Tiny brain, but great vision

The mantis shrimp, a marine crustacean of the order Stomatopoda, is one of the top predators in coral reefs and other shallow warm water environments.

It is both fierce and fast, thanks to its unique pair of eyes with stereoscopic vision that move independently of each other, and its ability to take in an amazing amount of information.

In fact, says Justin Marshall from the Queensland Brain Institute (QBI), Australia, it has the most complex visual system of any living animal.

“Mantis shrimp have four times as many colour receptors as we humans do: we have three – red, green, and blue – and they have 12,” he says.

“They sample light we can’t see, and they also sample light we do see in a completely different way, and as a result, mantis shrimp have much more visual information coming in than we do.”

What hasn’t been clear, however, is how they can process and remember so much information when they have such tiny brains. 

Marshall and colleagues from QBI, the University of Arizona and University of Washington in the US, and Sweden’s Lund University may now have some clues after mapping a region of the mantis shrimp brain called the reniform (“kidney-shaped”) body. 

And they say it may help researchers better understand the evolution of colour vision in the animal kingdom.

Using a variety of imaging techniques, they traced connections made by neurons in the reniform body and discovered that it contains a number of distinct, interacting subsections. One is connected to a deep visual centre called the lobula, which is structurally comparable to a simplified visual cortex. 

“Mantis shrimp most likely use these subsections of the reniform body to process different types of colour information coming in and organise it in a way that makes sense to the rest of the brain,” says QBI’s Hanne Thoen, the lead author of a paper in the Journal of Comparative Neurology.

“This would enable them to interpret a large amount of visual information very quickly.”

One of the study’s crucial findings was that neural connections link the reniform bodies to centres called mushroom bodies, structures of arthropod brains that are required for olfactory learning and memory.

“The fact that we were now able to demonstrate that the reniform body is also connected to the mushroom body and provides information to it suggests that olfactory processing may take place in the context of already established visual memories,” says Arizona’s Nicholas Strausfeld.

The reniform body has been identified in other species, including shore crabs, shrimp and crayfish, but not to date in insects, and it may be uniquely crustacean attribute, the researchers say. 

Alternatively, it might be homologous to the lateral horn found in insect brains, which sits between the optic lobes and the mushroom bodies. 

Strausfeld points out that fruit fly research done by other groups shows that the lateral horn is crucial in assigning values to learned olfactory information.