As food

Historically, firefly squid was never eaten raw in Japan, due to risk of the nematode parasite '' Crassicauda giliakiana''. However, with the use of modern

refrigeration Refrigeration is any of various types of cooling of a space, substance, or system to lower and/or maintain its temperature below the ambient one (while the removed heat is ejected to a place of higher temperature).IIR International Dictionary of ...

, raw firefly squid can now be served after passing food safety standards set by the

Ministry of Health, Labour and Welfare The is a cabinet level ministry of the Japanese government. It is commonly known as in Japan. The ministry provides services on health, labour and welfare. It was formed with the merger of the former Ministry of Health and Welfare or and th ...

. File:Boiled firefly squids (2014.03.19).jpg, Boiled firefly squid File:Boiled firefly squids, with vinegared miso.jpg, Boiled and served with vinegared

miso is a traditional Japanese seasoning. It is a thick paste produced by fermenting soybeans with salt and kōji (the fungus ''Aspergillus oryzae''), and sometimes rice, barley, seaweed, or other ingredients. It is used for sauces and spreads; p ...

File:Sashimi-and -ryugu-somen.jpg, As

sashimi is a Japanese cuisine, Japanese delicacy consisting of fresh raw fish or Raw meat, meat sliced into thin pieces and often eaten with soy sauce. Origin The word ''sashimi'' means 'pierced body', i.e., "wikt:刺身, 刺身" = ''sashimi'', whe ...

File:Tachiguizushi-Aburi-Hyakkan-Kaihinmakuhari_007.jpg, As

sushi is a traditional Japanese dish made with , typically seasoned with sugar and salt, and combined with a variety of , such as seafood, vegetables, or meat: raw seafood is the most common, although some may be cooked. While sushi comes in n ...

File:W. scintillans in Toyama 2019-12-09 (1) sa.jpg, Mixed with squid ink and ''

shiokara , is a food in Japanese cuisine made from various marine animals that consists of small pieces of meat in a brown viscous paste of the animal's heavily salted, fermented viscera. The raw viscera are mixed with about 10% salt, 30% malted rice ...

'' File:Watasenia scintillans-1.jpg, Served in

soy sauce Soy sauce (sometimes called soya sauce in British English) is a liquid condiment of China, Chinese origin, traditionally made from a fermentation (food), fermented paste of soybeans, roasted cereal, grain, brine, and ''Aspergillus oryzae'' or ''A ...

File:Firefly Squid okizuke.jpg, Marinated in ''okizuke'' (

vinegar Vinegar () is an aqueous solution of diluted acetic acid and trace compounds that may include flavorings. Vinegar typically contains from 5% to 18% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting ...

sake Sake, , or saki, also referred to as Japanese rice wine, is an alcoholic beverage of Japanese origin made by fermenting rice that has been polished to remove the bran. Despite the name ''Japanese rice wine'', sake, and indeed any East Asi ...

) File:Firefly squid with daikon oroshi.jpg, With grated

daikon Daikon 大根 (だいこん) or mooli, (مولی) ''Raphanus sativus'' var. ''longipinnatus,'' is a mild-flavored winter radish usually characterized by fast-growing leaves and a long, white, root. Originally native to continental East Asia ...

radish File:Sushiro---2022-04-29_013.jpg, As '' gunkanmaki''

References

External links

Tree of Life web project: ''Watasenia scintillans''

Sources

Patel, K. and D. Pee 2011. "Watasenia scintillans" (On-line), Animal Diversity Web. Accessed October 9, 2016 at http://animaldiversity.org/accounts/Watasenia_scintillans/ {{Taxonbar, from=Q303718 Bioluminescent molluscs Enoploteuthidae Molluscs described in 1911 Molluscs of Japan

Anatomy and morphology

Distribution

Diet and predators

Bioluminescence and vision

Background

Research

Mating

Background

Research

Commercial use

As food

See also

References

External links

Sources