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Many animals show conspicuous colour patterns that inform potential predators about their unprofitability due to their toxicity or unpalatability (aposematism). However, no systematic work has evaluated the colouration of crabs in terms of aposematism, even though many species exhibit conspicuous colouration. We conducted a laboratory study to compare predation risk from a fish predator, the barred mudskipper (Periophthalmus argentilineatus), in two sympatric fiddler crab species with different body colour patterns: Austruca perplexa with brownish-white colourations and Paraleptuca crassipes with red-and-black colour patterns. Fiddler crabs and mudskippers are active on tidal flats during low tides. A predation trial by stocking one crab and one mudskipper in a test arena was repeated five times for each species–sex combination. The animal behaviour was video recorded for 24 h, and crabs were confirmed to be alive or dead after 72 h. A. perplexa females were most vulnerable to predation from mudskippers. The attack intensity from mudskippers was stronger during the nighttime than daytime against P. crassipes females and males and A. perplexa males, suggesting that mudskippers may avoid them by visually detecting the body colouration in P. crassipes and the body structure in males of both species (i.e. a large weaponry claw) during the daytime. Mudskippers eventually preyed on and consumed some individuals in both species–sexes. Mudskippers are known to shift their main food components from small crustaceans through polychaete worms to crabs, such as fiddler crabs, and unpalatable polychaete worms tend to be brightly coloured. We suggest that the red colouration of P. crassipes may be incidental mimicry of the unpalatable red/orange-coloured polychaete worms that mudskippers might have learned to avoid. • We compared predation risk from mudskippers in fiddler crabs with different colours. • We employed brownish-white–coloured species and red-coloured species. • Females of brownish-white–coloured species were most vulnerable to predation. • Mudskippers avoided red-coloured species and males of both species. [ABSTRACT FROM AUTHOR]