Regional color variation in a marine crab species

by B. N. Sullivan

Last month we talked about some fishes that have color phases that change with developmental stage. We pointed out that these variations sometimes make it difficult to identify a species by color alone.

The same can be said for some kinds of marine invertebrates. In fact, some invertebrate species also vary in color from one location to another.

The crabs pictured on this page resemble each other in shape and size. Each is red with a carapace about three inches (8 cm) across. The shape of their bodies, appendages and claws are virtually identical; so is the spacing and positioning of the retractable eyestalks.

You could guess straightaway that they belonged to the same family (Xanthidae). Their similarity also might prompt you to surmise that they belonged to the same genus; but if you hesitated to identify the two as the same species, you would be forgiven. Although they both are red, the first crab has a much richer color, and a more striking pattern of spots on its carapace and legs, while the second crab is considerably paler, and is mottled with light grayish spots that look quite different than the darker spots on the first crab.

I photographed the first crab years ago in the Red Sea. By poring through published reference books, I was able to identify the species as Carpilius convexus.

Much later, and thousands of miles away, I photographed the second crab in the Pacific Ocean off the coast of Hawaii. When we first looked at the photo, we commented that it resembled Carpilius convexus from the Red Sea, so we assumed it must be a relative. What we did not realize at first was that the second, lighter colored crab was indeed a member of the same species.

It turns out that the coloration of each of these crabs is typical of the species in each location. If you look at reference photos from the Red Sea, you will see pictures of the darker red version with the well-defined spots. If you look at reference photos from Hawaii, you will see the paler colored crab with the grayish mottling.

These color variations are quite consistent within the populations at each location, but what we don't know is why the species developed these external differences. If you believe, as we do, that these kinds of characteristics evolve over time and become prevalent for reasons that relate to survival, then you have to wonder what those reasons might be.

Could it be that the richer coloration of the Red Sea variant somehow reflects its colorful environment? In the Red Sea, this crab species lives among very colorful Nephtheid soft corals, so perhaps its brighter coloration helps it to 'blend' better with its surroundings.

Conversely, the paler color variant may be more suited to living -- and hiding -- among the drabber hard corals that predominate on Hawaii's reefs. Here in Hawaii we have no brightly colored soft corals like the Nephtheids in the Red Sea.

This is conjecture on our part, of course. There may be other explanations for the regional color variation: diet perhaps? Maybe one day we will know the answer.

About the photos: Both photos show adult crabs of the same species, Carpilius convexus. The photo at the top of the page was taken in the Red Sea at Ras Um Sid, near Sharm el Sheikh, Egypt. And yes, the poor critter has lost one cheliped (arm). The second photo was taken in the Pacific Ocean off the coast of Hawaii's Big Island, near Puako.