Spearfisherman Greg Caterino was chasing a black grouper off Pacific Reef Light on Dec. 23 when his quarry disappeared under a 90-foot-deep ledge. Suddenly, the fish’s head popped out from the hole where it was hiding, and the diver from Tavernier shot it.

But when Caterino and dive buddy Wayne Grammes of Palmetto Bay retrieved the 15-pounder, they saw it wasn’t a black grouper at all. It had an odd-shaped head, a hump back and polka dots. They swam back to Grammes’ boat, piloted by Dave Cantrell of Deerfield Beach, shaking their heads in surprise the whole way.

“We were like, ‘Didn’t I see a black grouper going in there?’ ” Caterino said. “Wayne and I were looking at each other like, ‘What’s going on?’ ”

They photographed the fish, put it in a cooler and went home to consult the Internet to try to identify it. Caterino figured the black grouper he was chasing must have muscled the mystery fish out of the hole the two were sharing.

Grammes’ Internet search identified the fish as a humpback grouper, scientific name: Chromileptes altivelis — also known as panther grouper, or barramundi cod. According to the references, the fish had reached its maximum size. And it was very, very far from home: the species is native to the Indo-West Pacific.

Realizing they had just bagged a large, exotic species from southeast Florida waters, Grammes contacted Lad Akins, director of special projects for Key Largo-based Reef Environmental Education Foundation (REEF), and showed him the fish. Akins measured the fish at 27 inches, took tissue and organ samples, and positively identified it as a male humpback grouper — one of the largest of a handful sighted on both the east and west coasts of Florida since the mid 1980s.

Akins, who has dedicated plenty of time and resources to knocking down populations of exotic Pacific lionfish in Florida over the past several years, said the humpback — like the lionfish — probably was an aquarium pet that grew too large for its tank and got dumped in the ocean.

“This is about as big as the fish gets,” Akins said. “It is likely this was released as a much smaller fish. We’ve known the fish have been sighted in Florida waters and to see something this big that’s not native that’s been able to grow to this size is very disturbing.”

Akins said some experts believe the humpback, which has a voracious appetite and likes to hide beneath ledges and in caverns, could be the next big invasive problem predator after the lionfish, which has been spotted throughout the Atlantic, Gulf and Caribbean. He said anyone who spots one should shoot a photo and report the location to reef.org.

“If there are others out there, we need to try to remove these fish before they gain a foothold,” he said.

Divers should only collect a specimen if they can positively identify it, because shallow water grouper harvest is prohibited in South Atlantic waters from Jan. 1 through April 30, and some divers have mistaken the native species of marbled grouper and soapfish for the exotic humpback.

As for Grammes, he filleted the catch and stored it in his freezer for later dining. It is reputed to be an excellent food fish.