Electronic spies come in all shapes and sizes, but none is as funny looking as an oyster impersonator called the Flex Spy now infiltrating the waters off western France.

Looking for all the world like the bivalves it is protecting, the plastic imposter is fitted with a circuit board that allows it to snitch on thieves.

Invented by French startup Flex-Sense, the device has been on the market since September.

After the first prototypes were tested in Vietnam, the gadgets are now making their appearance in the oyster beds off France's Atlantic coast, with a major deployment planned in February.

Several dozen tons of oysters are stolen each year out of France's total production of 100,000 tons.

"It may not be a big proportion, but it is a lot for the operator who is robbed" after seeing much of his production wiped out by a mystery disease for the past several years, said oyster farmer Gerald Viaud, president of the national shellfish farmers' association.

Theft is a "real problem" in the sector, which is "always on the lookout for solutions", from surveillance cameras to ground, sea and air patrols, he said.

One quirkier approach is to fill an oyster shell with cement stamped with the farmer's phone number in the hope that a vendor who finds it among stolen oysters will contact the victim.

Enter Flex-Sense, which was founded some 18 months ago specializing in wireless telemetry in complex environments.

Initially it was interested in offering shellfish farmers a way to monitor water temperature, salinity and oxygen concentration from a distance to enable them to limit the mortality rates of their mussels and oysters.

But customers were also interested in ways to prevent thefts, which spike ahead of the holiday season.

After months of development, the electronic oyster was hatched.

The electronic spy kicks into action if it detects suspicious movement, transmitting an alert to the oyster farmer's phone or computer.

The user can then track the oysters' movements for up to a week.

It is too early to judge the device's effectiveness, however, since no thieves have yet been caught.

The company wants to go on to adapt the device for use in the construction industry, said Sylvain Dardenne, co-founder and commercial director of Flex-Sense.