Abstract The present study investigated the population of the xanthid brachyuran crab, Macromedaeus distinguendus (De Haan, 1835), at a toxic hydrogen sulfide, shallow hydrothermal vent area at Turtle Island, off northeastern Taiwan. This is the first record of this species from this extreme environment, and M. distinguendus is only the second brachyuran discovered in this area. The wet weight of male and female M. distinguendus varied from 0.11 to 1.22 g (mean ± standard deviation: 0.56 ± 0.39 g) and 0.06 to 0.62 g (0.37 ± 0.15 g), respectively. The wet weight: carapace width ratio was significantly correlated in both sexes. The local distribution patterns of M. distinguendus were different from those of the obligate hydrothermal crab Xenograpsus testudinatus N. K. Ng, J.-F. Huang & Ho, 2000 (Xenograpsidae) in this area. An analysis of resource use and habitat partitioning revealed that the two crab species were able to co-exist within the study area.