NUS Study Unveils Mechanism Behind Face-Banded Crab's Blue Strip

SINGAPORE – Researchers from the National University of Singapore (NUS) have made a groundbreaking discovery regarding the striking electric blue strip found on the face of the face-banded crab (Parasesarma eumolpe), a species commonly inhabiting mangrove environments. This study, led by Associate Professor Peter Todd and Dr. Ian Chan from the Department of Biological Sciences, reveals that the blue band’s concave shape acts as a natural reflector, enhancing brightness without relying on bioluminescence or iridescence. The findings were published in the 2025 edition of the Journal of Crustacean Biology.

The face-banded crab, typically measuring between 2.5 cm and 4 cm in width, is prevalent in mangrove ecosystems across Singapore, Malaysia, Indonesia, Thailand, and China. Unlike known examples of reflective structures in animal eyes, which aid in vision, the blue strip serves as a signaling mechanism, optimizing communication among crabs in their dimly lit habitat. According to Professor Todd, “The band reflects light most intensely at the eye level of other face-banded crabs, facilitating effective signaling.”

This research marks a significant advancement in the understanding of animal signaling mechanisms. The study's implications extend to behavioral ecology, as the researchers suggest the brightness of the bands may be crucial for territorial defense, mate attraction, and predator deterrence.

Past research has shown that male face-banded crabs typically display blue bands while females exhibit green. The coloration is attributed to carotenoids sourced from their diet, primarily consisting of decaying mangrove leaves and organic matter, rather than structural coloration. “Our findings indicate that the vibrancy of these bands correlates with the crabs' health, as they fade during starvation and regain brightness when adequately fed,” stated Professor Todd.

The research team conducted field studies at Mandai Kechil mangrove, meticulously recording interactions among a sample of 30 male and 26 female crabs. They faced challenges in accurately measuring the reflectivity of the bands due to their small size, which necessitated the development of a specialized probe. This innovative approach involved modifying an existing reflectance spectrometer to obtain precise readings of light reflected from the facial bands at various angles.

The study also included controlled experiments demonstrating that crabs preferentially approached images of their kind with brighter bands. This behavior suggests that the brightness of the bands plays a significant role in visual communication, enhancing the crabs’ ability to relay information to one another. Interestingly, some crabs exhibited same-sex attraction towards individuals with similar colored bands, a behavior that remains unexplained and contrasts with findings in closely related species.

Professor Todd emphasizes that this discovery raises new questions about communication among face-banded crabs, remarking, “In science, we must always have more questions when we find answers. While our findings are fundamental, they open avenues for future research into the nuances of crab communication.”

As the study concludes, the researchers plan to delve deeper into understanding the specific messages conveyed through the crabs' vibrant bands. This research not only broadens the scientific understanding of animal signaling but also highlights the importance of preserving coastal ecosystems that support such unique species. The findings underscore the interconnectedness of dietary habits, health, and communication in the animal kingdom, paving the way for further explorations in behavioral ecology and the adaptations of marine life.

The ongoing efforts of researchers at NUS exemplify the institution’s commitment to advancing knowledge in biological sciences and ecological conservation, ensuring that such critical studies continue to contribute to our understanding of biodiversity and environmental sustainability.