Yes, the picture gets more complicated when you look at Pacific eels, with Japanese, Australian and New Zealand eels all having different, but shorter, breeding routes. But I would guess that the mechanism was somewhat similar: they started off with a much smaller migration route and, probably, year-round breeding. Then owing to geographical/oceanic changes (maybe changes in water depth? Temperature?) they were forced to make a longer migration route. There's a lot left to discover!
I admit I know very little of eels. I didn't even know there were other eels in the Pacific than our local NZ eel but I figured there probably were. However I do know that the Pacific Ocean itself has been shrinking for a long time though, a quick check suggests longer than eels have existed. So there wouldn't have been a central point to gradually drift from, extending the journey, like there was in the Atlantic.
My first impression is that a warm salty water temperature is the initial starting condition and then eels seek out land and fresh water where-ever they can find it. Sort of like how frogs have to go back to water to lay eggs and be tadpoles.
> "the present distance back to the breeding grounds is an accident of geography."
That wouldn't explain Pacific eels though, would it?
Yes, the picture gets more complicated when you look at Pacific eels, with Japanese, Australian and New Zealand eels all having different, but shorter, breeding routes. But I would guess that the mechanism was somewhat similar: they started off with a much smaller migration route and, probably, year-round breeding. Then owing to geographical/oceanic changes (maybe changes in water depth? Temperature?) they were forced to make a longer migration route. There's a lot left to discover!
I admit I know very little of eels. I didn't even know there were other eels in the Pacific than our local NZ eel but I figured there probably were. However I do know that the Pacific Ocean itself has been shrinking for a long time though, a quick check suggests longer than eels have existed. So there wouldn't have been a central point to gradually drift from, extending the journey, like there was in the Atlantic.
My first impression is that a warm salty water temperature is the initial starting condition and then eels seek out land and fresh water where-ever they can find it. Sort of like how frogs have to go back to water to lay eggs and be tadpoles.
Anyway, just my layman's 2 cents.