Florida stone crabs (Menippe mercenaria) are known to diners for their sweet, meaty claws. And octopuses also seem to relish these delicacies. Reports are coming out of Florida that the stone crab fishery is way down this year—and many think local common octopuses (Octopus vulgaris) are to blame.

The crabs are caught in traps, most of which have a main funnel-shaped entrance and a bait pouch inside, which lures the crustaceans with tasty morsels such as fish heads or pigs' feet. For octopuses, however, it's the trap's quarry that is the lure.

Octopuses are voracious—hunting and consuming everything from crabs to mussels to snails to one another. So if the opportunity presents itself for a live meal that’s already caught, all the better for the octopus. If "a crab or two comes plopping in, [the octopus] says, 'hey, this is not so bad—the food comes to me,' so they'll set up shop," says Ryan Gandy, a research scientist with the Florida Fish and Wildlife Conservation Commission who studies the stone crab fishery.

And those crabs are a pretty swanky free meal. A plate of meaty Florida stone crab claws, served up at a tony Miami restaurant, will run a human diner upward of $70.

For the crab fishery in Florida, however, the octopuses' feasting threatens to cost fishers a chunk of change. The state brings in nearly all of these crabs that are sold in the U.S. Its stone crab catch in the October 2011 to May 2012 season yielded about 2.9 million pounds, which is valued at about $26.5 million. As quantities of crabs decline, the price for human foodies soars.

The Florida stone crab fishery has been operating at maximum capacity for years, Gandy says. But octopuses might actually do more damage to the stone crab population than do humans. After all, octopuses consume the whole creature. In contrast, to harvest the prized claw meat from caught crustaceans for restaurants, fishers simply pull the big claw off a crab and toss it back in. The crabs can regrow their claws, although crabs missing a claw have about a 28 percent higher chance of dying [pdf] than those that have both their claws. In that way the stone crab fishery is somewhat sustainable (it has earned a "best choice" recommendation from the Monterey Bay Aquarium's Seafood Watch sustainability ranking).

The octopuses, although they are much larger than the five-to-six-inch-wide crabs, are flexible and can squeeze in and out of tiny holes. "Man, they're wily," says James Wood, a Florida-based marine biologist who studies cephalopods. In addition to a free lunch, the octopuses also use the traps as shelter. Gandy notes that he has seen plenty of crab traps housing an octopus with no evidence of a crab shell that's been sucked clean (octopuses can drill into hard shells with their mouths, inject a muscle-weakening toxin and essentially suck the contents of a carapace out, discarding this shell).

Because of the octopus's contortionist's abilities, "I don't think you're going to be able to economically octopus-proof a trap," Gandy says. Wood agrees: "It would be really hard to make a trap that would catch a crab and exclude an octopus—they are way more flexible than a crab and a lot smarter," he notes.

Crab fishers frequently find octopuses in their crab traps. But some fishers are reporting more—and larger—octopuses than usual. One fishing boat fleet owner told Reuters that instead of finding two-pound octopuses, some fishers were pulling up eight-pound cephalopods in their traps. Other folks in the fishing industry report that many more than usual of their traps are coming up with an octopus—rather than crabs—inside.

So could a booming octopus population be responsible for taking a bite out of the stone crab fishery? Scientists note that it is exceedingly difficult to pin a particular change on a single member of the sea. Both crab and octopus population numbers fluctuate quite a bit, Gandy notes. Cold weather, for example, is good for the stone crab catch because it tends to put the crustaceans on the move—allowing them to populate other areas and getting them mobile so they come across more traps. Localized changes are common. But this is a rare year, Gandy says, in which octopuses have been hitting crab populations all the way up Florida's Gulf coast—from the Keys to the Panhandle.

And octopuses are tricky beasts to track and count. They're reclusive, solitary and often well camouflaged. The Florida Fish and Wildlife Conservation Commission does not keep consistent records on octopus size, but Gandy says that in their crab monitoring they have encountered more octopuses than usual. And fishers often have a better—if less official—beat on the seas. "Fishermen are in the water every single day," Wood says. "So they see a lot of things that scientists don't see."

Whether or not octopuses are the main cause of the stone crab population dip is unclear. The ocean "is a complex system, with millions of things interacting with each other, so causation is really difficult [to determine] in the field," Wood notes. Small changes elsewhere in the food chain could be leading to a boom in octopus populations; or it could be that another change elsewhere is already weakening the stone crab population.

In the meantime, Florida crab catchers are allowed to sell any octopuses they inadvertently catch, although they don't bring in quite as much cash as the crabs the animals have been eating—especially if those crabs could have been contributing claws for a few years. No one, however, seems to be talking about starting up an octopus fishery. "Most of the world eats them—except North America," Woods notes. Although it has become a popular appetizer on some New York City menus, octopus has yet to become a locally prized catch in U.S. waters. Maybe it's somehow crueler than pinching claws off of crabs, but octopus arms also grow back, and they do have eight of them…(only kidding, of course.)

Illustration courtesy of Ivan Phillipsen