MIAMI — They eat anything that fits in their mouths. They reproduce copiously and adapt effortlessly. And they have become as ubiquitous and pesky as rats — only prettier and more conniving.

Nearly three decades after a lone venomous lionfish was spotted in the ocean off Broward County — posing as a bit of eye candy back then and nothing more — the species has invaded the Southern seaboard, staking a particular claim on Florida, as well as the Gulf Coast, the Caribbean, and even parts of South America. Spreading gradually at first, and then frenetically from 2005 onward, lionfish have become the most numerous marine nonnative invasive species in the world, scientists said. Along the way, the predators, which hail from the other side of the world and can grow here to 20 inches long, are wreaking havoc on delicate reefs and probably further depleting precious snapper and grouper stocks.

There is no stopping them now, salt-water experts said. But hoping to at least slow them down, marine biologists and government agencies have been intensifying efforts recently to spearfish them out of certain areas that harbor fragile reefs and figure out how they became a threat so quickly and so successfully in the Atlantic Ocean.