TAMPA, FLORIDA—The pinnacle of beauty to most people is a symmetrical face, one without any major left-right differences. But for blind Mexican cavefish (Astyanax mexicanus), asymmetry may be a lifesaver. That’s because their lopsided skulls may help them feel their way along dark cave walls—similar to a person navigating by touch in the dark. That behavior, presented here this week at the annual meeting of the Society for Integrative and Comparative Biology, suggests being a little “off” can have evolutionary benefits.
Lots of cave dwellers are a bit unbalanced. Cave fish tend to have one eye that is larger than the other, for example, and cave crickets have different size antennae. Some researchers wondered whether left-right differences might help these creatures get around.
They scanned the skulls of A. mexicanus fish from three caves in Mexico. Their computerized tomography scans revealed most fish skulls bent slightly to the left, giving the right side of their faces slightly more exposure. Other tests showed these fish tended to drift along the right-hand side of cave walls, presumably using the larger side of their faces to feel their way in the dark.
Next, the researchers counted mechanical sensors known as neuromasts in the heads of embryonic fish. These sensors, or “nerve buttons,” detect water flow and sometimes vibrations. Blind fish had more—and larger—neuromasts than fish of the same species that lived on the surface, they reported.
To discover the genetic basis for the difference, they then bred the surface fish with the cave fish; those descendants with extra neuromasts tended to have a higher expression of a gene called Mn1. In mice, defective Mn1 leads to abnormal facial bones. In the fish, the extra neuromasts usually show up where bones have grown cock-eyed, leading the scientists to think neuromasts may somehow cause developing bone to misform.
That could have implications for fish, mice, and even humans, who are affected by 700 so-called “craniofacial” syndromes. As the researchers continue to investigate the connection between neuromasts, bone asymmetry, and behavior, they hope to clarify how developing sensory and skeletal tissues interact to cause irregularities in the skull—some of them more useful than others.