Crab Signaling 

DISCOVERIES 

Crab Signaling 

Some animals have reflectors inside their eyes, like cats and crocodiles, to help them see better in low-light conditions.  

Now, scientists have found that a species of crab has developed a built-in headlamp structure on its face that brightens its visual signals in the dark mangrove forest where it lives, to help it communicate with other crabs. 

“These facial bands are, to our knowledge, the first example of light-focusing, external, macro signaling structures in animals,” researchers wrote in a new study. 

Previous research had already noticed unusually bright facial bands in the crabs (Parasesarma eumolpe), with pigments derived from diet-dependent carotenoids, and that the colors fade when crabs are starved and brighten when they are fed. It was not clear, however, how the color remained bright in the dark. 

Understanding what the bands are used for is important to trace how they have evolved in the crabs, and researchers speculated that they could need them for social interactions. 

To study how facial bands convey increased brightness and why, researchers observed the behavior of 56 adult crabs at the Mandai Kechil mangrove in Singapore. Here, they measured body orientation, signaling angles, and spatial distances between interacting individuals. 

This showed that the concave structure of these bands works like a headlamp reflector, boosting their brightness at the ideal angle for crab-to-crab signaling. 

Then, researchers used 10 crabs of each sex to test whether brightness and band colors played a role in influencing the crabs’ ability to communicate using their facial bands. They used a spectrometer to shine light on the colored bands on their face, which scientists already knew were blue for males and green for females. 

The study found that crabs were more likely to approach crabs with brighter bands and preferred band colors that matched their own. 

Researchers also realized that most of the light striking the bands is reflected forward in a narrow, concentrated beam. When crabs face each other at the typical interaction distance, which is between 2 and 20 inches, most of the light is in the other crab’s eyes, which makes the face bands seem much brighter compared with the rest of the crab, according to Science. 

These facial bands are not only used as visual markers but also as optical devices, directing light in a specific direction to improve signal detection in dark environments. 

The study didn’t decode the meaning encrypted in the band messaging but researchers say that the dimly lit environment of the mangrove forest likely favored this angular signal design, which allows crabs to communicate from sheltered positions, reducing the risk of encountering predators.