The crab is spotted

June 2, 2016 • 10:30 am

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.

Crab on wet sand, Playa Pan Dulce, Osa Peninsula, Costa Rica
Crab on wet sand, Playa Pan Dulce, Osa Peninsula, Costa Rica

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.

Hermit crabs (Coenobita) on Playa Pan Dulce, Osa Peninsula, Costa Rica
Hermit crabs (Coenobita) in periwinkle shells on Playa Pan Dulce, Osa Peninsula, Costa Rica

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.)

Playa Pan Dulce, Osa Peninsula, Costa Rica
Playa Pan Dulce, Osa Peninsula, Costa Rica

If anyone can help identify the camouflaged crab, the periwinkle (?Littorina sp.), or the species of the hermit crab, please weigh in.

6 thoughts on “The crab is spotted

  1. I am going to guess that the hermit crab is Pagurus, and I only know the genus b/c of the fantastic childrens’ book “Pagoo, which is about a hermit crab. Even adults would enjoy that book.

  2. Lounging o a beautiful beach in the Maldives, my late wife and I watched a hermit crab move to a new shell. It spent about twenty minutes examining its intended new home, than took less than a second to actually transfer. We did not know that they could move so fast.

  3. The crab is spotted, indeed! Also if you double click and get a good look at it, the crab is seen to have a pattern of another crab on its back! That was a neat feature I had not noticed before.

  4. Have you ever been outbluffed by a crab?

    I was wandering across the rocks one day and a number of fair-sized (4″) crabs were sunning themselves – of course they popped out of sight the instant they saw me. If I stood very still, they cautiously and very slowly re-emerged.

    So, moving like a leopard stalking its prey, I very quietly crept to within a few feet of where the largest one had been and sat down as motionless as a rock and waited. And very slowly and hesitantly, over the space of five minutes, Mr Crab inched out into full view. We looked at each other for several minutes. Eventually, tiring of this, I just stood up to go – and the crab didn’t move. I jumped up and down and shouted at it – and still it didn’t move. Feeling a lot less important, I slunk off to my car.


  5. Incidentally, was the sand unusually dense? That degree of darkness often goes with a high proportion of magnetite/ ilmenite grains compared to the (nearly) white quartz and feldspar grains more often seen.
    Asking if it’s a “volcanic” region isn’t much help because most of the western side of the Americas is volcanic to some degree or another (California and it’s environs getting relief at the expense of having lots of earthquakes instead.)
    [Googles] Volcan Baru, just over the Panama border. Ohh, I see a city built on a lahar (volcanic mudflow) fan – that’s not healthy. More volcanoes further north.

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