Crabs are one of the most diverse animals on the planet with a variety of difference species. Plus, there is the somewhat funny (but also somewhat true) theory that evolution ‘crabifies’ everything, meaning that over a long enough time period, all living things eventually evolve to become crabs. this is a process scientifically known as Carcinization.

So, when researchers in the mangroves of Singapore were looking at the face-banded crab (Parasesarma eumolpe), they likely weren’t all that surprised to learn that it had a very unique feature. They have the ability to reflect light from the bands of color on their face. This is a similar process that cats, alligators, and other animals do with the reflective material on the inside of their eyes. For these crabs, however, it is mounted externally. This is the only example of an externally mounted reflector currently known.

The bands of color on the face of these crabs are likely used in things like mate selection and competition between males. The researchers found that the colors will change based on a variety of factors including how much food they are getting and their overall health. So, if you are a strong male face-banded crab and you see another male that has very faded looking bands of color on their face, you can attack them knowing that they are in a weakened state.

The authors of a study published in the journal Ecology comment on this, saying:

“…this past research did not explain the anatomical basis of the exceptional facial band brightness we saw in the field. Understanding the mechanism of this brightness, potentially micro- or macro-morphological features, could help in determining the function of the crabs’ facial bands, for example, whether they are used for communication with conspecifics or otherwise, and thereby shed light on its evolution by pinpointing the trait that has presumably been selected for.”

As part of the study, the team took these crabs and immobilized them while keeping them in cold water. They then shined light on the bands of color on their faces. This would allow them to test the reaction of other crabs. Interestingly, they found that crabs were drawn to other crabs that had bright colors as well as those that had colors that mirrored their own. The authors of the study explained:

“Here, we demonstrate that the concave macrostructure of these bands acts in a manner analogous to the reflector of a headlamp, increasing their apparent brightness at the same range of angles that are optimum for signaling between two crabs. Scanning electron microscopy indicated no iridescence or structural coloration. These facial bands are, to our knowledge, the first example of light-focusing, external, macro signaling structures in animals.”

The current understanding is that this is the only species of crab that evolved in this way, making them an excellent opportunity for further research.

Thought that was fascinating? Here’s another story you might like: Why You’ll Never See A Great White Shark In An Aquarium