Abstract. The occurrence of hypoxia becomes more prevalent in coastal and marine waters due to ocean warming and human-induced eutrophication. While hypoxia is expected to hamper calcification via metabolic depression, recent studies showed that some calcifying organisms can maintain normal shell growth. The underlying mechanism is unclear, but may be associated with energy reallocation or mineralogical plasticity which reduces the energy demand for calcification. We tested the hypothesis that shell growth can be maintained under hypoxia by compromising the mechanical strength of shells as the trade-off, or changing the mineralogical properties of shells. The respiration rate, clearance rate, shell growth rate and shell properties of a calcifying polychaete (Hydroides diramphus) were determined under normoxia or hypoxia in two contexts (life-threatening and unthreatened conditions). Despite the reduced respiration rate and clearance rate under hypoxia, harder and stiffer shells were still produced at a higher rate under life-threatening conditions. The maintenance of this anti-predator response is possibly attributed to the reduced energy demand for calcification by altering mineralogical properties (e.g. increased calcite to aragonite ratio). Our findings suggest that this compensatory mechanism may enable calcifying organisms to maintain calcification under hypoxia and acclimate to the future metabolically stressful environment.