Why we can trace a baby eel home but never a baby turtle

A sea turtle hatches on a beach, scrambles into the surf, and disappears. For years no one knows where it is. Biologists call this stretch the lost years. The turtle turns up again as a dinner-plate-sized juvenile far out in the ocean, and eventually comes back as an adult to nest near the same beach it hatched on. So a natural question is whether you could take one of those lost-years turtles and run the tape backward to the exact beach it came from.

The answer is no, and not because it is hard. The information is physically gone. That surprised us, because the sister version of this question, for baby eels, comes out the other way. You can trace a young eel back toward its birthplace. The difference between the two animals turns out to follow one clean rule, and that rule is the interesting part.

The ocean is an eraser

Turbulent water mixes. Two hatchlings that start on beaches a few kilometers apart get stirred together fast. We measured how fast three separate ways: a computer twin experiment, the spreading of thousands of real ocean drifters, and the tracks of real tagged turtles. They all point to about two weeks. Within roughly two weeks, the uncertainty about where a turtle started grows larger than the spacing between nesting beaches. After that, many possible birthplaces map onto the same spot in the ocean. Run the drift backward and it fans out to everywhere. That is why the field assigns turtle origins with genetics, not by tracing currents.

So why does it work for eels?

Eel larvae get caught when they are only about eight days old. Turtles are observed months to years into their drift. Timing is the whole game. Catch an animal early enough and the ocean has not finished erasing where it came from. Catch it late and the record is gone.

One inequality for every drifting animal

This is not special to eels or turtles. Whether you can reconstruct any drifting animal’s origin depends on three numbers. How old it is when you observe it. How vigorously its patch of ocean mixes. And how far apart the candidate birthplaces are that you need to tell apart. Reconstruction is possible only while the age at observation stays below the spacing squared, divided by twice the mixing rate. We call that line the invertibility frontier. Eel larvae, observed young, sit well below it, so their origin is still readable. Lost-years turtles, observed old, sit well above it, so their origin is erased.

The nice thing about a rule like this is that it predicts. Give us the age, the mixing, and the spacing for a species nobody has tried, and it says up front whether tracing the origin is even possible. It says young bluefin tuna larvae should be traceable and spiny lobster larvae, which drift for months, should not. It turns a one-off result into something you can falsify.

What we do and do not claim

This is a first-order map, not a precise calculator. Real ocean mixing is messier than the simple version, and it varies a lot from place to place, so each animal is really a smudge on the diagram rather than a dot. If anything the messy reality makes tracing fail sooner, so the frontier is a generous bound. And there is a useful corollary. The exact beach is unrecoverable within weeks, but the coarse region a turtle came from survives much longer, which is why combining a weak genetic signal with a weak drift signal can still place most turtles in the right general area.

The forward problem and the backward problem are not mirror images. We can predict where the lost years go. We can prove you can never run that backward to a birthplace. And now we can say, for any animal the ocean carries, exactly which side of the line it lands on.