This Ancient Crab Had Unusually Huge Eyes

To figure out how Callichimaera used its eyes, Ms. Jenkins and Dr. Luque used the abundance of available Callichimaera specimens to put together a growth sequence. They compared this with 14 living species from across the crab family tree. They were surprised to find that — unlike other crab species — Callichimaera retained its large eyes into adulthood.

In fact, their calculations showed that Callichimaera’s compound eyes grew faster than those of the modern crabs the team sampled. At their final size, their eyes took up around 16 percent of their body, the equivalent of a person walking around with eyes the size of soccer balls.

Animals with compound eyes have an essentially pixelated view of the world, Ms. Jenkins said, with each facet of the eye delivering a separate pixel. The higher the pixel count, the sharper the vision. The team’s analysis of Callichimaera’s eyes suggests that it had unusually sharp vision for a crab — closer to efficient, cleareyed predators like dragonflies and mantis shrimps.

“Whatever this animal was doing, it must have used such big eyes actively,” Dr. Luque said. ‘They’re a huge drag in the water, and they’re vulnerable. So whatever drawbacks that are there for such big eyes, they must have been nothing compared to the advantages.”

When combined with the paddle legs and streamlined body, Dr. Luque said, these fast-growing, powerful eyes suggested that adult Callichimaera preyed on smaller creatures. And they did so by maintaining their predatory larval form into adulthood, rather than making the final transformation into the flat and scuttling shape favored by other crabs.

Callichimaera is also the youngest-known fossil arthropod with both eyes and preserved neural tissue. Most arthropods with fossilized brains come from sites that are half a billion years old, where it is rare to get a good look at an animals’ visual processing system. “Usually, you can find elements of the brain, but the eyes are gone, or vice versa. But Callichimaera has both,” Dr. Luque said.

The site that produced Callichimaera is likely to have many more secrets to share.

“There’s a huge gap in the fossil record, because we’re not collecting enough fossils or doing much fieldwork in the tropics,” Dr. Luque said. “Usually these places are covered in vegetation, and the rocks weather very fast. Finding such exquisite preservation there is opening new avenues to study the fossil record with fresh eyes. No pun intended.”

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