Recent observations of unusual mass stranding and mortality of two Indian Ocean crustacean species, the swimming crab Charybdis smithii and the mantis shrimp Natosquilla investigatoris, are documented and analysed. Strandings of C. smithii were observed for the first time in the equatorial Indian Ocean, the main area of its pelagic distribution. Strandings of mantis shrimps are reported from throughout the western Indian Ocean; occurrences of mass stranding in the Maldives Archipelago mark an extension of the known range of N. investigatoris into the central Indian Ocean. Mortality of crabs probably represents a ‘catastrophic event’. In contrast, mantis shrimp strandings, which were always associated with a sudden increase of its biomass (‘blooms’), are apparently post-reproduction mortalities indicating potential semelparity for this species.