How does an octopus decide what to eat?

Excerpt: Under the sea, octopuses spend a lot of time hunting their dinner. Their choice of prey ranges from clams to crabs to fish. When picking from this extensive menu, do octopuses care more what the food looks like or what it smells like? Scientists now have an answer.

Study: Maselli, Valeria, et al. “Sensorial Hierarchy in Octopus vulgaris’s Food Choice: Chemical vs. Visual.” Animals 10.3 (2020): 457.

Imagine this: there’s a new bakery in town, and you decide to check it out. You find a charming little pastry shop with ornate cakes on display behind tall windows. Inside, it smells like vanilla and strawberries. There are lemon bars, chocolate cupcakes, fruit tarts, and too many types of cookies to count. All these desserts look so delicious, but how do you decide what to get?

Octopuses or octopi? The origin of the word “octopus” is Greek rather than Latin, so “octopi” is incorrect – although people still use it! Photo by Isabel Galvez on Unsplash

Before we even taste our food, we evaluate it on how it looks, how it smells, and (sometimes) how it feels to touch. Our brains integrate this visual, chemosensory, and tactile information, and then use it to pick the yummiest morsel – like a cherry danish from a selection of pastries. We humans tend to rely on vision, choosing foods that look the tastiest. But how do animals who live underwater, where appearances are often distorted, resolve this conundrum?

Scientists at the University of Naples Federico II, Italy, studied how the common octopus, Octopus vulgaris, chooses its food. They discovered that just like humans, the intelligent mollusks used their senses of vision and smell to decide on their meal. But of these two senses, only one was a clear favorite.

No nose? No problem! 

Can octopuses thoroughly examine their food options? Yes – because they have a sophisticated nervous system.   

These marine mollusks possess complex eyes that detect light and maybe even colors. But their eyes aren’t their only depository for visual information. Octopuses can also detect light with their skin, which is how they decide to change their color and camouflage to their environment. Because octopuses’ light-sensing organs are so well developed, scientists have long considered them to predominantly rely on vision to navigate the underwater world.

However, octopuses also enjoy an acute sense of olfaction. Underwater, scents can carry quite far, and octopuses have a variety of organs to take advantage of that. Arm suckers, the mouth, and solitary nerve cells all detect chemical cues – as if octopuses had lots of tiny noses scattered throughout their bodies. 

Vision and olfaction are both very important to octopuses, but when it comes to picking food, is one sense more useful than the other? The research team set out to answer this question. 

A tentacle in the anchovies jar

The scientists recruited their four study subjects by fishing them out of the Bay of Naples. The octopuses were then housed individually in the lab.

The scientists gave the octopuses jars with either anchovies, clams, or mussels. Some jars prevented the animals from seeing their food, and some from smelling it. Image from Maselli et al., Animals 10.3 (2020), in open access.

To research how the octopuses selected food, the scientists first had to know what food octopuses like. When given a choice of anchovies, clams, and mussels, the octopuses preferred anchovies above all of the foods, didn’t mind clams, and turned their beaks up at mussels. Armed with this knowledge, the team set up five different food selection experiments. 

In the first experiment, the researchers filled three clear jars with anchovies (favorite food), clams, or mussels (least favorite food), and offered them to each octopus one by one. The lids of the jars had holes in them, so that the contents could emanate their smell into the water. Here, the octopuses could use both vision and olfaction to pick their snack and decide which jar to open.

In the second experiment, the researchers put the food in the jars without holes in the lids, so the octopuses saw the treats but couldn’t smell them. In the third experiment, the foods were in non-transparent jars with holes in the lids, so the octopuses could smell the food but not see it.  

For the fourth experiment, the researchers tricked the octopuses. The foods were once again in non-transparent jars with holes, but this time, the jars were labeled with images of food that didn’t correspond to their contents. For example, a black jar had a picture of anchovies on it, but actually contained clams. Here, the octopuses had to reconcile conflicting smells and images.

Finally, the researchers sealed the three types of food into non-transparent jars without holes. The octopuses had no choice but to guess their contents.

In each trial, the researchers recorded which jars the octopuses opened first. They also measured how long each octopus took to open the jar with anchovies.

Smells fishy

The octopuses chose their favorite snack most accurately when they both saw and smelled it in the first experiment. This accuracy was preserved when the octopuses could only smell the jar contents, but dropped when the octopuses could only rely on visual cues. 

In the fourth experiment, the octopuses didn’t fall for misleading labels, but instead were guided by their sense of smell to find the anchovies jar. When neither smells nor visual cues were available in the fifth experiment, the octopuses opened the jars at random.

The octopuses in the study preferred anchovies to all other seafood and used their keen sense of smell to find the jars with their favorite snack. Photo by Diane Helentjaris on Unsplash

However, when the researchers analyzed how long it took the octopuses to find anchovies, the importance of olfactory clues became evident.

Surprisingly, the octopuses opened anchovies jar the fastest in the fourth trial, when the jars were mislabeled with incorrect images – even faster than when they could both see and smell the treats in the first trials! The smart molluscs were almost as fast to find their preferred food in the third experiment, when they could only smell, but not see, the treats.

The researchers concluded that the sense of smell plays a critical role in food selection for octopuses – a more important one than eyesight does. This finding challenges the previous view that octopuses are primarily “visual” creatures. 

It makes sense that olfaction is so important for marine animals. By keeping their beaks peeled for chemical cues, underwater creatures can detect a source of food long before they see it.

But it is likely that octopuses take advantage of all their senses to make such important decisions as meal selection. After all, we humans primarily rely on our eyesight to navigate the world, but we like to smell our food too – although maybe not if it’s anchovies.

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