About 95 million years ago, the chimera crab lived in the warm, tropical waters of what is now Colombia during the mid-Cretaceous period. The tiny crab was already known to have efficient, paddle-like legs, but a new study shows its eyes accounted for 16 percent of its body. If a human sported this eye-to-body ratio, they would have eyes the size of dinner plates.
In combination with its streamlined body features, the new discovery about its eyes suggests the coin-sized crustacean was a predator with sharp vision, reports Phillip Kiefer for Popular Science. In a study published this month in the journal iScience, scientists compared nearly 1,000 fossils and living crabs to create a growth sequence that tracked the ancient predator's development, reports the New York Times. Some of the fossils were so well-preserved that researchers were able to discern what the shape of the nerves connecting the eyes and brain looked like.
"I'm 5'2". If my eyes were this big, they'd be a little over 9 inches in diameter," Kelsey Jenkins, a graduate student at Yale University and the study's first author, tells Jim Shelton for Yale News. "If something has eyes this big, they're definitely very highly visual. This is in stark contrast to crabs with tiny, vestigial eyes where they may only be 1 to 3% of the animal's body size."
When first described in 2019, researchers suspected the crab was in its last larval stage. Crabs in this stage are young, free-swimming predators with large eyes and stay in this form before growing into its large, armored final form, reports Asher Elbein for the New York Times. In the new study, researchers found that the crab actually retained its bulging eyes into adulthood.
"Larval crabs have all these swimmy features, and they're just floating in the water column like plankton," Jenkins tells Popular Science. "It's not until they become adults that they take on more adult features. This crab just looks like a giant baby."
Paleontologists also revealed that the eyes grew faster than most modern crabs, which means it could see almost as well as a dragonfly and more sharply than a mantis shrimp, Popular Science reports.
"Crabs whose eyes are growing very quickly are more visually inclined — likely they're very good predators who use their eyes when hunting — whereas slow-growing eyes tend to be found in scavenger crabs that are less visually reliant," Derek Briggs, an invertebrate paleontologist and one of the study's authors said to Yale News.
In combination with the chimera crab's clear vision and fast-swimming body, the research team suspects the crab was a predator that zoomed through sunlit water to catch its prey. Instead of transforming to a flatter, adult-like shape seen in modern crabs, its baby-like features seemed to be an advantage for the critter, the New York Times reports.
"We don’t have any crabs that are swimming as well as this thing probably was,” Jenkins tells Popular Science. “It’s almost like a lost lifestyle.”