Ecological consequences of the algal symbiosis in reef-building corals: Analysis of an underlying mechanism driving species richness patterns

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Tomás López-Londoño, Kelly Gómez-Campo and Roberto Iglesias-Prieto, Department of Biology, The Pennsylvania State University, University Park, PA, Xavier Hernández-Pech and Susana Enríquez, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Cancún, QR, Mexico

Background/Question/Methods
The processes that regulate biodiversity patterns has been a controversial topic in ecology. Coral reefs, being one of the most biodiverse ecosystems with consistent biodiversity patterns across environmental gradients, have been ideal candidates for studying these processes. The fruits of such efforts, however, remain inconclusive. The species–energy hypothesis (SEH) attempts to explain species richness gradients as a function of energy availability. This hypothesis has been difficult to test in coral communities due to unclear proxies of energy availability or difficulties for measuring them. We used a mechanistic model based on underwater optics and physiological principles to test whether predicted changes in the energetic contribution of algal symbionts (zooxanthellae) to the animal respiration (CZAR) explains documented patterns of coral species richness with depth. The model was tested against published datasets from studies conducted on reefs with contrasting evolutionary history and environmental conditions, encompassing the three centers of coral diversity. With a view of confronting an alternative model with empirical data, we compared our CZAR model with a linear model using solar irradiance as predictor of change in coral richness with depth.
Results/Conclusions
Irradiance did not explain species richness variation with depth in sites with unimodal richness-depth relationships, where a disproportional reduction in species number towards the shallow high-energy portion of reefs was observed. Changes in irradiance explained richness variation in sites with a monotonic increase or decrease in species number with depth. At these sites, a strong correlation, although of opposite nature, was found between irradiance and species richness. These findings support a site-specific local component of this relationship, where the water optical properties may play a decisive role. Conversely, the CZAR model provided a strong fit to the empirical data in all locations and throughout the full depth gradient. The consistency of the CZAR model minimized the effect of local evolutionary, environmental, and ecological differences on the variation of species number across depths. Our analysis supports the validity of the SEH for explaining biodiversity patterns in reef coral communities, underlining the fundamental role of the energetic contribution of algal symbionts to their coral hosts at both the organismal and community levels. These results indicate that a better understanding of corals bioenergetics and its dependence on specific coral-algae symbioses is fundamental to explain key ecological processes and current responses to local and global threats in coral reef ecosystems.