The Southern Ocean is considered to be the canary in the coal mine with respect to the first effects of ocean acidification (OA). This vulnerability is due to naturally low carbonate ion concentrations that result from the effect of low temperature on acid-base dissociation coefficients, from the high solubility of CO 2 at low temperature, and from ocean mixing. Consequently, the two calcium carbonate polymorphs, aragonite and calcite, are expected to become undersaturated in the Southern Ocean within 50 and 100 years, respectively. Marine invertebrates such as echinoderms, whose skeletons are classified as high-magnesium carbonate (>4% mol MgCO 3 ), are even more vulnerable to OA than organisms whose skeletons consist primarily of aragonite or calcite, with respect to both increased susceptibility to skeletal dissolution and further challenge to their production of skeletal elements. Currently, despite their critical importance to predicting the effects of OA, there is almost no information on the Mg-calcite composition of Antarctic echinoderms, a group known to be a major contributor to the global marine carbon cycle. Here we report the Mg-calcite compositions of 26 species of Antarctic echinoderms, representing four classes. As seen in tropical and temperate echinoderms, Mg-calcite levels varied with taxonomic class, with sea stars generally having the highest levels. When combined with published data for echinoderms from primarily temperate and tropical latitudes, our findings support the hypothesis that Mg-calcite level varies inversely with latitude. Sea stars and brittle stars, key players in Antarctic benthic communities, are likely to be the first echinoderms to be challenged by near-term OA. © 2011 by The University of Chicago. All rights reserved.