- Octopuses use their eight tentacles to envelop and shovel prey into their mouths.
- Special receptors on their tentacles help octopuses taste objects just by touching them, a new study found.
- This touch-taste sense helps an octopus detect hidden prey and retreat from objects that taste toxic.
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Octopuses are famous for being intelligent escape artists: Their camouflage skills are second to none. Each of an octopus's eight arms is also governed by a mini brain, which helps the animal probe the ocean floor in search of food.
Scientists know that this tactile probing is critical to helping octopuses distinguish between something inanimate and a tasty meal of shrimp, fish, or crab. But how, precisely, these cephalopods glean the flavor of an object was a mystery until now.
The secret is two types of sensory cells — one for touch, one for taste — in the suckers that line their tentacles, according to a Harvard University study published Thursday in the journal Cell.
Both taste and touch-oriented cells are critical to helping octopuses decide when to hunt and when to retreat, the study found.
"Our results demonstrate that the peripherally distributed octopus nervous system exhibits exceptional signal filtering properties that are mediated by highly specialised, sensory receptors," the study authors wrote.
When an octopus's sensory receptors determine that an object is prey, the animal will use its tentacles to envelop and shovel the creature into its mouth, as seen in the video below.
Using touch to detect hidden prey
The Harvard research team first looked at two female California two-spot octopuses in laboratory tanks.
They offered each octopus either her favorite food, a fiddler crab, or an inanimate object through a hole in the tank. Upon touching the latter, the octopuses would let go. But tentacles that encountered a crab would draw the prey in closer.
From there, the researchers peered more closely at the suckers on each tentacle. They found sensory cells — cells that detect light, sound, touch, or taste, then signal their presence to the brain — on the surface of each sucker.
The skin on the suckers had two types of detectors: mechanosensory cells (cells found in other animals that help them sense pressure or vibrations) and chemosensory cells, which contain receptors that help octopuses detect chemicals emitted by their underwater prey.
Mechanosensory cells, which send signals only at the first moment the octopus makes contact with an object, help the animal discern a rock from a fiddler crab. Since the inanimate object isn't moving, the signal stops right away. But a squirming crab prompts the signal to be sent again and again, each time a sucker makes new contact.
"This is highly useful for the octopus to detect prey hidden within seafloor crevices or areas inaccessible from its traditional sense organs," Nicholas Bellono, senior author of the new study, told ScienceAlert.
Octopuses avoid things that taste dangerous
To see what types of molecules the chemical-detecting receptors reacted to, Bellono's team extracted cells from the octopuses' suckers and exposed them to compounds emitted by other aquatic creatures.
The receptors didn't respond to common chemicals that produce a smell and taste sensation in most animals.
Instead, the receptors were only sensitive to chloroquine, a compound that humans find bitter, and molecules called terpenoids — sometimes-toxic chemicals emitted as a defense or warning signal by marine animals like jellyfish, sponges, mollusks, and crabs.
Octopuses might avoid toxic prey by detecting these signals, according to the researchers.
When a California two-spot octopus in the tank touched a surface infused with terpenoids, it withdrew its arms and avoided that area of the tank, the team found.
While genes underpinning these taste-by-touch receptors were found in three different octopus species examined by the researchers, octopus relatives like squid do not appear to use their suckers to taste their environment in the same way.
"We're really interested in how this unique sensorimotor system evolved in other cephalopods," Bellono told Science Alert.