This ancient crab has unusually large eyes

To figure out how Callichimaera uses its eyes, Ms. Jenkins and Dr. Luque used a variety of existing Callichimaera specimens to form a growth sequence. They compared this with 14 species of crabs. They were surprised to discover that – unlike other crabs – Callichimaera retains large eyes as an adult.

In fact, their calculations showed that Callichimaera’s compound eyes developed faster than the eyes of the modern crabs the team sampled. At their final size, their eyes make up about 16% of their body, equivalent to a person walking around with eyes the size of soccer balls.

Animals with compound eyes see the world in essentially pixels, with each aspect of the eye providing a separate pixel, says Ms. The higher the pixel count, the sharper the vision. The team’s analysis of Callichimaera’s eyes revealed that it had unusually sharp vision for a crab – close to clear, effective predators like dragonflies and mantis shrimp.

Dr Luque said: ‘No matter what this animal is doing, it has to actively use such big eyes. “They are a huge drag in the water, and they are vulnerable. So whatever the downside to such big eyes, they must be nothing compared to the pros.”

Combined with the flippers and slim body, these fast-growing and powerful eyes suggest that adult Callichimaeras were hunting for smaller creatures, said Dr. Luque. And they did so by maintaining their predatory larval form as adults, rather than making the final transformation into the flat and scaly shape favored by other crab species.

Callichimaera is also the youngest known fossil arthropod with both eyes and nerve tissue preserved. Most arthropods with fossilized brains come from sites half a billion years old, where a clear view of the animal’s visual processing systems is rarely available. “Usually, you can find elements of the brain, but the eyes disappear, or vice versa. But Callichimaera has both,” said Dr. Luque.

The Callichimaera production site may have more secrets to share.

“There’s a big gap in the fossil record, because we didn’t collect enough fossils or do a lot of fieldwork in the tropics,” said Dr Luque. “Usually these places are covered with vegetation and the weather rocks very quickly. Finding such delicate preservation there is opening new avenues to study the fossil specimen with fresh eyes. No pun intended.” This ancient crab has unusually large eyes

Fry Electronics Team

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