Thermal optima in the hypoxia tolerance of marine ectotherms: Physiological causes and biogeographic consequences

Hypoxia traits imprinted in otolith δ13C from individual to global scales

Hypoxia tolerance and its variation with temperature, activity, and body mass, are critical ecophysiological traits through which climate impacts marine ectotherms. To date, experimental determination of these traits is limited to a small subset of modern species. We leverage the close coupling of carbon and oxygen in animal metabolism to mechanistically relate these traits to the carbon isotopes in fish otoliths (δ13Coto). The model reproduces the major empirical patterns in δ13Coto at individual to global scales. The weak dependence on body size and strong, non-linear, dependence on temperature reflect the same balance between metabolism and ventilatory gas exchange that underlies organisms' hypoxia tolerance. The global relationship between temperature and δ13Coto records both the fractionation by aragonite precipitation and the variation in hypoxia traits across ocean biomes. Because hypoxia tolerance is imprinted on both otolith geochemistry and species biogeography, the model allows the aerobic limits of species geographic ranges to be predicted from fish δ13Coto. This physiologically grounded model provides a foundation for the use of otolith chemistry to reconstruct modern spatial patterns and paleoceanographic changes in key traits that shape aerobic habitat of aquatic species.

Deep-water first occurrences of Ediacara biota prior to the Shuram carbon isotope excursion in the Wernecke Mountains, Yukon, Canada

Ediacara-type macrofossils appear as early as ~575 Ma in deep-water facies of the Drook Formation of the Avalon Peninsula, Newfoundland, and the Nadaleen Formation of Yukon and Northwest Territories, Canada. Our ability to assess whether a deep-water origination of the Ediacara biota is a genuine reflection of evolutionary succession, an artifact of an incomplete stratigraphic record, or a bathymetrically controlled biotope is limited by a lack of geochronological constraints and detailed shelf-to-slope transects of Ediacaran continental margins. The Ediacaran Rackla Group of the Wernecke Mountains, NW Canada, represents an ideal shelf-to-slope depositional system to understand the spatiotemporal and environmental context of Ediacara-type organisms' stratigraphic occurrence. New sedimentological and paleontological data presented herein from the Wernecke Mountains establish a stratigraphic framework relating shelfal strata in the Goz/Corn Creek area to lower slope deposits in the Nadaleen River area. We report new discoveries of numerous Aspidella hold-fast discs, indicative of frondose Ediacara organisms, from deep-water slope deposits of the Nadaleen Formation stratigraphically below the Shuram carbon isotope excursion (CIE) in the Nadaleen River area. Such fossils are notably absent in coeval shallow-water strata in the Goz/Corn Creek region despite appropriate facies for potential preservation. The presence of pre-Shuram CIE Ediacara-type fossils occurring only in deep-water facies within a basin that has equivalent well-preserved shallow-water facies provides the first stratigraphic paleobiological support for a deep-water origination of the Ediacara biota. In contrast, new occurrences of Ediacara-type fossils (including juvenile fronds, Beltanelliformis, Aspidella, annulated tubes, and multiple ichnotaxa) are found above the Shuram CIE in both deep- and shallow-water deposits of the Blueflower Formation. Given existing age constraints on the Shuram CIE, it appears that Ediacaran organisms may have originated in the deeper ocean and lived there for up to ~15 million years before migrating into shelfal environments in the terminal Ediacaran. This indicates unique ecophysiological constraints likely shaped the initial habitat preference and later environmental expansion of the Ediacara biota.

Metabolic responses to cold and warm extremes in the ocean

Temperature influences the geographical distribution of species, but its mechanisms are much debated. A new study in PLOS Biology suggests that metabolic constrains can arise in both warm and cold waters at the geographical range limits of marine species.