Ionic status, calcium uptake, and Ca2+-ATPase activity during early development in the purple sea urchin (Strongylocentrotus purpuratus). Ionic status during early development was investigated in the purple sea urchin. Whole body cation concentrations (Ca(2+), Na(+), K(+), Mg(2+)), unidirectional Ca(2+) uptake rates measured with (45)Ca(2+), Ca(2+)-ATPase activity, and growth were examined at 12h intervals over the first 96h of development. Whole body Ca(2+) concentration was low initially but increased steadily by >15-fold through to the pluteus stage. Whole body Mg(2+), K(+) and Na(+) levels exhibited diverse patterns, but all increased at 72-96h. Ca(2+) uptake rates were low during initial cell cleavages at 12h but increased greatly at blastulation (24h) and then again at gastrulation (48h), declining thereafter in the pluteus stage, but increasing slightly at 96h. Ca(2+)-ATPase activity was initially low but increased at blastulation through gastrulation (24-48h) but declined thereafter in the pluteus stage. Embryonic weights did not change over most of development, but were significantly higher at 96h. Overall, the gastrulation stage displayed the most pronounced changes, as Ca(2+) uptake and accumulation and Ca(2+)-ATPase levels were the highest at this stage, likely involved in mineralization of the spicule. Biomarkers of Ca(2+) metabolism may be good endpoints for potential future toxicity studies.