When octopuses extend their eight arms into hidden nooks and crannies in search of a meal, they are not just feeling around in the dark for their food. They are tasting their prey, and with even more sensory sophistication than scientists had already imagined.

Researchers reported on Tuesday in the journal Cell that octopus arms are fine-tuned to “eavesdrop into the microbial world,” detecting microbiomes on the surfaces around them and deriving information from them, said Rebecka Sepela, a molecular biologist at Harvard and an author of the new study.

Where octopus eyes cannot see, their arms can go to identify prey and make sense of their surroundings. Scientists knew that those eight arms (not tentacles) sense whether their eggs are healthy or need to be pruned. And the hundreds of suckers on each arm have over 10,000 chemotactile sensory receptors each, working with 500 million neurons to pick up that information and relay it throughout the nervous system.

Yet, what exactly the octopus is tasting by probing and prodding — and how its arms can distinguish, say, a rock from an egg, a healthy egg in its clutch from a sick one or a crab that’s safe to eat from a rotting, toxic one — has long baffled scientists.

What about the surfaces are they perceiving?

For Dr. Sepela, this question was heightened when her team discovered 26 receptors along the octopuses’ arms that didn’t have a known function. She supposed those receptors were tuned only to molecules found on surfaces, rather than those diffused in water.

So she and her colleagues collected swaths of molecules coating healthy and unhealthy crabs and octopus eggs. They grew and cultured the microbes from those surfaces in the lab, then tested 300 microbial strains, one by one, on two of those 26 receptors.