Firefly squid

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Firefly squid
Watasenia scintillans.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Order: Oegopsida
Family: Enoploteuthidae
Genus: Watasenia
Ishikawa, 1914
Species:
W. scintillans
Binomial name
Watasenia scintillans
(Berry, 1911) [2]
Synonyms
  • Abralia (Compsoteuthis) nishikawae Pfeffer, 1912
  • Abralia japonicaIshikawa, 1929
  • Abraliopsis scintillans Berry, 1911

The firefly squid (Watasenia scintillans), also commonly known as the sparkling enope squid or hotaru-ika in Japan, [3] is a species of squid in the family Enoploteuthidae. [4] W. scintillans is the sole species in the monotypic genus Watasenia. [4]

Contents

These tiny squid are found on the shores of Japan in springtime during spawning season, but spend most of their lives in deeper waters between 200 and 400 metres (700 and 1,300 feet; 100 and 200 fathoms). [5] 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. [3]

The firefly squid is a predator and actively hunts its food, which includes copepods, small fish, and other squids. [3] 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. [5]

Anatomy and morphology

Diagram illustrating the basic features of a generic squid. The mantle, eyes, arms, tentacles, buccal membrane, and typical suckers are all shown in this diagram. Composite diagram illustrating basic squid features, ventral aspect.jpg
Diagram illustrating the basic features of a generic squid. The mantle, eyes, arms, tentacles, buccal membrane, and typical suckers are all shown in this diagram.

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 7.5 cm (3 in) in length. [3] They are brown/red in color, but emit blue and green light by their photophores. Firefly squid possess three types of photophores. [6] 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. [7] [6] 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. [8] The reactant luciferin and the necessary enzyme luciferase are located in a crystalline structure within rod-like bodies in their photophores. [9] [10] 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. [11] 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. [11] The photophores on the tips of its fourth arm pair produce a very intense light that can be seen by the naked eye. [12]

Distribution

The firefly squid inhabits the waters off the coast of Japan. [13] [14] 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, [14] as they are one of the several species of squid that participates in diel vertical migration. [14] [15] For this reason, they also experience a significant change in environmental temperatures 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). [14] The firefly squid is especially well known for its yearly migration to the coastal waters of Toyama Bay for the purpose of reproduction. [13] [14] [15]

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. [16]

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. [13] [17] 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. [18] As a participant in diel vertical migration, firefly squid primarily feed during the night. [14] [15] 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. [14] [15]

Bioluminescence and vision

Principle of the squid's counter-illumination camouflage. When seen from below by a predator, the bioluminescence helps to match the squid's brightness and colour to the sea surface above. Squid Counterillumination.png
Principle of the squid's counter-illumination camouflage. When seen from below by a predator, the bioluminescence helps to match the squid's brightness and colour to the sea surface above.

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 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. [19]

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. [20] [21] The presence of two or more visually active pigments have only been found in the eyes of other organisms capable of color discrimination. [22] 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. [20] 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. [19]

Mating

Background

Cephalopods species have historically been polyandrous, in which a female mates with multiple mates, through common reproductive traits and life history. [23] 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. [24] 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. [13] 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. [1]

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. [24] They also observed a gradual decrease in the quantity of sperm during the reproductive season. [24] 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. [24] Further data collection confirmed that a single male's sperm fertilized all of the female's eggs. [24] 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. [24] 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. [25]

Commercial use

Boiled firefly squid, as served at a restaurant. Firefly squid are caught in bulk during spawning on the shores of Japan and are offered in many restaurants and grocery stores. Boiled firefly squids (2014.03.19).jpg
Boiled firefly squid, as served at a restaurant. Firefly squid 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. [26]

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. [27] W. scintillans quickly returned to its normal state only minutes after being transferred into fresh seawater at the final destination. [27] The transported animals maintained their photophore-flashing capabilities, a key focus for researchers. [27]

See also

Related Research Articles

<span class="mw-page-title-main">Squid</span> Superorder of cephalopod molluscs

A squid is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting these criteria. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.

<span class="mw-page-title-main">Cephalopod</span> Class of mollusks

A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.

<span class="mw-page-title-main">Bioluminescence</span> Emission of light by a living organism

Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bioluminescent bacteria, and terrestrial arthropods such as fireflies. In some animals, the light is bacteriogenic, produced by symbiotic bacteria such as those from the genus Vibrio; in others, it is autogenic, produced by the animals themselves.

<span class="mw-page-title-main">Vampire squid</span> Species of cephalopod

The vampire squid is a small cephalopod found throughout temperate and tropical oceans in extreme deep sea conditions. The vampire squid uses its bioluminescent organs and its unique oxygen metabolism to thrive in the parts of the ocean with the lowest concentrations of oxygen. It has two long retractile filaments, located between the first two pairs of arms on its dorsal side, which distinguish it from both octopuses and squids, and places it in its own order, Vampyromorphida, although its closest relatives are octopods. As a phylogenetic relict, it is the only known surviving member of its order.

<span class="mw-page-title-main">Photophore</span> Glandular organ that appears as luminous spots on various marine animals

A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, however unlike an eye it is optimized to produce light, not absorb it. The bioluminescence can variously be produced from compounds during the digestion of prey, from specialized mitochondrial cells in the organism called photocytes, or, similarly, associated with symbiotic bacteria in the organism that are cultured.

<span class="mw-page-title-main">Viperfish</span> Genus of fishes

A viperfish is any species of marine fish in the genus Chauliodus. Viperfishes are mostly found in the mesopelagic zone and are characterized by long, needle-like teeth and hinged lower jaws. A typical viperfish grows to lengths of 30 cm (12 in). Viperfishes undergo diel vertical migration and are found all around the world in tropical and temperate oceans. Viperfishes are capable of bioluminescence and possess photophores along the ventral side of their body, likely used to camouflage them by blending in with the less than 1% of light that reaches to below 200 meters depth.

<span class="mw-page-title-main">Stomiidae</span> Family of fishes

Stomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes. They are quite small, usually around 15 cm, up to 26 cm. These fish are apex predators and have enormous jaws filled with fang-like teeth. They are also able to hinge the neurocranium and upper-jaw system, which leads to the opening of the jaw to more than 100 degrees. This ability allows them to consume extremely large prey, often 50% greater than their standard length.

<i>Malacosteus niger</i> Species of fish

Malacosteus niger, commonly known as the black dragon fish, is a species of deep-sea fish. Some additional common names for this species include: northern stoplight loosejaw, lightless loosejaw, black loosejaw, and black hinged-head. It belongs to the family Stomiidae, or dragonfishes. It is among the top predators of the open mesopelagic zone. M. niger is a circumglobal species, which means that it inhabits waters ranging from the tropics to the subarctics. Not many studies have been conducted on its feeding habits, but recent research suggests that M. niger primarily feed on calanoid copepods which is a form of zooplankton. Indeed, it appears that M. niger primarily prey on zooplankton despite its apparent morphological adaptations for the consumption of relatively large prey. Another unique adaptation for this species is its ability to produce both red and blue bioluminescence. Most mesopelagic species aren't capable of producing red bioluminescence. This is advantageous because most other species cannot perceive red light, therefore allowing M. niger to camouflage part of itself to its prey and predators.

<span class="mw-page-title-main">Blackbelly lanternshark</span> Species of shark

The blackbelly lanternshark or lucifer shark is a shark of the family Etmopteridae found around the world in tropical and temperate seas at depths between 150 and 1,250 meters. Compared to other mesopelagic fish predators and invertebrates, the blackbelly lanternshark is thought to reside in shallower, more southern waters. E. lucifer can reach up to 47 centimeters in length and consumes mesopelagic cephalopods, fish, and crustaceans. Blackbelly lanternsharks are bioluminescent, using hormone controlled mechanisms to emit light through ventral photogenic organs called photophores and are presumed to be ovoviviparous. The blackbelly lanternshark has been classified as "Not Threatened" within the New Zealand Threat Classification System.

<i>Taningia danae</i> Species of cephalopods

Taningia danae, the Dana octopus squid, is a species of squid in the family Octopoteuthidae. It is one of the largest known squid species, reaching a mantle length of 1.7 m (5.6 ft) and total length of 2.3 m (7.5 ft). The largest known specimen, a mature female, weighed 161.4 kg (356 lb).

<span class="mw-page-title-main">Fringefin lanternshark</span> Species of shark

The fringefin lanternshark is a shark of the family Etmopteridae found in the western central Atlantic from Texas to Florida, northern Gulf of Mexico, and Mexico. It is endemic to this area. It is a deep water shark and is found about 220 to 915 meters below the surface, on the upper continental slopes of the Gulf. E. schultzi is a small shark, about 27–30 cm long and feeds on squid. It is also bioluminescent, which counter-illuminates it and helps with intraspecific interaction. Due to its limited range and the difficulty of collecting deep water species, it has not been evaluated by the IUCN Red List, but due to recent oil spills in the Gulf of Mexico, it is likely that fringefin lanternsharks have decreased in population.

<span class="mw-page-title-main">Splendid lanternshark</span> Species of shark

The splendid lanternshark is a shark of the family Etmopteridae found in the western Pacific at depths between 120 and 210 m. Through the classification of Etmopterus species into several clades based on the positioning of their bioluminescent photophores, the splendid lanternshark can be considered a member of the Etmopterus pusillus clade.

<span class="mw-page-title-main">Counter-illumination</span> Active camouflage using light matched to the background

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.

<span class="mw-page-title-main">Neon flying squid</span> Species of squid

The neon flying squid, sometimes called the red flying squid, akaika, and red squid is a species of large flying squid in the family Ommastrephidae. They are found in subtropical and temperate oceanic waters globally.

<i>Heteroteuthis dispar</i> Species of mollusc

Heteroteuthis dispar, also known as the odd bobtail, is a small deep water squid found in the North Atlantic Ocean and the Mediterranean Sea.

Aristostomias is a genus of barbeled dragonfishes native to the ocean depths in the Pacific, Atlantic and Indian oceans.

Pachystomias microdon, the smalltooth dragonfish, is a species of barbeled dragonfish found in the oceans at depths of from 660 to 4,000 metres. This species grows to a length of 22.1 centimetres (8.7 in) SL. This species is the only known species in its genus.

<i>Histioteuthis heteropsis</i> Species of squid

Histioteuthis heteropsis, also known as the strawberry squid, is a species of small cock-eyed squid. The scientific nomenclature of these squid stems from their set of differently sized eyes, one being small and blue and the other being large and yellow. It is thought that the large eye is used to see objects against dim light, while the smaller eye is more able to view bioluminescent light sources. The squid's vernacular name arose due to its rich red skin pigmentation and the presence of photophores along its body, making it appear like a strawberry with seeds.

A micronekton is a group of organisms of 2 to 20 cm in size which are able to swim independently of ocean currents. The word 'nekton' is derived from the Greek νήκτον, translit. nekton, meaning "to swim", and was coined by Ernst Haeckel in 1890.

References

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Sources

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