Abstract
As the devastating impacts of global climate change and local anthropogenic stressors on shallow-water coral reefs are expected to rise, mesophotic coral ecosystems have increasingly been regarded as potential lifeboats for coral survival, providing a source of propagules to replenish shallower reefs. Yet, there is still limited knowledge of the capacity for coral larvae to adjust to light intensities that change with depth. This study elucidates the mechanisms underlying plasticity during early life stages of the coral Porites astreoides that enable survival across broad depth gradients. We examined physiological and morphological variations in larvae from shallow (8–10 m) and mesophotic (45 m) reefs in Bermuda, and evaluated differences in survival, settlement patterns and size among recruits depending on light conditions using a reciprocal ex situ transplantation experiment. Larvae released from mesophotic adults were found to have significantly lower respiration rates and were significantly larger than those derived from shallow adults, indicating higher content of energetic resources and suggesting a greater dispersal potential for mesophotic larvae compared to their shallow counterparts. Additionally, larvae released from mesophotic adults experienced higher settlement success and larger initial spat size compared to larvae from shallow adults, demonstrating a potential connection between parental origin, offspring quality, and recruitment success. Although both shallow and mesophotic larvae exhibited the capacity to survive and settle under reciprocal light conditions, all larvae had higher survival under mesophotic light conditions regardless of parental origin, suggesting that conditions experienced under low light may enable longer larval life, further extending the dispersal period. These results indicate that larvae from mesophotic Porites astreoides colonies are likely capable of reseeding shallow reefs in Bermuda, thereby supporting the Deep Reef Refugia Hypothesis.
Highlights
By creating complex 3-dimensional calcium carbonate frameworks, reef building corals support the most productive and biologically diverse marine ecosystems on Earth
Rate of respiration differed significantly between experimental treatments (p < 0.001; Analysis of Variance (ANOVA)), where respiration rates based on total larval volume were higher for wells containing larvae released by shallow colonies compared to those released by mesophotic colonies (Figure 3)
Settlement success differed significantly by depth of parental origin (p = 0.007; ANOVA), where percent of settlement success relative to total number of larvae per chamber was higher in chambers containing larvae released by mesophotic corals than those released by shallow corals
Summary
By creating complex 3-dimensional calcium carbonate frameworks, reef building corals support the most productive and biologically diverse marine ecosystems on Earth. Coral reefs are a critical habitat for a wide variety of species, and provide indispensable ecosystem services for millions of people (Burke et al, 2011). Despite their great importance, coral reefs are facing significant challenges due to human activities (e.g., pollution, overfishing, and the physical destruction of reefs) (Burke et al, 2011). Changes in environmental features determined by ocean warming and seawater acidification are seriously threatening the survival of entire coral reef ecosystems (Hoegh-Guldberg et al, 2017). It is hypothesized that MCEs may act as refugia for corals, serving as potential sources to reseed shallower reefs through larval exchange and thereby increasing overall reef resilience [termed the Deep Reef Refugia Hypothesis “DRRH”; (Bongaerts et al, 2010; Lesser et al, 2018)]
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