Japanese Eel
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The Japanese eel (''Anguilla japonica''; ) is a species of anguillid
eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
found 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 ...
,
Korea Korea is a peninsular region in East Asia consisting of the Korean Peninsula, Jeju Island, and smaller islands. Since the end of World War II in 1945, it has been politically Division of Korea, divided at or near the 38th parallel north, 3 ...
, Taiwan,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
, and
Vietnam Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about and a population of over 100 million, making it the world's List of countries and depende ...
, as well as the northern
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
. Like all the eels of the genus ''Anguilla'' and the family
Anguillidae The Anguillidae are a family of ray-finned fish that contains the freshwater eels. All the extant species and six subspecies in this family are in the genus ''Anguilla'', and are elongated fish of snake-like bodies, with long dorsal, caudal and a ...
, it is
catadromous Fish migration is animal migration, mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few ...
, meaning it spawns in the sea but lives parts of its life in freshwater. This food in Japan is called '' unagi''; they are an essential part of the food culture, with many restaurants serving grilled eel called ''
kabayaki is a preparation of fish, especially '' unagi'' eel,, vol. 7,"kabayaki" by describes it as being used principally or almost always for ''unagi'' (「もっぱら鰻」) where the fish is split down the back the Japanese dictionary says ''kab ...
''. However, presumably due to a combination of overfishing and habitat loss or changing water conditions in the ocean interfering with spawning and the transport of their leptocephali this species is endangered.


Breeding

Between April and November, the Japanese eels leave their freshwater river habitats in
East Asia East Asia is a geocultural region of Asia. It includes China, Japan, Mongolia, North Korea, South Korea, and Taiwan, plus two special administrative regions of China, Hong Kong and Macau. The economies of Economy of China, China, Economy of Ja ...
to breed larvae in the ocean near the
North Equatorial Current The North Equatorial Current (NEC) is a westward wind-driven current mostly located near the equator, but the location varies from different oceans. The NEC in the Pacific and the Atlantic is about 5°-20°N, while the NEC in the Indian Ocean is v ...
in the western North Pacific. Adult Japanese eels migrate thousands of kilometers from freshwater rivers in
East Asia East Asia is a geocultural region of Asia. It includes China, Japan, Mongolia, North Korea, South Korea, and Taiwan, plus two special administrative regions of China, Hong Kong and Macau. The economies of Economy of China, China, Economy of Ja ...
to their spawning area without feeding. The eels are able to travel this long distance without nutrients because of the oils they collect in their bodies before the migration out to sea. The spawning area for this species is approximately 15°N, 140°E, a location corresponding with the location of a salinity front separating the north equatorial current from the tropical waters. This front is an indicator to tell the eels that they are in their preferred spawning location. The North Equatorial Current assists the eels in migrating from the center of the Pacific Ocean to the coast of Asia; without this indicator, the larvae would end up in the Mindanao Current. The discovery of the Japanese eel breeding location was a new finding in 1991 when R.V. Hakuho Maru performed a research cruise. Before this research cruise, little was known about how eels breed, and there is still so much to learn as these eels remain a mystery to many scientists. Until the late 20th century, scientists hypothesized that the different stages of the eel's life cycle were entirely different species. Then in 2005, the same team of Japanese scientists at the
University of Tokyo The University of Tokyo (, abbreviated as in Japanese and UTokyo in English) is a public research university in Bunkyō, Tokyo, Japan. Founded in 1877 as the nation's first modern university by the merger of several pre-westernisation era ins ...
found a more precise location of spawning based on genetically identified specimens of newly hatched pre-larva only 2 to 5 days old in a small area near the Suruga Seamount to the west of the
Mariana Islands The Mariana Islands ( ; ), also simply the Marianas, are a crescent-shaped archipelago comprising the summits of fifteen longitudinally oriented, mostly dormant volcanic mountains in the northwestern Pacific Ocean, between the 12th and 21st pa ...
(14–17° N, 142–143° E). The larvae, also known as leptocephalius, hatch from the egg approximately 36 hours after fertilization. These leptocephalius grow from 7.9 to 34.2 mm, growing by 0.56 mm daily. After riding the north equatorial current, the leptocephali take and head northward by the
Kuroshio Current The , also known as the Black Current or is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Ku ...
to east Asia. These eels live in rivers, lakes, and estuaries until they return to the ocean as adults to breed and die off.


Life cycle

The Japanese eel metamorphoses into five stages throughout its life cycle, all with their distinct names. In the open ocean,
leptocephalus A leptocephalus (meaning "slim head") is the flat and Transparency (optics), transparent larva of eels and other members of the superorder Elopomorpha. This is one of the most diverse groups of teleosts, containing 801 species in 4 orders, 24 f ...
, the first stage after the egg, feed on
marine snow In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
. Approximately 18 months after hatching, they metamorphose into "glass eels", a name derived from their clear appearance. As these glass eels reach their freshwater habitats from December to April they become known as Elvers. The migration time of this species corresponds with the moon as it affects the tide. This time is during a tide that occurs at night and simulates a flood making it easier for the eels to survive their migration. An eel at this stage in its life cycle is 6 cm long with an intense instinct to swim upstream. This instinct allows them to scale any scenario and attempt to make it to their permanent habitat. During this migration, the eels are not presented with many predators as they are not commonly preyed on during the eel's life cycle stage. This low number of predators could also affect the nocturnal nature of the eels during this time. They chose to swim upstream during the night hours and hide under banks and rockets of the river during the day. Two weeks into this migration, the eels develop a black coloring and metamorphosis from elvers to brown-stage eels to continue their journey. The brown-stage, also known as the yellow stage, stage of the eel's life lasts for 5–10 years, and during this time, the eel feeds on worms and insects. The characteristics of this stage include a dull pigment with a grey, brown, and greenish top and white underbelly. This pigment is related to the environment as it relies significantly on the color of the water. The eels grow to around their adult size in this stage as well during this time, which is up to 57 – 60 cm for females and 35 cm for males. At 30 cm, the eels grow sexual organs for the first time and prepare to make their great migration. Once these eels reach adulthood, they develop a silvery color under their skin. This change in appearance signaling that they are entering their last stage of life, the silver stage. During this time, the eels prepare to migrate to the spawning area by naturally producing more oil in their body. This oil is stored in the muscles of the eels and is approximately 20% of their body mass. Once the appropriate content of the oil is reached, the eels stop feeding. During the autumn and generally on the last quarter of the moon, the eels migrate downstream to the center of the pacific.


Life history and habitat

The Japanese eel and other anguillid eels live in freshwater and
estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...
, an area where a freshwater river meets the ocean. Since then, more pre-leptocephali were collected at sea, and even Japanese eel eggs have been collected and genetically identified on the research vessel. The collections of eggs and recently hatched larvae have been made along the western side of the seamount chain of the West Mariana Ridge. Mature adults of the Japanese eel and giant mottled eel were captured using large midwater trawls in 2008 by Japanese scientists at the Fisheries Research Agency. The adults of the Japanese eel appear to spawn in the upper few hundred meters of the ocean, based on the recent catches of their spawning adults, eggs, and newly hatched larvae. The timing of catches of eggs and larvae and the ages of larger larvae have shown that Japanese eels only spawn during the few days just before the
new moon In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse. ...
period of each month during their spawning season. After hatching in the ocean, the leptocephali are carried westward by the North Equatorial Current and then northward by the Kuroshio Current to East Asia. before they metamorphose into the
glass eel Eels are any of several long, thin, bony fishes of the order Anguilliformes. They have a catadromous life cycle, that is: at different stages of development migrating between inland waterways and the deep ocean. Because fishermen never caught anyt ...
stage. The glass eels then enter the estuaries and headwaters of rivers and many travel upstream. In fresh water and estuaries, the diet of yellow eels consists mainly of shrimp, other crustaceans, aquatic insects, and small fishes.


Behavior

When experimentally fed to dark sleepers, most Japanese eels attempted to escape up the digestive tract into the gills and esophagus. Some even successfully escaped via the gills. Others that were fully encased in the stomach circled inside, as if searching for a way out.


Conservation


Threats

In the case of the Japanese eel's spawning is likely affected by the north–south shifts of a salinity front created by an area of low-salinity waters resulting from tropical rainfall. The front is thought to be detected by the adult spawning eels and to affect the latitudes at which they spawn. A northward shift in the front that occurred over the past 30 years appears to have occurred, which could cause more larvae to be retained in eddies offshore in the region east of Taiwan, and southward shifts in the salinity front have been observed in recent years that could increase southward transport into the Mindanao Current that flows into the Celebes Sea. These types of unfavorable larval transport are thought to reduce the recruitment success of the Japanese eels that reach river mouths as glass eels.The decline of the population of the Japanese eel is also directly related to the strong connection that the eel's life cycle has with the temperature of the water. This species relies on this environmental signal to know when to migrate in and out of their fresh water habitat; thus, the change in water temperature is directly proportional to the decline in population size. These rising water temperatures to
ocean acidification Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ...
and the thinning of the
ozone layer The ozone layer or ozone shield is a region of Earth's stratosphere that absorption (electromagnetic radiation), absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the a ...
. These eels are having a more challenging time knowing when to migrate. When migrating into their freshwater habitats, they prefer 8 to 10 degrees Celsius, a temperature usually reached in the autumn months but is getting later and later. Another factor that is effecting the Japanese eels is habitats loss. The coast is becoming a more preferred location for humans to live as the climate and pollution continues to worsen taking away from the habitat of these eels. On average from 1970s–2010s, 76.8% of the Japanese eels habitat is lost to human development.


Status

The Japanese Eel is considered an
endangered species An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching, inv ...
by
IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...
. The last time that this species was assessed in November 2018 and according to that assessment the population size is still decreasing.


Efforts

There are multiple preservation effectors that the Japanese government is undertaking to slow or stop the extinction of this highly important species for their consumption. In 2015 the Inland Water Fishery Promotion Act was put in place preventing the fishing and culturing of eels without the proper permit. The goal of this act was to slow down the over fishing of the Japanese eel and control the number of eels being taken into captivity. In the future there is a law being placed that in December 2023 that makes fishing glass eels without a permit punishable with up to 3 years in prison or a 30 million Japanese yen fine. There have also been efforts made to stop the habitat loss of this species through the Nature-oriented river works. The high price and demand of this species means that there is also need for the Act on "Ensuring the Proper Domestic Distribution and Importation of Specified Aquatic Animals and Plants" that prohibits harvesting and culturing these eels without a permit that was put in place in 2020.


Aquaculture

The culturing of eels was started in Japan in 1894 and has become the most significant eel culture industry in the world since. Japan is the biggest consumer and producer of eels generating approximately 70–90% of the eel population as of 1991. The farming of the Japanese eel is challenging due to the breeding habits. Scientists and farmers have never been able to breed an eel, so this species' agriculture relies heavily on their catch in their elver stage. A net is strewn across the rivers that these eels migrate up in the early autumn, and then they are transported to cultured ponds to grow to commercial size. The female eel grows to a much bigger size and has a longer life span than the male eel therefor; the cultured population is made up of 90% female eel. The heavy farming of this species has negatively impacted the conservation of the species; however, the production and consumption have not slowed.


Scientific and medical use

The Japanese eel has the ability to produce the protein UnaG which makes it unique among vertebrates. This protein is only found in the muscles of this species of eels making it a rare commodity. UnaG has demonstrated utility in life sciences and can be used to fluorescently label cells and tag proteins when exogenously expressed. This protein has been used in an experimental diagnostic test to assess liver function.


Consumption

As a food product, the Japanese eel is commonly referred to as '' unagi'' or ''
kabayaki is a preparation of fish, especially '' unagi'' eel,, vol. 7,"kabayaki" by describes it as being used principally or almost always for ''unagi'' (「もっぱら鰻」) where the fish is split down the back the Japanese dictionary says ''kab ...
'', with the latter being the method which the eels are often prepared. Kabayaki eels are prepared by being cut into fillets, deboned, skewered, marinated in sweet
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 ...
(usually
tare sauce is a general term in Japanese cuisine for dipping sauces often used in grilling (''yakitori'' and ''yakiniku'', especially as ''teriyaki'' sauce) as well as with ''sushi'', ''nabemono'', and ''gyoza''. It can also be used to make the soup for ...
) and then grilled. Japanese eels that have been grilled without tare sauce and seasoned only with salt are referred as ''Shirayaki''. Eels are eaten all year round in Japan. Dishes made with Japanese eel include ''unajū,'' a dish consisting of the better cuts of eel served in a lacquered box over steamed rice, and '' unadon'', a
donburi is a Japanese "rice-bowl dish" consisting of fish, meat, vegetables or other ingredients simmered together and served over rice. ''Donburi'' meals are usually served in oversized rice bowls which are also called ''donburi''. If one needs to ...
type dish where fillets of eels are served over rice in a large bowl. Japanese eel is also served 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 ...
, commonly called ''unagi sushi''. Some notable types include ''unakyu,'' a type of sushi containing eel and cucumber, and ''rock and roll,'' a western-style sushi made with eel and avocado. The Japanese eel contains a protein toxin in its blood that can cause harm to any mammals that ingest it, including humans. However, there is no need for any special procedures as temperatures of destroy the toxin. Thus, Japanese eels are always cooked before consumption, even unagi sushi. Eels intended to be used in sushi are usually sold in pre-cooked fillets by many sushi suppliers. Due to the decline of the population of the Japanese eels, they are often being substituted with European or
American eel The American eel (''Anguilla rostrata'') is a facultative catadromous eel found on the eastern coast of North America. Anguillidae, Freshwater eels are fish belonging to the Elopomorpha, elopomorph superorder, a group of Phylogenetics, phylogen ...
s, even within Japan, where Japanese eels were commonly used. Japanese eels are a good source of a wide range of vitamins and minerals. A serving of 100 grams contains roughly 120% of vitamin B12, 53% of
vitamin D Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. In humans, the most important compo ...
and 126% of the recommended daily value of
vitamin A Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most not ...
. They are also a source of vitamins such as B1, B2, E, D and
niacin Nicotinic acid, or niacin, is an organic compound and a vitamer of vitamin B3, an essential human nutrient. It is produced by plants and animals from the amino acid tryptophan. Nicotinic acid is also a prescription medication. Amounts f ...
. Minerals such as
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
,
selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...
,
zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
and
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
are also present, along with traces of
magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
,
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
, and
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
. Additionally, they are rich in dietary protein and contain a good amount of
omega-3 fatty acid Omega−3 fatty acids, also called omega−3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond three atoms away from the terminal methyl group in their ...
s, albeit not as much as other seafood, like
sardine Sardine and pilchard are common names for various species of small, oily forage fish in the herring suborder Clupeoidei. The term "sardine" was first used in English during the early 15th century; a somewhat dubious etymology says it com ...
s. They contain relatively low quantities of mercury.


References


External links

* {{Authority control Japanese eel Fish farming Fish of Japan Freshwater fish of Asia Marine fauna of East Asia Endangered fish Japanese eel Taxa named by Coenraad Jacob Temminck Taxa named by Hermann Schlegel