No, I’m not talking about unearthing some hidden family secret (you can exhale a sigh of relief, mom). Here I refer to fruit flies growing up in the dark as participants in a 62-year long experiment at Kyoto University in Japan. More than 1,500 generations of flies have been reared in total darkness ever since Syuiti Mori shut the blinds on his flies in 1954 starting one of the longest laboratory experiment on evolution.
In nature, there are actually quite a few animals who encounter absolutely no sunlight living deep in caves or the lowest depths of the sea. One such animal is the cave crayfish, a blind, pale, small lobster-like creature (please click over to Wildscreen Arkive for some great photographs that I am not able to share here). Cave crayfish have been found in caves all over the United States, including (much to my excitement) Mammoth caves in Kentucky, which just happens to be my next vacation destination! Cave crayfish are considered blind, but perhaps a more accurate term would be eyeless. That’s right, the cave crayfish have adapted to darkness not just by no longer using, but by losing their eyes.
Losing an eye, not accidentally but evolutionarily, is not something that happens overnight. Eyes are complex organs that develop specialized cells that work with neurons and muscles and require thousands of genes to properly form and function. Cave crayfish retain some muscle and neurons where their eyes should be, in primitive “eye buds”, but the cells are not nearly as numerous nor as complex as found in the eyes of their surface-dwelling crayfish cousins. Interestingly, these muscle and neuron cells are entirely functional when stimulated in a lab, begging the question of whether they still perform some sensory function for the blind crayfish.
Cave crayfish are exquisitely sensitive to vibration, which is a primary way they sense their environment. So it’s possible that facial neurons in their eye buds help them sense the presence of predators nearby. One hypothesis for having some functional muscles in the crayfish eye bud is that it might provide spatial orientation cues much like how the inner ear works in mammals. Such that, in addition to physical sensations, spatial cues through these sensors could allow a crayfish to know when it’s sitting flat versus at an incline on a rock in the pitch black–knowledge of which could save a crayfish’s life if a predator were to pounce.
Adding to what we’ve discovered about adaptation in the natural world, the 60+ year “dark-fly” experiment has also revealed some interesting findings on the evolution of the eye and sensory organs in a dark environment. For example, although the dark-fly looks very much like their light-fly counterparts (and retains eyes–for now), their sensory head bristles are longer. Much like Daredevil has “extra-sensory” hearing, perhaps the dark-fly has enhanced physical sensation to compensate for its lack of sight.
With modern DNA sequencing methods, genetic factors involved in dark adaptation may now be identified and explored. In the Kyoto groups’ most recent analysis, they pinpointed 100 genes that have changed in the dark-fly. It will be fascinating to understand the role these genes play in evolution and I very much look forward to these researchers shedding some light on the dark side.
1. Catherine Offord. Notebook: Feeling Around in the Dark. The Scientist Magazine, May 1, 2016.
2. Mellon and Lnenicka. Structure and electrical properties of eye muscles in cave- and surface-dwelling crayfishes. Journal of Experimental Biology, 84:187, 1980.
3. Izutsu et. al. Dynamics of Dark-Fly Genome Under Environmental Selections. G3 (Bethesda) 6:365, 2015.