Think of giant pythons from southeast Asia, ending up in the Florida everglades and suffocating any small mammal they could find. Think of cane toads from South America, relentlessly marching over Australia, swallowing bird eggs and native frogs. Think of rats from pretty much any mainland country, stowing away onto pristine islands and eating their way through the helpless local birds. These are all classic examples of invasive species.

Here is another, and it’s very different. It’s a microscopic alga called Symbiodinium trenchii. Unlike the python or the cane toad or the rat, this tiny brown bauble seems fairly benign—even beneficial. It lives in the cells of corals and provides them with food, by harnessing the sun’s energy