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Friday, December 2, 2022

The eyes of this ancient crab were unusually large

Modern adult crabs are not particularly well-known for their swimming skills or their ability to see in the dark. Their movements throughout the seafloors are almost entirely dependent on little eyes as they forage or graze on the seafloors of silence.

Although it is thought to have existed 95 million years ago, a unique crab flitted with amazing elegance through the tropical seas of what is now Colombia. The species, Callichimaera perplexa, was about the size of a quarter, with flat, oarlike legs and a keeled body that reminded me of a spider. It also had a large set of eyes, which was a plus.

In a research published last month in the journal iScience, a group of palaeontologists demonstrated that Callichimaera’s eyes were exceptionally acute, indicating that the creature lived as a free-swimming predator and had incredibly fine eyesight.

The species was first identified in 2005 by Javier Luque, a Colombian palaeontologist who is currently a research fellow at Harvard University. Luque was the first person to find it. His research into fossil-rich rocks in Colombia’s Boyacá department, which is how the country’s regions are termed, began when he came upon an outcrop that included a collection of intricately detailed fossil arthropods while still an undergraduate.

After years of work, Dr. Luque and his colleagues amassed over a hundred specimens, many of which were exceptionally well-preserved. “It’s a really large sample size,” Dr. Luque said emphatically. “It’s not often that you have 100 specimens.”

Seven of the specimens had detailed eyes that could be seen with the naked eye. Dr. Luque said that these, however, provided a dilemma since living crabs normally have small compound eyes that are located at the end of a stalk and covered by a protective orbit. The compound eyes of Callichimaera were huge and lacked a stalk and orbits.

Crabs go through a number of developmental phases, starting as small, shrimp-like animals and eventually developing into their ultimate armoured forms. Young crabs are free-swimming predators with comparatively large eyes during the last larval stage, the megalopa, which occurs in the last larval stage. When Dr. Luque and his colleagues published their official description of the fossil in 2019, they speculated that it may have been a crab in its last larval stage, although an extraordinarily enormous one, at the time.

Ms. Jenkins and Dr. Luque utilised the plethora of Callichimaera specimens available to them to piece together a development chronology in order to find out how the creature utilised its eyes. They compared this to 14 extant species from throughout the crab family tree to see which was the most similar. Their surprise was that, unlike other crab species, Callichimaera kept its huge eyes well into maturity, something they had not expected.

They discovered that Callichimaera’s compound eyes developed at a higher rate than the compound eyes of contemporary crabs that they had collected for comparison. Their eyes took up almost 16 percent of their whole body volume at their ultimate size, which is the equivalent of a human walking around with eyes the size of soccer balls.

Animals with compound eyes, according to Ms. Jenkins, have an essentially pixelated vision of the environment, with each aspect of the eye transmitting a different pixel of information. With a larger pixel count, the eyesight becomes sharper as well. According to the team’s research of Callichimaera’s eyes, the crab has extraordinarily fine vision for a crab, putting it on par with other efficient, clear-eyed predators such as dragonflies and mantis shrimp.

Dr. Luque believes that when these fast-growing, strong eyes are joined with the paddle legs and streamlined body, it suggests that adult Callichimaera hunt on smaller species. And they were able to do so by retaining their predatory larval form throughout maturity rather than undergoing the final change into the flat and scuttling shape preferred by the majority of other crabs.

Callichimaera is also the earliest-known fossil arthropod to have both eyes and retained brain tissue, making it the youngest-known fossil arthropod. The majority of arthropods with fossilised brains originate from locations that are more than half a billion years old, making it difficult to gain a clear look at an animal’s visual processing system in such ancient environments.

As Dr. Luque said, “there is a large vacuum in the fossil record because we are not collecting nearly enough fossils or doing nearly enough research in the tropics.” “Normally, these areas are densely vegetated, and the rocks deteriorate quickly due to the weather. The discovery of such excellent preservation is opening up new opportunities for researchers to look at the fossil record with fresh eyes. “There was no irony intended.”

A Boyle
I cover Science related topics for The National Era
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