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. 2025 May;106(5):e70107.
doi: 10.1002/ecy.70107.

Nature's headlamps: A unique light-focusing structure in Parasesarma de Man, 1895 mangrove crabs

Peter A Todd  1 , Ian Zhi Wen Chan  1 , Wendy Yanling Wang  1   2 , Zuze Boh  1 , Hui Shan Soh  1 , Huiwen Huang  1
Affiliations

Nature's headlamps: A unique light-focusing structure in Parasesarma de Man, 1895 mangrove crabs

Peter A Todd et al. Ecology. 2025 May.
No abstract available

Keywords: 3D light model; animal communication; behavior model; binary choice experiments; brightness enhancement; facial bands; visual signaling.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Modeling the signaling behavior of Parasesarma eumolpe crabs. (A) A P. eumolpe individual with its facial band indicated by the white arrow. Scale bar: 10 mm. (B) Scanning electron micrograph image of the cross section of a facial band (indicated by the white arrow) reveals its concave macrostructure (and lack of microstructure). Scale bar: 100 μm. (C) Schematic of the model describing the signaling behavior between two crabs in terms of interaction distance, D, eye stalk length, L, incident and reflected light angles, θi and θr, and crab body angle, θc. (D) At typical interaction distances, D, of 50–500 mm, the optimal reflected light angle, θr, is between +0.5° and +5.1° (blue arrows). (E) A composite 3D light reflection computer model of the facial band of a P. eumolpe crab where light is simultaneously provided from nine different angles on a vertical plane (from −60° to +60° inclusive at intervals of 15°). The model simulates the paths of light rays reflecting off the band based on the curvature of its surface. (F) The appearance of the crab when viewed from an angle of 0° under each of the nine lighting angles. In each case, the greater intensity of light reflecting off the facial bands compared to other areas of the crab remains constant. Image and illustration credits: (A) Peter A. Todd, (B) Huiwen Huang, and (C, D) Zuze Boh.
FIGURE 2
FIGURE 2
The facial bands of both male and female Parasesarma eumolpe crabs focus the light they reflect within the 0°–5° range (highlighted in blue for males and green for females), which is optimal for enhancing signaling to conspecifics. Angles ±15° around the illuminating fiber‐optic cable could not be measured due to the diameter of the probes used (details in the Appendix S1). Illustration credit: Zuze Boh.
FIGURE 3
FIGURE 3
Behavioral choice experiments with Parasesarma eumolpe demonstrate that the crabs use their facial bands in signaling. (A) Both female and male specimens preferentially chose crab images with a brighter face band (p < 0.01). There was no significant difference between the responses of the two sexes (p = 0.26). (B) A significantly larger proportion of specimens made a choice in actual experimental trials (when images were presented to them) than in negative controls (where no images were presented) (p < 0.001), demonstrating that the majority of the specimens in our trials were actively making a choice. (C) Specimens preferentially chose androgyne crab images with facial bands that bore the color of the same sex, that is, females chose female bands and males chose male bands (p < 0.001). (D) The same behavior as in (C) was observed when live P. eumolpe crabs were used as stimuli for positive controls (p < 0.001). **p < 0.01; ***p < 0.001.
See this image and copyright information in PMC

References

    1. Boon, P. Y. , Yeo D. C. J., and Todd P. A.. 2009. “Sound Production and Reception in Mangrove Crabs Perisesarma spp. (Brachyura: Sesarmidae).” Aquatic Biology 5(2): 107–116.
    1. Brakefield, P. M. , Marples N. M., and Veelen W. V.. 1994. “The Relative Importance of Colour, Taste and Smell in the Protection of an Aposematic Insect Coccinella septempunctata .” Animal Behaviour 48: 967.
    1. Chan, I. Z. W. , Stevens M., and Todd P. A.. 2019. “PAT‐GEOM: A Software Package for the Analysis of Animal Patterns.” Methods in Ecology and Evolution 10(4): 591–600.
    1. Chan, I. Z. W. , Wang W. Y., and Todd P. A.. 2020. “Facial Band Colour in the Mangrove Crab Parasesarma peninsulare Shahdadi, Ng & Schubart, 2018 Plays a Role in Mate Recognition.” Estuarine, Coastal and Shelf Science 248: 106721.
    1. Endler, J. A. 1987. “Predation, Light Intensity and Courtship Behaviour in Poecilia reticulata (Pisces: Poeciliidae).” Animal Behaviour 35: 1376–1385.

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