Meta-analysis suggests negative, but 𝘱�CO₂-specific, effects of ocean acidification on the structural and functional properties of crustacean biomaterials

Abstract

Crustaceans comprise an ecologically and morphologically diverse taxonomic group. They are typically considered resilient to many environmental perturbations found in marine and coastal environments, due to effective physiological regulation of ions and hemolymph pH, and a robust exoskeleton. Ocean acidification can affect the ability of marine calcifying organisms to build and maintain mineralized tissue and poses a threat for all marine calcifying taxa. Currently, there is no consensus on how ocean acidification will alter the ecologically relevant exoskeletal properties of crustaceans. Here, we present a systematic review and meta-analysis on the effects of ocean acidification on the crustacean exoskeleton, assessing both exoskeletal ion content (calcium and magnesium) and functional properties (biomechanical resistance and cuticle thickness). Our results suggest that the effect of ocean acidification on crustacean exoskeletal properties varies based upon seawater 𝘱CO₂ and species identity, with significant levels of heterogeneity for all analyses. Calcium and magnesium content was significantly lower in animals held at 𝘱CO₂ levels of 1500–1999 µatm as compared with those under ambient 𝘱CO₂. At lower 𝘱CO₂ levels, however, statistically significant relationships between changes in calcium and magnesium content within the same experiment were observed as follows: a negative relationship between calcium and magnesium content at 𝘱CO₂ of 500–999 µatm and a positive relationship at 1000–1499 µatm. Exoskeleton biomechanics, such as resistance to deformation (microhardness) and shell strength, also significantly decreased under 𝘱CO₂ regimes of 500–999 µatm and 1500–1999 µatm, indicating functional exoskeletal change coincident with decreases in calcification. Overall, these results suggest that the crustacean exoskeleton can be susceptible to ocean acidification at the biomechanical level, potentially predicated by changes in ion content, when exposed to high influxes of CO₂. Future studies need to accommodate the high variability of crustacean responses to ocean acidification, and ecologically relevant ranges of 𝘱CO₂ conditions, when designing experiments with conservation-level endpoints.