An explosion in the population of the predatory lionfish in Caribbean waters, where it has no natural predators, is posing a widespread threat to marine wildlife.
Just off the north shore of Little Cayman, I sink into the blue abyss.
I am descending the vertical coral wall at Bloody Bay Marine Park.
Straight ahead and straight down there is nothing but blue – a dizzying empty space where sunlight streams down and down into darker places well beyond my reach.
But up close, the wall of coral is covered in giant barrel sponges as tall as a man, bright purple vase sponges, green and red corals and creatures that creep, crawl and swim within and among them.
I spot a seahorse, clinging to a whip coral by its tail, a spider crab with legs almost 3ft (1m) wide and a baby hawksbill turtle rocketing to the surface for a breath of air.
And there, spiralling up from the depths come three graceful Caribbean reef sharks, curious and skittish.
Suddenly, I notice Peter Hillenbrand, my diving buddy, gesticulating angrily – he points with one hand and pulls the trigger on an imaginary gun.
It is not the sharks he is angry at, but a brightly coloured fish covered in feathery spines.
I recognise it immediately as a lionfish – distinctive with its tracery of red-brown stripes and the high venomous spikes all along its back and protruding from its pectoral fins.
I have seen thousands of these fish in the Indian and Pacific Oceans, but they should not be here in the Caribbean.
“These fish are like Godzilla,” Peter tells me on the boat after we surface.
“Two or three years ago we would see the odd one here and there, but now on every dive they’re there.
“I’ve been diving these reefs for over 30 years and I’m worried that these fish are taking over,” he says.
On Little Cayman, I visit the Central Caribbean Marine Institute, where scientists are studying the invasive population of lionfish, more properly known as Pterois volitans.
Morgan Edwards, a post-graduate researcher at the Institute invites me to watch her dissecting them.
Even in death they are beautiful, with a maze of dark red stripes all over their head and body, protective spines covered in a web of fine skin and delicate frilly tassels of skin hanging from their mouths.
After measuring its length, weight and other details, Morgan removes the stomach from each fish and opens it to find out what they have been eating – shrimps, baby grouper, damsel fish and crabs.
“Their stomach can expand up to 30 times its volume,” Morgan explains.
“And they can swallow any other fish up to two thirds their own body length.
“But they seem to have no natural predators here in the Caribbean.”
The facts about lionfish are frightening.
A female can produce 30,000 eggs every four days. The eggs are unpalatable to other fish.
And lionfish are growing larger than they do in their native waters – up to 18in long (47cm), and they are stealthy ambush predators.
No-one knows how lionfish got into the Caribbean.
One theory says they escaped from a Florida aquarium during a hurricane about 10 years ago.
Others believe that tropical fish keepers released their pets into the sea when they grew too large to keep at home.
It does not really matter how they got here, but since 1992 they have spread from Florida up the east coast of the United States as far north as Long Island in New York.
About six years ago they crossed the western Atlantic to Bermuda and then drifted south to Bahamas, Jamaica and Cuba.
In 2008 the first lionfish was spotted at Bloody Bay on Little Cayman.
One year later there were hundreds, and now thousands. And they have now gone south as far as Venezuela.