Differential susceptibility of reef-building corals to deoxygenation reveals remarkable hypoxia tolerance.

Ocean deoxygenation threatens the persistence of coastal ecosystems worldwide. Despite an increasing awareness that coastal deoxygenation impacts tropical habitats, there remains a paucity of empirical data on the effects of oxygen limitation on reef-building corals. To address this knowledge gap, we conducted laboratory experiments with ecologically important Caribbean corals Acropora cervicornis and Orbicella faveolata. We tested the effects of continuous exposure to conditions ranging from extreme deoxygenation to normoxia (~ 1.0 to 6.25 mg L-1 dissolved oxygen) on coral bleaching, photophysiology, and survival. Coral species demonstrated markedly different temporal resistance to deoxygenation, and within a species there were minimal genotype-specific treatment effects. Acropora cervicornis suffered tissue loss and mortality within a day of exposure to severe deoxygenation (~ 1.0 mg L-1), whereas O. faveolata remained unaffected after 11 days of continuous exposure to 1.0 mg L-1. Intermediate deoxygenation treatments (~ 2.25 mg L-1, ~ 4.25 mg L-1) elicited minimal responses in both species, indicating a low oxygen threshold for coral mortality and coral resilience to oxygen concentrations that are lethal for other marine organisms. These findings demonstrate the potential for variability in species-specific hypoxia thresholds, which has important implications for our ability to predict how coral reefs may be affected as ocean deoxygenation intensifies. With deoxygenation emerging as a critical threat to tropical habitats, there is an urgent need to incorporate deoxygenation into coral reef research, management, and action plans to facilitate better stewardship of coral reefs in an era of rapid environmental change.

Sex-specific stress tolerance, proteolysis, and lifespan in the invertebrate Tigriopus californicus.

Because stress tolerance and longevity are mechanistically and phenotypically linked, the sex with higher acute stress tolerance might be expected to also live longer. On the other hand, the association between stress tolerance and lifespan may be complicated by tradeoffs between acute tolerance and long-term survival. Here we use the copepod Tigriopus californicus to test for sex differences in stress resistance, proteolytic activity and longevity. Unlike many model organisms, this species does not have sex chromosomes. However, substantial sex differences were still observed. Females were found to have superior tolerance to a range of acute stressors (high temperature, high salinity, low salinity, copper and bisphenol A (BPA)) across a variety of treatments including different populations, pure vs. hybrid crosses, and different shading environments. Upregulation of proteolytic capacity - one molecular mechanism for responding to acute stress - was also found to be sexually dimorphic. In the combined stress treatment of chronic copper exposure followed by acute heat exposure, proteolytic capacity was suppressed for males. Females, however, maintained a robust proteolytic stress response. While females consistently showed greater tolerance to short-term stress, lifespan was largely equivalent between the two sexes under both benign conditions and mild thermal stress. Our findings indicate that short-term stress tolerance does not predict long-term survival under relatively mild conditions.