NUS researchers shed light on local mangrove crab's electric blue strip

Light-focusing band believed to be first such external structure used by animals for signalling

- Zachary Lim

A black mangrove crab peers from a muddy bed of fallen leaves in Mandai Kechil, its striking cyan "monobrow" set in an alien-like glare.

The piercing blue strip on the face-banded crab (Parasesarma eumolpe) is neither bioluminescent—it does not glow in the dark—nor iridescent, meaning that its colours do not shift at different angles.

Instead, researchers here have discovered, it is the band's concave shape that focuses reflected light to enhance brightness, much like the reflective surface behind the bulb in car headlights.

What is more, no matter what angle the source light comes from, the crab's band reflects light most intensely at the eye level of other face-banded crabs to optimise signalling, or conveying information about itself.

The face-banded crab—a small freshwater crustacean with a shell typically between 2.5cm and 4cm wide commonly found in Singapore, Malaysia, Indonesia, Thailand and China—has been the focus of over a decade of research by scientists from NUS.

NUS researchers Peter Todd and Jan Chan, from the university's Department of Biological Sciences, believe this is the first known example of such light-focusing external structures used for signalling in animals.

Their new study, co-authored with four other researchers from the department, details the band's structure that they term "nature's headlamps"—finally shedding light on the mystery behind its luminance and brightness.

Similar reflective features have previously only been documented in the eyes of animals, like dogs, cats and crocodiles, but these enhance vision in the dark and are not used for signalling.

"It is exciting that such a structure has evolved to enhance signalling in these crabs but has not been described previously," said Associate Professor Todd.