Natures wonders: eel metamorphosis and migration: (Introduction)

by David Turell , Thursday, January 19, 2023, 20:26 (672 days ago) @ David Turell

Metamorphosis in eels is outlined by not fully understood, while the migration of European eels to the Sargasso is confirmed:

https://www.newscientist.com/article/mg25734222-000-how-we-finally-tracked-european-eel...

"RIGHT now, millions of sinuous, silvery fish are swimming determinedly across the Atlantic Ocean. They are snake-like, more than a metre long and have huge, bulbous eyes. They left their homes in Europe in late autumn and have been navigating westwards ever since, often swimming against the currents that once carried them the other way. They travel alone at a languid pace, never stopping to rest. By night they are near the surface; by day in the depths. Their journey will take more than a year. Many won’t make it. But those that do have a reward awaiting them: sex and death in the Sargasso Sea.

"This is the ultimate goal, and fate, of the European eel (Anguilla anguilla), a remarkable and enigmatic species that has nourished the human imagination, and belly, for millennia. Their life cycle is fascinating and their final journey, the details of which have only recently been discovered, is jaw-dropping. “This is a species which is notoriously difficult to understand,” says eel expert Jack Wootton. What we do know for certain is that the European eel is critically endangered and needs help to recover, or woe betide it and the ecosystems it nourishes.

"This species starts life far from Europe in the Sargasso Sea, a region in the western Atlantic that is defined by the four ocean currents that form its boundaries. From December to May adult eels spawn there, and their larvae – known as leptocephali – start a long journey to Europe and North Africa. They are largely carried by the prevailing currents, which drag on their leaf-shaped bodies, but they also navigate. “There’s some active swimming,” says Ros Wright of the UK’s Environment Agency.

"From the Sargasso Sea, the leptocephali fan out and eventually arrive across the western shores of Eurasia and Africa, from Iceland to Morocco, while also penetrating deep into the Mediterranean and Black Sea. By now they have reached the next stage of their life cycle, becoming glass eels, which are less than 10 centimetres long, thin and transparent, but distinctly eely (see diagram below).

"For the next few months, glass eels wash in and out of estuaries, feeding and growing and gradually transforming into elvers, which are dark brown and about 12 cm long. At this point they are ready to swap the sea for freshwater and make their way up rivers and streams to find a place to grow up. Once settled in a lake or river, they transform again, into yellow eels. “This life stage can be decades long,” says Wootton. “And this is usually what we see when we see eels within our rivers and lakes and lochs.”

"Eventually, though, it is time to return. Following an unknown cue, our yellow eel begins its final metamorphosis into a silver eel. “It doesn’t fully mature yet, but it’s starting the process,” says Wootton. “It has an incredible metallic silver belly, a really dark back, its eyes grow and enlarge, pectoral fins grow, its digestive tract changes and its sexual organs start to develop.” “They’re so silver it’s ridiculous, a really blue silver,” says Andy Don, also of the UK’s Environment Agency. The silver eels then head for the Sargasso Sea, where they mate for the first and last time, spawn and die.

***

"One big surprise was that the leg of the journey from the Azores to the Sargasso Sea takes the best part of a year. The presumption was that silver eels go hell for leather to reach the Sargasso Sea for the first spawning season after their departure. But the truth is that they dawdle and meander for months, travelling an average of just 6.5 kilometres a day, skipping the first season and joining the next one.

***

"From 1980 to 2010, glass eel recruitment declined 15 per cent a year. In 2011 the decline stopped, and recruitment has been bumping along the bottom ever since, but it is still in a terrible state, says Kerr. “This is a critically endangered fish”.

"The consequences of this decline ripple through the whole ecosystem. Eels once represented a huge influx of nutrients into wetland environments, providing food for species such as otters and bitterns. “All this work that we do on eels has got wider biodiversity benefits and that’s just so important now,” says Wright.

***

"Then there is the small matter of how they navigate back to their birthplace, across up to 10,000 km of open ocean. “The navigation, I think, is still something we really don’t know,” says Wright. It could be guided by magnetic fields, or maybe visual cues from the sky – hence the huge eyes and shallow swimming by night. It could be genetic: American eels also spawn in the Sargasso Sea, but head east as their European counterparts head west; hybrids occasionally arise and head north. Those route-finding skills could be down to something else entirely. But that is a mystery that the European eel may well take to its watery grave."

Comment: migration mechanism not solved. I'll bet it is our magnetic field in use. Ells are like butterflies, not only migrating but metamorphosing. I feel it is something so complex it must be designed. The article has magnificent illustrations to make this all clear.