Resembling nothing so much as a ribbon of smoke, larval eels swims off the eastern shore of Bali.

During their larval phase, eels lack red blood cells. Their organs are small and their internal tissues transparent. This helps them avoid predators and earned a telling scientific designation: Any larval eel is a leptocephalus, latin for “slim head.” Only their heads are clearly visible.

Starting with Aristotle’s declaration that eels emerged from the droppings of earthworms, eels have been veiled in mystery and misinformation. Only in the late 18th century were they recognized as fish. Only in the late 19th century, when French zoologist Yves Delage watched leptocephali mature, did scientists show that larvae were actually larvae rather than a species distinct from eels.

Over the 20th century, biologists searched for the at-sea breeding grounds of various eel species, which migrate thousands of miles from inland waters to specific open-ocean locales. The journey is made in reverse by their offspring, with the translucent larvae becoming literally more substantial as they swim towards an adult home.

Via @DrCraigMc & Deep-Sea News

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