Boon Peiya shared more insights into these colourful mangrove crabs which I've been seeing on Pulau Semakau and Sungei Buloh recently. First of all, the Face-banded sesarmine crabs are Perisesarma eumolpe and Perisesarma indiarum! The old name was Chiromantes eumolpe.
She also shared photos on how to tell the two species apart.
Peiya adds that P. indiarum has a more bluish tinge on the legs compared to P. eumolpe.
Peiya also shared a paper by Huang Huiwen et al which explores the relationship of these colourful face bands to species, sex and size.
In this study, the researchers found that males of both species had more intense blue facial bands, whereas green was more pronounced in female facial bands. These colour differences may play a role in sexual recognition within the species. Bigger females had more intense blue in their facial bands, suggesting that s indicates of their maturity (and possibly body condition). In large males, facial band colours contrast strongly against the surrounding mudflat and may play an important role in signalling to other males during territorial disputes or competition for females.
"Inter- and intra-specific variation in the facial colours of Perisesarma eumolpe and Perisesarma indiarum (Crustacea: Brachyura: Sesarmidae)" by H. Huang, P. A. Todd, D. C. J. Yeo in Hydrobiologia (2008) 598:361–371 DOI 10.1007/s10750-007-9169-z (Abstract on SpringerLink)
From their study I also learnt that Perisesarma crabs eat sediment, leaves of various mangrove trees, green algae and small invertebrates. The study suggests pigments from their food may contribute to their colouration and thus could be an honest signal of an individual’s foraging ability, age or state of health. Indeed, their study found the bands turned grey or black when the crabs are dead. Females with eggs also had duller bands.
In fact, Peiya did a study of what these crabs ate! In the paper by Boon Peiya et al, the researchers found both P. eumolpe and P. indiarum are mainly sediment grazers, but also feed on mangrove leaves and roots and occasionally animals. Both crab species prefer Avicennia alba leaves to other, locally common, mangrove species, i.e., A. officinalis, A. rumphiana, Rhizophora apiculata, Bruguiera gymnorhiza. There is, however, no significant preference for leaves of differing ages in the two species; and no difference in amount of fresh leaves eaten by the two crab species.
A point I found most interesting in Peiya's paper is that the genus Perisesarma comprises one of the highest biomasses of mangrove crabs and play an important ecological role. As they feed on mangrove leaves, they recycle nutrients in the mangrove forest. Quickly breaking down the leaves for others in the food chain to eat, e.g., animals that eat the fragments left over by the crabs, what comes out of the crab after it eats the leaves, and of course, the crab itself!
"Feeding ecology of two species of Perisesarma (Crustacea: Decapoda: Brachyura: Sesarmidae) in Mandai Mangroves, Singapore" by Boon Pei Ya, Darren C. J. Yeo, and Peter A. Todd in Journal of Crustacean Biology, 28(3): 480–484, 2008 (Abstract on BioOne)
I've updated the wild fact sheet on the Face-banded sesarmine crab with the information so kindly shared by Peiya. She adds that it's nice to let Singaporeans know the extent of research being done on our local flora and fauna. I do agree with her and thank her for sharing this information!