For an animal that lives its whole life in a deep, totally dark cave, eyesight is pretty useless. And for an animal that doesn't see -- or even have eyes -- the parts of the brain that enable vision are just wasted space. That's why cave-dwelling creatures that have slowly lost their vision over time are now ditching the the visual centers of their brain as well, according to study published Tuesday in BMC Neuroscience.
The researchers took three blind, cave-dwelling species of Malacostraca, the large crustacean group that includes common crabs and shrimp, and compared their brains to living relatives and known ancestors. Unlike the surface-dwelling crabs we snack on, these crustaceans have lived in dark caves for long enough that they've shed their eyes. Some still possess eye stalks where their ancestors once kept peepers, but the tiny creatures gave up vision long ago.
The animals were preserved rather than living, so the team could not observe their tiny brains in action. But by looking at the physical shape of the brain, and making comparisons with what we know about how the brain works in their evolutionary relatives, the researchers were able to assign jobs to the various lobes, lumps and spindly structures they could see under the microscope.
Once they'd pinpointed the parts of the brain that should be used to see, the researchers were able to assess how they'd changed since diverging from a common ancestor with seeing crustaceans. In just 200 million years -- a relatively short time, in the evolutionary scheme of things -- the lobes of the brain associate with vision shrank away notably. Meanwhile, regions of the brain devoted to touch and smell stayed the same, or even got bigger.
That indicates that the brains of these species got the memo fairly quickly and ditched unnecessary vision centers.
"The reduction is much more dramatic than for other crustaceans of this group," lead author Martin Stegner, of the University of Rostock told the BBC. "It's a nice example of life conditions changing the neuroanatomy."
Brain atrophy isn't the only energy saver a blind cave-dweller can look forward to. A previous study found that some species of blind, cave-dwelling fish -- who've lost their eyes and pigment, both unnecessary in the dark -- have given up the normal circadian rhythms that make our bodies behave differently based on time of day. The change made them much more efficient than seeing fish, whose metabolic systems were still slave to the light-dark cycle.
The new study is a nice reminder of the fact that the living world is constantly evolving. The changes may occur too slowly for us to catch, but that doesn't mean we aren't still subject to natural selection. The rigors of life still pressure animals to take on new adaptations and ways of life, changing the form of their descendants for better or worse.
You can read more about the findings in Stegner's blog post for BMC.
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