Do sharks see optical illusions?

I really like optical illusions: interesting effects such as parallel straight lines looking like curves, or the identical lines looking longer or shorter depending on the background. Our brain makes us believe that we see something that is actually not there.

Maybe not surprisingly, monkeys and many other mammals with brains quite similar to ours can also see optical illusions. But other animals with less similar brains, like birds and even some ray-finned fishes also respond to them.

Recently I came across an article in which sharks were tested to check whether they can also “see the magic”. Sharks are one of the oldest groups of vertebrates. They are less closely related to ray-finned fishes (like goldfish) than birds are to mammals. Their brain has an organization and structure that differs greatly from ours, although some regions are similar.

Testing whether sharks respond to optical illusions can tell us something about the evolutionary origins of brain functions. If most animals descending from the same ancestor (some early vertebrate) possess a specific trait, it is very likely that this trait existed also in that ancestor.

How do you check what sharks can see?

Researchers tested young grey bamboo sharks. They are relatively small sharks (up to 75 cm length) that live in the shallow coastal waters of the Indian Ocean and West Pacific.

First, researchers trained sharks to recognize different shapes. Sharks could choose between two shapes randomly located either on the left or on the right. If they swam to a correct shape, they would get a reward (food), otherwise not. When sharks learned which shape is rewarded, they were shown optical illusions to check whether they would recognize them and treat like the shapes they experienced in training.

Figures that are not there

First sharks were tested on Kanizsa figures – illusionary figures that only seem to be there because their corners are formed by the negative spaces in Pacman shapes. Humans not only see white figures, but even report that these shapes seem brighter than the background white.

Sharks were taught that swimming to open figures is rewarded with food. Then they were tested whether they would swim towards Kanizsa figures when the alternative was one of the random arrangements of “Pacman shapes.
One group of sharks was trained to swim towards squares, and the other towards triangles. Then they were tested whether they can distinguish between a Kanizsa square and triangle.

Almost all sharks preferred (that is, swam towards) Kanizsa figures that resembled the symbol they earlier learned to associate with a reward (in both experiments shown on the figures above).

Sharks were even able to see figures with subjective contours created by shifting straight lines.

Sharks were trained to distinguish squires from rhomboids and then tested whether they could see the subjective figures defined by using phase-shifted grating.

Longer and shorter lines?

Next the researchers tested whether sharks are fooled by the Müller-Lyer deception – lines of the same length that seem longer if surrounded by inverted arrow heads (see figure below). Although it is treated as illusion, the line with inverted arrows actually isslightly lengthened by the arrow heads because they’re attached on the outsides of the line.

While many people are fooled by this illusion, there seems to be a cultural component. Europeans are more susceptible than Inuit, Aboriginal Australians or some Africans. But many birds and mammals actually see the length difference between the lines.

Sharks were trained to swim towards longer lines, independent of the arrow heads surrounding them. Afterwards they were tested on whether they would choose lines marked with inverted arrow heads.

And sharks? Only one shark (out of 8) consistently chose the seemingly longer line during testing. Lack of clear preference in other sharks is probably not a matter of lack of visual acuity since at least some sharks were able to learn a difference between simple 5 cm and 6 cm lines.

It is unclear why different species or even humans from different cultures respond differently in this test.

We are not so different

Even though sharks are not our close relatives and live in completely different conditions, they see some of the same illusions we and many other vertebrates do. It seems that the visual system and the brain works similarly in different species in order to make sense of the world.

While seeing things that are not there may seem bad, it can actually be a result of a very useful ability to recognize objects even if they are not fully visible (for example a predator partly hidden behind a bush). The brain of many species completes partly obscured shapes and might be first assessing an object as a whole before, or instead of, paying attention to its parts.

All in all, the study suggest that the way visual information is processed in the vertebrate brain has old evolutionary origins.

Shark photo: By © Citron, CC BY-SA 3.0,

Figures of optical illusions adjusted from the original article: Fuss et al. (2014) The brain creates illusions not just for us: sharks (Chiloscyllium griseum) can “see the magic” as well.

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