1 Variation in evolved “limits to life” precludes universal tolerance indices: a critique of the “Respiration Index” Brad A. Seibel 1 , Peter R. Girguis 2 , and James J. Childress 3 1. Corresponding author, Biological Sciences, University of Rhode Island, Kingston, RI 02891, seibel@uri.edu , ph. 401-874-7997 2. Organismal and Evolutionary Biology, Harvard University, Cambridge, MA 02138, pgirguis@oeb.harvard.edu , ph. 617-496-8328 3. Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106, childres@lifesci.ucsb.edu , ph. 805-893-3203 Human emissions of carbon dioxide will affect marine ecosystems directly via ocean acidification (2) and indirectly via expanding oxygen minimum zones (3). Understanding the extent and severity of the impact requires knowledge of levels of both oxygen and carbon dioxide that are critical to organisms. Such critical levels represent species- specific adaptations in oxygen uptake and acid-base regulation that have evolved for each species within the specific habitats. One cannot set a single index for tolerance of these parameters because individual species have evolved unique tolerances to their habitats, and these limits are usually far from the ultimate limits that can be evolved in “extreme” habitats. The “Respiration Index ” (RI, = Log 10 {pO 2 /pCO 2 }), proposed by Brewer and Peltzer (1) as a metric to define the operational limits of marine life, is theoretically invalid