Mating?

[thedigitalimager]

I took this photograph several years ago in the Red Sea. I came upon these two while walking the reef top looking for dead shells. I always assumed that they were mating, but I'm no expert. Also, I'm curious to know why one maintained a dark coloration and the other light. They stayed this way the entire time I observed them which was almost an hour. Any suggestions or opinions?



Thanks, Geary