Freshwater bivalves evolved by sending their larvae up rivers in the gills of spawning salmon. Now, like their ocean ancestors, they live out some of the most obscure lives on the planet, clasped in a darkness of their own creation, sometimes for up to a century or more. “Under a firmament of nacre,” wrote the French poet Francis Ponge about the oyster. A firmament, yes, because, like the sky, it is vast and ancient. Because, like the sky, you can get lost in it.