Metabolic rate and the vulnerability of mollusks to hyperthermal-driven extinction events

Climate change can be a major driving mechanism behind mass extinctions. The combined multistressor effects of rapid global warming, ocean acidification, and hypoxia are devastating to marine faunas. Such episodes in Earth’s history, dubbed ‘hyperthermals’, serve as natural experiments that can provide insight into the effects of climate warming on marine ecosystems in the past as well as today. As water temperatures rise and oxygen solubility decreases, metabolic rates, and, consequently, the oxygen demands of organisms increase. This suggests that organisms with higher metabolic rates, already requiring more oxygen overall, should be more vulnerable to deoxygenation associated with rapid climate warming. However, more active organisms generally have physiologies less vulnerable to hypercapnia resulting from CO2 buildup in the oceans during hyperthermal conditions. Previous work on activity levels of fossil taxa disagree whether more active organisms are selected for (i.e., less vulnerable) or selected against during major hyperthermal-driven extinction events.Here, we explore the effects of resting metabolic rate, body size, and temperature preference on extinction vulnerability in gastropods and bivalves during post-Paleozoic hyperthermals. We estimate metabolic rates with a general model of metabolic rate originally derived by Gillooly et al. (2001), using published biomass estimates and location-specific sea surface temperatures from published climate models. Following Reddin et al. (2020), we then calculate relative hyperthermal vulnerability (RHV), the difference between the risk of extinction at intervals associated with hyperthermal conditions versus baseline conditions, in order to determine how an organism’s metabolism may affect patterns of taxonomic extinction and survival across hyperthermal-driven extinction events. RHV can be preferable to more direct comparisons of extinction selectivity in that it allows for comparisons among groups with very disparate basal turnover rates. Preliminary results for bivalves indicate that a higher metabolic rate is associated with a reduced risk of extinction during hyperthermal conditions. These results also seem to suggest that the driving force behind this pattern of selectivity is the B0 standard metabolic rate coefficient, estimated using experimental data on respiration rates in modern bivalve and gastropod clades. Future work will focus on whether the range of variation in the experimental data underlying the B0 estimates lines up with what is expected of fossil taxa, and ultimately, whether these data can be used to evaluate how metabolic rate can affect species vulnerability to stress or extinction risk.