Life history of Japanese eel
Prof. Ahn's research theme (in Ph.D)
Program-Specific Assistant Professor Ahn retired from Hokkaido University in March 2023.
Topic outline
Intro
Eels, although often considered as “river fish,” are actually migratory fish that travel between the sea and river.
The spawning area of Japanese eel is located near Guam, approximately 2,000 km from Japan.
Hatched larvae were drifted by ocean current to their habitats (Taiwan, China, Korea and Japan), grew up until they were fully matured and made large scale migration back to the sea where they were born for spawning event.
Eels are catadromous fish that can live in both seawater and freshwater, and very interesting species that change their appearance depending on their growth stage.
Let me introduce the life stages of Japanese eel.
Larvae
Japanese eel larvae were born in the middle of the tropical sea, approximately 3 mm in size.
They have long yolk sac and oil droplet under their body.
Yolk sacs and oil droplets are the only “food” they can take until the larvae start the first feeding.
The yolk and oil droplet are consumed in approximately one week after hatching, similar timing of the first feeding (exogenous consumption).
The eels before and after the first feeding are called preleptocephalus and leptocephalus, respectively.
The preleptocephalus and leptocephalus eels are collectively called larvae.
Japanese eels spend approximately half a year in the sea during the larval stage.
The timing to define the preleptocephalus and leptocephalus period differs depending on the scholar.
Some use the general concept of early and late larval stages, thereby using the absorption of the egg yolk and oil droplet and the start of feeding as the criteria (approximately one week after hatching), while others call the eels preleptocephalus until they exhibit a leaf-like shape, which is unique to leptocephalus larvae (approximately one month after hatching).
To avoid confusion, I prefer using the term “larvae” without separating the two.
Among the mysterious life history of eels, the larval stage has been a mystery for many years.
To begin with, their larvae are never found in the river because they spend the larval stage in the sea.
Even if they are caught in the sea, who can even imagine they are eels?
Larvae have a completely different shape from the adult eels, don’t they?
Aristotle, an ancient Greek scholar who tried to find the eel larvae but failed, said, “Eels are born in the mud.”
With the advancement in molecular biology techniques, genetic assessments of species found these interestingly shaped creatures to be eel larvae.
Juvenile
Since larvae have weak swimming ability, they are drifted in the ocean currents along the coasts of Japan, South Korea, Taiwan, and China for approximately half a year.
The larvae that have reached their maximum body length transform to an elongated cylindrical shape, which is the shape of the eels we all know (glass eel).
This is called “metamorphosis,” and the eels do not eat any food during this period.
Once they have completed metamorphosis, they recruited in the river mouth area, and their body becomes pigmented as they start eating food (elver).
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Yellow eel and Silver eel
Elvers continue to grow larger and become the “eels” we are familiar with
The back of an eel has a greenish bronze-like color, and the belly is white but slightly yellow.
This stage is called “yellow eel,” perhaps because the band-like yellow color between the back and belly of the eels might have been impressive to the person who named it.
(However, eels at this stage exhibit many color variations.)
This yellow eel stage, eels are consumed for food, such as Kabayaki.
Yellow eels, those have grown for 5–10 years, metamorphose again when they reach sexual maturity.
(This change is not as dramatic as the one from the larva to glass eel.)
Their eyes and fins became bigger to enable them to swim 2,000 km to the spawning area, and the color of their body also changes.
Because their skin changes to a metallic color at this time, they are called "silver eels” and this metamorphosis is called “silvering.”
The stages of Yellow and Silver eels are further subdivided according to the characteristics of their body: Yellow eel stage 1 (Y1), Yellow eel stage 2 (Y2), Silver eel stage 1 (S1), and Silver eel stage 2 (S2).
After metamorphosing into S1 stage with the beginning of sexual maturity, eels turn into S2 stage while they swim toward the spawning ground.
Without sleeping or eating, eels swim toward the spawning area, the place where they were born, and end their life after breeding.