by Greg Mayer
I think most readers probably spotted the crab (which happens to be spotted) rather quickly, in part because it was in the dead center of the photo, which is how I deliberately composed the shot. More than a challenge to readers’ spotting ability, I wanted to illustrate the delightfully exact camouflage that the crab’s spotted pattern achieved on the sand background. If you look at the original post, you’ll note that the crab has stopped near the edge of the crest of the little dry sand hillock it has ascended. Because the crab’s front side is raised higher than its posterior, the light, coming from behind the crab, casts a slight shadow to the front of the crab. This shadow would ordinarily make the crab stand out a bit as a dark spot. But because it is near the crest, its shadow connects with the shadow that the crest casts in the lee of the light, and the crab is less visible. I wondered whether it chose that position deliberately to aid its camouflage, but my observations were insufficiently prolonged for me to determine whether its position was intentional or accidental.
After allowing me to take a number of pictures of it atop the dry sand hillock, the crab moved off to wet sand, and its camouflage is nearly as good.
The lightness of the crab’s underparts makes it a bit more visible on the darker wet sand, but it seems to my eye that the dark spots of the crab have darkened somewhat, enhancing its camouflage. Again, my observations were not extensive enough to resolve the question of color change, but blowing both pictures up, and comparing them side by side, the spots do indeed look darker on wet sand.
More common than the well-camouflaged crabs were hermit crabs (Coenobita sp.), most of which were in the shells of periwinkles, live specimens of which were common lower in the intertidal zone.
Finally, in this last picture, I’m not sure if the camouflaged crab, or one of its companions, is present– it’s a real spot the crab problem, since I don’t know if there is one. (The hole, and accompanying excavated hillock of dry sand, I suspect is the work of a larger, unseen crab.)
If anyone can help identify the camouflaged crab, the periwinkle (?Littorina sp.), or the species of the hermit crab, please weigh in.