Scientists Figured Out How Eels Reproduce (2022)

Until recently, nobody knew how eels reproduce. On August 14, 2021, TikTok user ColeTheScienceDude published a video named We Don’t Know Where Eels Come From. The user shares information about how little is known about the eel species Anguilla anguilla, also referred to as freshwater eels or the European eel, reproductive cycle. He says, “We have been to the moon, and yet we still do not know how eels sexually reproduce.” The user then describes more qualities of eels that experts find puzzling, including: “If you dissect one and you look inside, you will not find sexual organs.” He added. “We have never observed eels mating in captivity or the wild.” Most Anguilla anguilla eels lack reproductive organs; however, scientists can bring them to sexual maturity by treating them with hormones. With hormone treatment, eels can be brought together in captivity to reproduce. In the wild, however, during the last phase of their complex life cycle, they do develop reproductive organs. This final stage is known as the silver stage, and during this stage, the eels’ stomach dissolves before their sexual organs develop. This means the eels die soon after reproducing, which has made the mystery of eel reproduction even more challenging to work out. Born in the remote Sargasso Sea, these eels begin life as transparent larvae, resembling tiny leaves with fish heads. Carried by Atlantic currents, they embark on a voyage spanning thousands of miles, eventually reaching the shores of eastern Europe. Here, they transform into bootlace-sized young, known as glass eels or elvers. As they mature, they settle in shallow coastal seas, undergoing another transformation into yellow eels, marked by a change in color and habitat as they migrate into rivers and lakes. They can stay in these inland waters for decades, silently growing and waiting. Finally, they enter the silver stage, retracing their paths back to the Sargasso Sea, where their reproductive organs develop, culminating in a life dedicated to this ultimate journey. After decades of speculation, researchers have successfully tracked European eels back to their breeding grounds in the Sargasso Sea, following their journeys thousands of kilometers along one of nature’s most incredible animal migrations. Scientists are ecstatic because this is the first confirmation of a long-suspected stage of the eels’ life cycle, which was proposed about 100 years ago. Fish biologist Kim Aarestrup from the Technical University of Denmark, the study’s author, stated on Twitter that “eels have piqued the curiosity of scientists for millennia.” North Atlantic Ocean’s Sargasso Sea is where eels congregate to breed, yet until recently, no eels or eggs have been discovered. “For the first time, we followed eels to their spawning grounds,” added Aarestrup. In the early 1920s, a Danish biologist named Johannes Schmidt found eel larvae in the Sargasso Sea. This was a long way from where eels live in Europe and Africa, where they reside in freshwater, estuaries, and along the coasts. Schmidt’s findings, published in 1923, predicted the next century of research to comprehend how eels reproduce, summarizing his efforts as “disappointment alternating with encouraging discoveries and periods of rapid progress with others during which the solution of the problem seemed wrapped in deeper darkness than ever.” Since then, researchers have been trying to determine the precise location of the eels’ breeding grounds. This endeavor has proven challenging due to the various obstacles that eels must overcome, including weirs, dams, habitat loss, pollution, and overfishing. Moreover, the effort is increasingly urgent due to the dramatic decline in European eel populations since the 1980s, marked by a staggering drop of over 95% in the number of glass eels arriving at European coasts. The species is now highly endangered, facing multiple threats that complicate conservation efforts. Migration Barriers: Dams and other man-made structures block essential migration routes, disrupting the natural life cycle of eels and preventing them from reaching crucial habitats. Illegal Trading: The illegal trade of glass eels, driven by high demand for rearing in fish farms, further exacerbates the problem. These practices not only reduce the wild populations but also undermine legal conservation efforts. Addressing these challenges requires a coordinated approach that includes stricter enforcement of trade regulations and innovative solutions to allow safe passage for migrating eels. European eels travel between 3,100 and 6,210 miles (5,000 to 10,000 kilometers) to breed at sea, and then their larvae float back to land and the safer environment of rivers. The researchers behind this recent discovery used satellite tags to follow 21 female European eels as they navigated the final phase of their incredible journey southwest from the Azores, the volcanic archipelago of the North Atlantic Ocean west of Portugal. Previous studies revealed that eels from across Europe congregate in the Azores before heading for the Sargasso Sea. In 2018 and 2019, the eels were captured from the Azores islands, tagged with removable satellite trackers, swabbed for DNA tests, and released back into the Atlantic Ocean. With their satellite trackers still attached, six eels arrived at the Sargasso breeding grounds months later; data from 15 other eels were gathered along the route. The most significant distance measured in a straight line was 1,410 miles (2,275 kilometers). “Their journey will reveal information about eel migration that has never been known before,” explained Ros Wright, a fisheries biologist at the UK Environment Agency who led the study. It is still being determined how eels reach the Sargasso Sea or how long their breeding season lasts. However, the eels in this study’s swimming speed, which averaged 6.8 kilometers (4.2 miles) per day, and the length of their marathon voyage – which lasts more than a year – indicate that these long-distance travelers must undertake a meticulous migration. Wright and his colleagues wrote in the study: “Rather than make a rapid migration to spawn at the earliest opportunity, European eels may instead make a long, slow spawning migration at a depth that conserves their energy and reduces mortality risk. This timing would enable the completion of their reproductive maturation before they arrive at the spawning area.” “It’s also incredible to know they go way deeper than 1,000 meters on the way!” tweeted James Maclaine, senior fish curator of the UK National History Museum. That is plummeting more than 3,280 feet into complete darkness! The Anguillidae family consists of approximately 15 distinct species of eels. These eels are characterized by their elongated, snake-like bodies. Many of these species embark on migrations, moving between oceanic environments and inland freshwater habitats at various stages of their life cycles. Questions Remain: However, questions remain about the timing and navigation of the eels across thousands of kilometers of open water to reach the Sargasso Sea. They may detect Earth’s magnetic fields, similar to how Chinook salmon (Oncorhynchus tshawytscha) return to the identical stream where they hatched. Other possibilities include sensing olfactory cues, tracking ocean currents, or observing temperature fronts. This new study completes the map of eel migrations, placing the Azores at the forefront of conservation efforts to prevent the eels’ decline, even though it may take some time to untangle those strands. José Manuel N. Azevedo, a fish ecologist at the University of the Azores and study author, concluded: “This discovery emphasizes the role of the Azores in the life cycle of eels. It will help scientists and conservationists to push for measures to restore eel habitats across the archipelago.” The findings were published on October 13, 2022, in Scientific Reports.