Mineralised CaCO3 exoskeletons exist in over half of marine invertebrate taxa and are vital for structural support and defence. Calcification of these structures involves seawater uptake of calcium and dissolved inorganic carbon (DIC) and net excretion of generated protons. In crustacea, such as the intensively farmed whiteleg shrimp (Penaeus vannamei), calcification is intermittent due to rhythmic moulting and rehardening of the mineralised exoskeleton. This results in periodically rapid ion and acid-base fluxes, suggesting that exoskeleton growth is tightly coupled to seawater salinity and carbonate chemistry. However, the relationship between seawater ion availability and calcification in crustacea remains poorly characterised. In this study we reveal that mass-specific calcium and DIC uptake in post-moult P. vannamei is among the highest ever recorded in an adult animal and can be maintained in remarkably dilute seawater. Experiments exposing P. vannamei to a range of seawater chemistries, reveal that maximum flux rates of calcium and acid-base equivalents over the 0-2-hr post-moult period are particularly susceptible to adverse seawater chemistry. Substrate-limiting uptake rates can be compensated for over the following 20 hours, although net uptake ceases by 22 hours post-moult, regardless of water chemistry. Importantly, we found that decreased seawater calcium/DIC uptake impacts exoskeletal macrostructure (ratios of mineral, protein, and chitin), as well as calcium carbonate mineralogy. These findings suggest that although P. vannamei exhibits a strong ability to obtain sufficient calcium and DIC for calcification even in extremely dilute seawater, this involves trade-offs to the speed of post-moult mineralisation and exoskeleton composition and structural integrity.