This Ancient Crab Had Unusually Huge Eyes

To figure out how Callichimaera used its eyes, Ms Jenkins and Dr Luque used the abundance of Callichimaera specimens available to put tog...

To figure out how Callichimaera used its eyes, Ms Jenkins and Dr Luque used the abundance of Callichimaera specimens available to put together a growth sequence. They compared this with 14 living species from the entire 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 the compound eyes of Callichimaera grew faster than those of modern crabs sampled by the team. At their final size, their eyes took up about 16% of their body, the equivalent of a person walking around with eyes the size of a football.

Animals with compound eyes have an essentially pixelated view of the world, Ms Jenkins said, with each facet of the eye delivering a distinct pixel. The higher the number of pixels, the sharper the vision. The team’s analysis of Callichimaera’s eyes suggests it had unusually sharp vision for a crab – closer to efficient, light-eyed predators like dragonflies and mantis shrimp.

“Whatever this animal is doing, it must have been actively using such large eyes,” Dr. Luque said. “They are a huge drag in the water and they are vulnerable. So whatever disadvantages exist for such large eyes, they should be nothing compared to the advantages.

When combined with the paddle legs and streamlined body, Dr. Luque said, those powerful, fast-growing eyes suggested that adult Callichimaera fed on smaller creatures. And they did this by retaining their predatory larval form until adulthood, rather than making the final transformation into the flat, scuttled form favored by other crabs.

Callichimaera is also the youngest known fossil arthropod with both eyes and preserved neural tissue. Most brain-fossilized arthropods come from sites half a billion years old, where good insight into an animal’s visual processing system is rare. “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 probably has many more secrets to share.

“There’s a huge void in the fossil record because we don’t collect enough fossils or do a lot of fieldwork in the tropics,” Dr Luque said. “Usually these places are covered with vegetation and the rocks weather very quickly. Finding such exquisite preservation opens up new avenues for studying the fossil record with fresh eyes. No pun intended.”

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Newsrust - US Top News: This Ancient Crab Had Unusually Huge Eyes
This Ancient Crab Had Unusually Huge Eyes
Newsrust - US Top News
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