The boy who started the fantasy journey is a beggar eel? !

Searching for flower eels - Environmental DNA ecological detective reveals the secrets for you

College of Marine Biology and Fisheries, Hainan University

Instructor: Song Yiqing

Student: Meng Yu

1. The Fantastic Journey of the Flower Eel: A Journey of Life Across Thousands of Miles

In the mysterious water world, the flower eel (Anguilla marmorata) is like a traveler in gorgeous clothes, embarking on an amazing journey of thousands of miles. It is a national second-class protected animal and was once a fishery "star" in Hainan Island, attracting much attention.

The footprints of the flower eel are all over tropical and subtropical oceans and coastal areas, just like it has its own performance on the ocean stage all over the world. It has a super complex "metamorphosis" life cycle. It starts as a small egg, then turns into a slender eel like a willow leaf, drifting in the vast ocean with the help of the ocean current, just like riding the waves in the huge playground of the sea. When it approaches the continental shelf, it transforms into a transparent glass eel and sneaks into the estuary area, as if to start a mysterious landing ceremony, gradually dyed with color, and turned into a thread eel.

Then, the line eel mustered up its courage, swam upstream, and headed for the freshwater river, as if to find its own secret base. It slowly grew up in the freshwater rivers and streams and turned into a yellow eel. After a few years, the yellow eel turned into a silver eel and started an ultra-long journey home, traveling all the way to a specific spawning ground. After completing its reproductive mission, it quietly left this world. The distribution range of the flower eel is so wide, from the east coast of Africa to the Marquesas Islands in the southeastern Pacific Ocean, spanning 180 degrees of longitude, like a long ribbon tied around the waist of the earth, and as far north as southern Japan. In my country, the estuary of the lower reaches of the Yangtze River and Hainan Island are its important "strongholds".

The mysterious journey of the flower eel is a continuation of life

2. The survival crisis of the flower eel: the former star is now in trouble. But now, this former ocean star has encountered many crises. The "big monster" of global climate change has hit, the sea temperature has risen, and the ocean currents have become unstable, just like disrupting the biological clock of the flower eel, affecting the hatching time and survival rate of fish eggs and willow leaf eels, and also reducing their diffusion range, making them face difficult challenges at the beginning of their lives.
With the rapid development of industry, agriculture and urbanization, pesticides and various pollutants have been mixed into water bodies, and eutrophication has become increasingly serious. Flower eels will accumulate these "bad guys" in their bodies if they are not careful, which will seriously threaten their health. Human land use activities have also changed their habitats, and the terrestrial and aquatic ecosystems have become fragmented. The passage for flower eels to migrate from freshwater to estuaries and then to the ocean has become rugged and difficult, and many sexually mature individuals have fallen before completing their mission.

There are also those invasive parasitic nematodes, like "assassins" hiding in the dark, that hinder the reproduction of flower eels. Coupled with the development of the global fishing industry and recreational fishing, overfishing has reduced the population resources in the marine food web, and destructive fishing gear has caused a devastating blow to the habitat of flower eels, and flower eels at all stages of development are not immune. In my country, dams, reservoirs, and hydropower stations block their upstream migration, and artificial breeding technology is immature. The "predatory" fishing of eel fry in the middle and lower reaches of the Yangtze River and the areas to the south has caused a sharp decline in the population resources of flower eels. Although it is a national second-level protected animal and one of the flagship species for biodiversity conservation on Hainan Island, its survival situation is still grim.

The flower eel's journey upstream is bumpy and difficult

3. eDNA detective appears: solving the mystery of the disappearance of the flower eel

Just as the traces of flower eels became increasingly difficult to find, the great detective of environmental DNA (eDNA) made a brilliant appearance. What is eDNA? Simply put, it is the sum of genetic material fragments extracted from environmental samples. When flower eels swim in the water, they leave some "clues", such as being released into the environment through feces, skin cells, etc. These genetic materials constitute eDNA.

The production of eDNA is not simple. It is related to the age, growth stage, diet and other biological characteristics of the flower eel, and is also closely related to its physiological state, life history and living environment. eDNA technology is like a super detector. It first extracts total DNA from environmental samples, then uses specific or universal primers for polymerase chain reaction (PCR), and finally through sequencing and analysis, it can accurately find out whether the target species - flower eel - exists in the environment.

This technology is amazing. It is non-intrusive and will not disturb the flower eels like traditional monitoring methods. It is also very efficient and can detect a large number of samples in a short period of time. The cost is also relatively low. It can replace or assist traditional monitoring methods to help researchers better understand the distribution and number of flower eels in the ecosystem. It is like giving researchers "clairvoyance" and "clairaudience", allowing them to track the traces of flower eels more easily and provide key clues for protecting flower eels. Let us look forward to the eDNA detective helping flower eels get out of trouble and regain their former vitality.

eDNA technology "great detective" escort for the protection of flower eels

References

[1] Jia Yihe. Molecular mechanism of salinity adaptation in juvenile eels (Anguilla marmorata)[D]. Nanjing Normal University, 2015.

[2]Arai, T. and Chino, N. 2018. Opportunistic migration and habitat use of the giant mottled eel Anguilla marmorata (Teleostei: Elopomorpha). Scientific Reports 8:5666: 1–10.

[3]Arai, T., Chino, N. and Le, DQ 2013. Migration and habitat use of the tropical eels Anguilla marmorata and A. bicolor pacifica in Vietnam. Aquatic Ecology 47: 57–65.

[4] Liu Chenhan, Peng Yougui, Wu Haolin, et al. Application and prospects of environmental DNA technology in biodiversity monitoring[J]. Southern Agriculture, 2024, 18(21): 107-114. DOI: 10.19415/j.cnki.1673-890x.2024.21.024.

[5] Yin Xiaoqiang, Zeng Jinfeng, Zhang Na, et al. Specific detection and biomass assessment of flower eel based on environmental DNA technology [J/OL]. Journal of Water Ecology, 1-11 [2025-02-21]. https://doi.org/10.15928/j.1674-3075.202310060273.