firefly squid

File:Composite diagram illustrating basic squid features, ventral aspect.jpg

The firefly squid belongs to the Cephalopoda class and the superorder Decapodiformes, commonly known as squid. Their body consists of a distinct head and mantle, and has a bilaterally symmetrical layout. They are soft-bodied organisms with 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 {{convert|7.5|cm|in|frac=4|abbr=on}} in length. They are brown/red in color, but emit blue and green light by their photophores. Firefly squid possess three types of photophores.{{Cite journal |last1=Teranishi |first1=Katsunori |last2=Shimomura |first2=Osamu |date=2008-05-01 |title=Bioluminescence of the arm light organs of the luminous squid Watasenia scintillans |journal=Biochimica et Biophysica Acta (BBA) - General Subjects |language=en |volume=1780 |issue=5 |pages=784–792 |doi=10.1016/j.bbagen.2008.01.016 |issn=0304-4165 |pmid=18294462}} 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.{{Cite journal |last=Tsuji |first=Frederick I. |date=2002-08-19 |title=Bioluminescence reaction catalyzed by membrane-bound luciferase in the "firefly squid," Watasenia scintillans |journal=Biochimica et Biophysica Acta (BBA) - Biomembranes |language=en |volume=1564 |issue=1 |pages=189–197 |doi=10.1016/S0005-2736(02)00447-9|pmid=12101012 |doi-access=free }} 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.{{Cite journal |last=Inamura |first=O. |date=1990-12-01 |title=Observations on Minute Photophores of the Firefly Squid, Watasenia scintillans |url=https://www.museum.yokosuka.kanagawa.jp/wp/wp-content/uploads/2020/12/s38-12_Inamura_1990.pdf |journal=Scientific Reports Tokosuka City Museum |volume=38 |pages=101–105}} The reactant luciferin and the necessary enzyme luciferase are located in a crystalline structure within rod-like bodies in their photophores.{{Cite journal |last1=Hamanaka |first1=Toshiaki |last2=Michinomae |first2=Masanao |last3=Seidou |first3=Masatsugu |last4=Miura |first4=Keiko |last5=Inoue |first5=Katsuaki |last6=Kito |first6=Yuji |date=2011-09-02 |title=Luciferase activity of the intracellular microcrystal of the firefly squid, Watasenia scintillans |url=http://doi.wiley.com/10.1016/j.febslet.2011.07.033 |journal=FEBS Letters |language=en |volume=585 |issue=17 |pages=2735–2738 |doi=10.1016/j.febslet.2011.07.033|pmid=21821032 |s2cid=37903489 |url-access=subscription }}{{Cite journal |last=Tsuji |first=F. I. |date=1985-07-01 |title=ATP-dependent bioluminescence in the firefly squid, Watasenia scintillans |journal=Proceedings of the National Academy of Sciences |language=en |volume=82 |issue=14 |pages=4629–4632 |bibcode=1985PNAS...82.4629T |doi=10.1073/pnas.82.14.4629 |issn=0027-8424 |pmc=390439 |pmid=16593580 |doi-access=free}} 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.{{Cite journal |last1=Kawahara |first1=M. |last2=Gleadall |first2=I. G. |last3=Tsukahara |first3=Y. |date=2010-04-10 |title=A note on the fibre-optic light-guides in the eye photophores of Watasenia scintillans |url=http://www.tandfonline.com/doi/abs/10.2989/025776198784126250 |journal=South African Journal of Marine Science |language=en |volume=20 |issue=1 |pages=123–127 |doi=10.2989/025776198784126250 |issn=0257-7615}} 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.{{Cite journal |last=Gleadall |first=Ian G. |date=1994 |title=A Model for Enhancing the Visual Information Available Under Low-Level Light Conditions: Multiple Contrast Channels Created by Stepwise Changes in Detector Parameters in the Banked Ventral Retina of the Firefly Squid. |url=http://www.jstage.jst.go.jp/article/iis/1/1/1_1_67/_article |journal=Interdisciplinary Information Sciences |language=en |volume=1 |issue=1 |pages=67–75 |doi=10.4036/iis.1994.67 |issn=1347-6157|doi-access=free |url-access=subscription }}

The firefly squid inhabits the waters off the coast of Japan.{{cite bioRxiv|last1=Sato |first1=Noriyosi |last2=Tsuda |first2=Sei-Ichiro |last3=Alam |first3=Nur |last4=Sasanami |first4=Tomohiro |last5=Iwata |first5=Yoko |last6=Kusama |first6=Satoshi |last7=Inamura |first7=Osamu |last8=Yoshida |first8=Masa-aki |last9=Hirohashi |first9=Noritaka |title=Polyandry is extremely rare in the firefly squid, Watasenia scintillans |date=2019 |biorxiv=10.1101/2019.12.13.875062}}{{cite journal|last1=Watanabe|first1=Hikaru|last2=Kubodera|first2=Tsunemi|last3=Moku|first3=Masatoshi|last4=Kawaguchi|first4=Kouichi|title=Diel vertical migration of squid in the warm core ring and cold water masses in the transition region of the western North Pacific|journal=Marine Ecology Progress Series|date=June 13, 2006|volume=315|pages=187–197|jstor=24870152|doi=10.3354/meps315187|bibcode=2006MEPS..315..187W|doi-access=free}} The depth at which these squids can be found varies ({{convert|300-400|m|ft|-2|abbr=on|disp=or}} during the day, and {{convert|20-60|m|ft|-1|abbr=on|disp=or}} 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.{{cite journal |last1=Omura |first1=Ayano |last2=Endo |first2=Hideki |title=The functional-morphological adaptive strategy of digestive organs of decapodiform cephalopods |journal=Journal of Veterinary Medical Science |date=2016 |volume=78 |issue=1 |pages=43–7 |doi=10.1292/jvms.15-0185|pmid=26369293 |pmc=4751115 }} For this reason, they also experience a significant change in environmental temperatures throughout the course of a day({{convert|3-6|C|F|disp=or}} during the day and {{convert|5-15|C|F|disp=or}} 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.

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.{{Cite journal|last1=Hayashi|first1=S.|last2=Hirakawa|first2=K.|date=1997|title=Diet composition of the firefly squid, Watasenia scintillans, from Toyama bay, southern Japan sea|url=http://agris.fao.org/agris-search/search.do?recordID=JP1998002745|journal=Bulletin of the Japan Sea National Fisheries Research Institute (Japan)|language=ja|issn=0021-4620}}

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.{{cite journal|last1=Mori|first1=Junta|last2=Kubodera|first2=Tsunemi|last3=Baba|first3=Norihisa|title=Squid in the diet of northern fur seals, Callorhinus ursinus, caught in the western and central North Pacific Ocean|journal=Fisheries Research|date=June 2001|volume=52|issue=1–2|pages=91–97|doi=10.1016/S0165-7836(01)00233-8}} 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.{{Cite journal |last1=Young |first1=R.E. |last2=Roper |first2=C.F. |year=1976 |title=Bioluminescent countershading in midwater animals: evidence from living squid |journal=Science |volume=191 |issue=4231 |pages=1046–8 |bibcode=1976Sci...191.1046Y |doi=10.1126/science.1251214 |pmid=1251214}} 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.

{{further|Bioluminescence|Counter-illumination}}File:Squid Counterillumination.png camouflage. When seen from below by a predator, the bioluminescence helps to match the squid's brightness and colour to the sea surface above.]]

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 bioluminescent. The shorter wavelengths of visible light 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 (A1) with a maximal absorption at 482 nm.{{Cite journal |last1=Kröger |first1=Ronald H.H. |last2=Gislén |first2=Anna |date=2004-08-01 |title=Compensation for Longitudinal Chromatic Aberration in the Eye of the Firefly Squid, Watasenia Scintillans |journal=Vision Research |language=en |volume=44 |issue=18 |pages=2129–2134 |doi=10.1016/j.visres.2004.04.004|pmid=15183679 |s2cid=12550230 |doi-access=free }}

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.{{Cite journal |last1=Michinomae |first1=M |last2=Masuda |first2=H |last3=Seidou |first3=M |last4=Kito |first4=Y |date=1994-08-01 |title=Structural Basis For Wavelength Discrimination in the Banked Retina of the Firefly Squid Watasenis Scintillans |url=https://journals.biologists.com/jeb/article/193/1/1/6820/STRUCTURAL-BASIS-FOR-WAVELENGTH-DISCRIMINATION-IN |journal=Journal of Experimental Biology |language=en |volume=193 |issue=1 |pages=1–12 |doi=10.1242/jeb.193.1.1 |pmid=9317205 |issn=1477-9145|doi-access=free }}{{cite web |title=Map of Life - 'Colour vision' in Firefly squid |url=http://www.mapoflife.org/topics/topic_76_Colour-vision-in-Firefly-squid/ |access-date=6 November 2012 |work=Convergent Evolution Online |publisher=University of Cambridge}} The presence of two or more visually active pigments have only been found in the eyes of other organisms capable of color discrimination.{{Cite book |last=Jacobs |first=G.H. |url=https://www.worldcat.org/oclc/1263028391 |title=Encyclopedia of biological chemistry |date=2021-01-01 |publisher=Elsevier |others=Joseph Jez |isbn=978-0-12-822040-5 |edition=3 |location=San Diego |oclc=1263028391}} 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. Light of specific wavelengths need to reach the specific photoreceptive cells in the retina to avoid longitudinal spherical aberration (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.

Cephalopods species have historically been polyandrous, in which a female mates with multiple mates, through common reproductive traits and life history.{{Cite journal |last1=Parker |first1=Geoff A. |last2=Birkhead |first2=Tim R. |date=2013-03-05 |title=Polyandry: the history of a revolution |url=http://dx.doi.org/10.1098/rstb.2012.0335 |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=368 |issue=1613 |pages=20120335 |doi=10.1098/rstb.2012.0335 |pmid=23339245 |pmc=3576588 |issn=0962-8436}} 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.{{Cite journal |last1=Sato |first1=Noriyosi |last2=Tsuda |first2=Sei-Ichiro |last3=Nur E. Alam |first3=Md. |last4=Sasanami |first4=Tomohiro |last5=Iwata |first5=Yoko |last6=Kusama |first6=Satoshi |last7=Inamura |first7=Osamu |last8=Yoshida |first8=Masa-aki |last9=Hirohashi |first9=Noritaka |date=2020-03-07 |title=Rare polyandry and common monogamy in the firefly squid, Watasenia scintillans |journal=Scientific Reports |language=en |volume=10 |issue=1 |pages=10962 |doi=10.1038/s41598-020-68006-1 |issn=2045-2322 |pmc=7334199 |pmid=32620906|bibcode=2020NatSR..1010962S }} 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 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.{{Cite journal |last1=Fromhage |first1=Lutz |last2=Elgar |first2=Mark A. |last3=Schneider |first3=Jutta M. |date=2005 |title=Faithful Without Care: The Evolution of Monogyny |url=http://dx.doi.org/10.1554/04-680 |journal=Evolution |volume=59 |issue=7 |pages=1400–1405 |doi=10.1554/04-680 |pmid=16153026 |s2cid=198155857 |issn=0014-3820|url-access=subscription }}

File:Hotaruika 002 for sale in Tokyo.jpg. They are caught in bulk during spawning on the shores of Japan and are offered in many restaurants and grocery stores]]

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.{{Cite journal |last=Daniel Hannah |date=2007 |title="Under a cloud": Silence, Identity, and Interpretation in Lord Jim |url=http://dx.doi.org/10.1353/cnd.0.0004 |journal=Conradiana |volume=40 |issue=1 |pages=39–59 |doi=10.1353/cnd.0.0004 |s2cid=170616297 |issn=1935-0252|url-access=subscription }}

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.{{Cite journal |last=Gleadall |first=Ian G. |date=2013-09-01 |title=Low dosage of magnesium sulphate as a long-term sedative during transport of firefly squid, Watasenia scintillans |url=https://linkinghub.elsevier.com/retrieve/pii/S0022098113000737 |journal=Journal of Experimental Marine Biology and Ecology |language=en |volume=447 |pages=138–139 |doi=10.1016/j.jembe.2013.02.021|url-access=subscription }} 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.

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, raw firefly squid can now be served after passing food safety standards set by the Ministry of Health, Labour and Welfare.

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

File:Sashimi-and -ryugu-somen.jpg|As sashimi

File:Tachiguizushi-Aburi-Hyakkan-Kaihinmakuhari_007.jpg|As sushi

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

File:Watasenia scintillans-1.jpg|Served in soy sauce

File:Firefly Squid okizuke.jpg|Marinated in okizuke (soy sauce, vinegar, sake)

File:Firefly squid with daikon oroshi.jpg|With grated daikon radish

File:Sushiro---2022-04-29_013.jpg|As gunkanmaki

• List of Special Places of Scenic Beauty, Special Historic Sites and Special Natural Monuments

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{{Commons category|Watasenia scintillans}}

• [http://tolweb.org/Watasenia_scintillans/19645 Tree of Life web project: Watasenia scintillans]

Patel, K. and D. Pee 2011. "Watasenia scintillans" (On-line), Animal Diversity Web. Accessed October 9, 2016 at http://animaldiversity.org/accounts/Watasenia_scintillans/{{Authority control}}

{{Taxonbar|from=Q303718}}

Category:Bioluminescent molluscs

Category:Enoploteuthidae

Category:Molluscs described in 1911

Category:Molluscs of Japan