Crab discovered with disco light on its head

A colorful mangrove crab can flash light beams from its eyes—at least, that's how it appears to observers. In fact, light beams are reflected and focused by special structures on the crab's head—similar to the curved reflective surfaces behind car headlights, as a team of researchers reports in the journal Ecology . The signals are therefore intended for potential partners or rivals.

Researchers led by Peter Todd from the National University of Singapore observed crabs of the species Parasesarma eumolpe in a mangrove forest in Singapore and then brought them back to the laboratory. There, the animals, which are only a few centimeters long, were illuminated with light on the colorfully pigmented bands on their faces, which look a bit like eyebrows. Males have blue stripes, females green, each about six millimeters wide and 1.6 millimeters high.

The researchers explain that most of the light striking such a stripe is reflected forward as a tightly focused beam. When a crab stands at a typical distance of 5 to 50 centimeters from a conspecific, a large portion of the rays reach its eyes. The stripe then appears significantly brighter than the rest of the other crab.

Conspecifics with bright shining stripes are particularly popular

In laboratory experiments, individual crabs were shown images of conspecifics with such reflections. They therefore prefer to approach animals with particularly bright stripes. The team also knows a possible reason: The pigments of the reflective zones come from carotenoids in the diet. They fade when the crabs are starved, thus providing an indication of the animal's physical condition. Those who glow brightly probably appear particularly fit and attractive.

The species Parasesarma eumolpe is widespread in mangrove forests along the Indo-Pacific coast, the scientists explain. Especially in such low-light environments, it could be advantageous to deliberately increase the intensity of visual signals, they write. The crabs can transmit their messages quite far from safe hiding places – thus reducing the risk of being preyed upon by one of the many predators.

Colors play an important role in the animal kingdom in camouflage, mate recognition and choice, as well as competition between rivals. Given the potential advantages of light-focusing reflective structures, it's surprising that similar structures haven't been found more frequently in animals, says Todd's team. Perhaps in many cases, it's more efficient to amplify a signal by using the entire body surface—as in the seven-spot ladybird, whose entire back is dotted. Alternatively, it might be more advantageous to develop structures that improve signal detection—for example, highly sensitive eyes for specific light ranges.