COZUMEL, Mexico (Reuters Life!) - Mexican dive master Martin Vera has one code for the intruders in his beloved waters: shoot to kill.
The intruders are lionfish, creatures with venomous tendrils, that are native to Indo-Pacific waters but are colonizing the Caribbean at a furious pace.
Vera said he saw his first lionfish just eight months ago.
"Then I would see one or two a week and then it started increasing, increasing," he said.
Now, on some sections of the magnificent reef that runs along the tropical island of Cozumel, the fish seem to be everywhere.
On a recent mid-February dive, Vera expertly "speared" about a dozen lionfish, gingerly discarding their corpses to avoid their venomous spines, which while not fatal to humans can still inflict a painfully poisonous sting.
"You see no other creatures on the coral where you see the lionfish. They eat everything that moves," he explained.
U.S. researchers say the lionfish are gobbling up many different species that are important for recreational, commercial and ecological reasons, such as juvenile grouper and snapper and parrot fish.
The spear Vera uses is more like a metal arrow or mini-harpoon about 18 inches (45 cms) in length that is propelled with a sling-shot-like device at point-blank range.
Vera skillfully removes the weapon from its metal sheath like an underwater archer, and then he sets sight on his prey, which on rare occasions elude him.
On one dive, several of the lionfish he nailed he kept for supper. With their tendrils removed they are apparently very good to eat and the Florida-based Reef Environmental Education Foundation (REEF) has released "The Lionfish Cookbook" to encourage people to eat the voracious invaders.
Human predation and consumption appear to be making little dent on their population explosion in the Caribbean and the stakes are high.
Cozumel is a scuba diving mecca and so any threat to its dazzling undersea life is a threat to a mainstay of the island's tourist industry.
Globally, climate change and invasive species form a "deadly duo" wreaking ecological and economic havoc on a global scale, according to a World Bank-funded report last year.
Lionfish were first spotted off Florida in the 1980s and U.S. researchers say they were almost certainly released by Florida aquarium owners into the state's waters.
Laddie Akins, director of special projects at REEF, told Reuters in a telephone interview that lionfish were first reported in the Caribbean in 2004, off the Bahamas.
He said a widely and repeatedly reported story that the lionfish was introduced into Florida waters during Hurricane Andrew in 1992 when an aquarium broke was "unproven." But aquarium owners are seen as the main culprits.
Currents are spreading their eggs and offspring far and wide in the Caribbean and they seem to have no natural predators.
"There may be incidental predation here and there but nothing is controlling their population," Akins said.
"You have incredible densities compared to their native ranges and lionfish of almost half a meter (yard), bigger than in their native range," he said.
In Indian Ocean dive sites off Africa for example, you see them occasionally and they are usually regarded by divers as a beautiful sight, their flowery tendrils or spines swaying gently in the currents as they hover around reefs.
But in the Caribbean, they are making divers see red and you can see a dozen or more on Cozumel dives now.
There are several species of lionfish and Akins said the Caribbean has been invaded by two that cannot be distinguished visually: the red lionfish and devil firefish.
Researchers hope more species don't show up.