Sri Lanka, April 22 -- In some Sri Lankan streams, what disappears first is not the water, but our understanding of what still lives there.

If you have ever stood beside a small stream, you will know how quickly this seems to make sense. A glance is usually enough.

Clean or not. Live or ordinary. Worth noticing or not.

We do this without thinking. It feels reasonable.

Small streams encourage it. They look complete. A little water, a few leaves, and everything appears to be in order. But a stream is not complete. It is a process. Each moment, it is quietly deciding what can live there, and what cannot.

And sometimes, within that quiet decision, there is a fish most of us have never heard of.

Jonklaas's Loach is a small freshwater fis...