Who needs eyes when you spend your whole life in completely dark caverns where the sun don’t shine? Researchers studying rare little cave-dwelling crustaceans have discovered that not only have they lost their eyes, it appears that the visual center in their brains have been shrinking dramatically over evolutionary time. The findings, published in BMC Neuroscience last week, is a very cool example of how changing ecological conditions can affect neuroanatomy.
Martin Stegner of Universität Rostock and colleagues studied three blind cave-dwellers from the crustacean group Malacostraca, which includes crabs, lobsters, and shrimp. These three inaccessible species—Spelaeogriphus lepidops collected from Table Mountain in South Africa, Mictocaris halope from Deep Blue Cave off the coast of Bermuda (pictured below), and Tethysbaena argentarii from Monte Argentario, Italy—live in complete darkness and have no eyes. Although, two of them do have rudimentary eyestalks that may have once bore an eye or two.
After scuba divers and cave explorers brought the animals back to the lab, the team combined thin tissue sectioning, immunolabeling, microscope work, and 3D modeling to describe their neuroanatomy. Like all crustaceans, their brains are located in the middle of their head and divided into three parts: protocerebrum, deutocerebrum, and tritocerebrum. Their optic center is in the protocerebrum, and in most other species, it’s made up of four substructures called neuropils.
“In a striking contrast, the optic center in the protocerebrum of S. lepidops is constituted by only a single neuropil, whose orientation toward the tip of the eye stalk is the only hint at a former visual-coordinative function. Optic nerves are absent,” Stegner writes in a BMC blog. There’s a comparable single neuropil in M. halope and T. argentarii, but it’s oriented away from the eyestalk. They think that, instead of the rudimentary optic center of S. lepidops, these two may have lost their optic center completely.
"The reduction is much more dramatic than for other crustaceans of this group," Stegner tells BBC. "It's a nice example of life conditions changing the neuroanatomy."
In fact, their most prominent nerves are connected to their first and second antennae, which help them detect food in the water. These long antennae are also equipped with numerous sensory hair-like structures, and some of them have additional chemosensory structures at the tip of the first antenna. Furthermore, in all three species, the olfactory lobe (devoted to the sense of smell) seems to have expanded in contrast.
The little critters had to have lived in pitch black for a long time for their brains to reduce the unnecessary parts—it may have taken as long as 180 million years.
Images: 2015 Stegner et al., BioMed Central (top), Tom Iliffe (middle)
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