User:Enabulei/Firefly squid

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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 in length. They are brown/red in color, but emit blue and green light by their photophores. Firefly squid have three types of photophores on its body. There are multiple (800 - 1000) very minute photophores over 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 ventral legs. 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 legs only produce blue wavelength light. undefined The reactant luciferin and the necessary enzyme luciferase are located in a crystalline structure within rod like bodies in their photophores. undefinedFirefly 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. The photophores on the tips of its fourth ventral legs produce a very intense light that can be seen by the naked eye.

Background
Cephalopods species have historically shown evidence of polyandry, in which a female mates with multiple mates, through common reproductive traits and life history. Firefly squids show rare evidence for monogamy in their reproductive cycle when they make a yearly migration to the coastal waters of Toyama Bay each spring during their mating season. The spawning season of the firefly squid runs from March to May. They gather here by the millions, and sometimes by the billions, to lay their eggs.This annual light show is so spectacular that the area where they gather has been designated as a special natural monument. 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. Once the squid's eggs have been fertilized and laid, it dies, having reached the end of its one-year lifespan.

Research
Sato et al conducted research 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. During this time, high numbers of males and virgin females were disappearing, indicating mating. 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. From these results, researchers tested the monogamy hypothesis in females first. They found that 95% of females tested stored sperm from a single male. Further data collection confirmed that the single male's sperm stored in the seminal receptacle fertilized all of the female's eggs. Both of these findings support monogamous reproductive 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.

Implications
Sato et al's findings are significant to establishing foundational support of monogamy in W. scintillans. It is known the female firefly squids can spawn eggs at various times in designated intervals within the reproductive season. Despite this, males cannot deposit more sperm to these females because they both disappear approximately a month before females are reproductively mature and that females do not spontaneous lose sperm that is already stored in their seminal receptacles. The research team concluded, based on their collected data and mathematical modeling, that female monandry was established first and subsequently males followed suit to create mutual monogamy in W. scintillans.

Background
The firefly squid resides in the deep waters of the ocean where very limited amounts of visual light penetrate from the surface. The shorter wavelengths of light are blue (450-495 nm) followed by green (495-570 nm) and yellow (570-590 nm). These shorter wavelengths have more energy and can penetrate deeper into the water column. W. scintillans uses both blue and green light emission as counter-illumination to reduce predation. The squid’s visual system is adapted to capture the maximal amount of light at these depths with each eye having a large pupil to allow maximal 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.

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 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.

Commercial Use
Fishers have long known that firefly squids congregate in Toyama Bay off the Japanese cost to spawn. They are often caught at night when they rise to the surface or in fishing nets that troll 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 squids 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.

References