Biology of Red Sea Corals: Metabolism, Reproduction, Acclimatization, and Adaptation

Coral reefs are the most abundant shallow water ecosystems in the Red Sea, harboring a high species diversity and habitat complexity over large environmental gradients. At the same time the semi-enclosed ocean basin and its partly extreme environmental conditions may promote species evolution being distinct from Indo-Pacific coral reefs. Extreme conditions are found in the southern Red Sea, where temperatures reach up to 33 °C in summer and where nutrient input is high. Mechanisms of organism adjustment to these conditions are of particular interest in the light of climate change research. Towards the north, conditions become more ‘coral-promoting’ finally reaching temperatures between 21–27 °C (winter-summer) and clear waters at the northern end of the Red Sea (Gulf of Aqaba). In this chapter, we summarize the current knowledge about the biology of shallow water, symbiotic, reef-building corals of the Red Sea. We start with an overview on the environmental conditions of the Red Sea, the history of coral reef research in this region and a general introduction into coral biology, before we describe the ecophysiology of Red Sea corals. Coral ecophysiology is presented in the context of varying environmental conditions over depth (e.g., light), between seasons, and over latitudes (e.g., light, temperature, nutrients). Mechanisms and patterns of coral reproduction are discussed in the context of seasonal and latitudinal environmental changes. Finally, we briefly describe anthropogenic influences on Red Sea coral reefs. Acclimatization mechanisms of corals to changing conditions over a depth gradient (mainly light reduction) have been well studied in the Gulf of Aqaba and include the following metabolic adjustments with depth: (i) an upregulation of light-harvesting pigments (chlorophyll a) and a downregulation of photo-protective pigments (xanthophyll), (ii) an increase of heterotrophy, and (iii) a decrease of metabolic activity (e.g., calcification and growth). In addition, a change in the symbiont composition (Symbiodinium clade and/or type) over depth was observed in some coral species. Seasonal environmental changes (mainly light availability, temperature, nutrients) lead to various metabolic responses of the corals, including (i) changes in zooxanthellae pigmentation and density and (ii) changes in the metabolic activity. In particular, changes in calcification and growth rates can be observed with lowest rates during low temperatures in winter. Interestingly, however, this reverses in the southern Red Sea, where calcification rates are higher in winter than in summer. This kind of latitudinal shift is also evident in the timing of reproduction, which occurs earlier in the year (January–March) in the south compared to the north (March–August). This indicates that growth and reproduction are strongly linked to temperature, following a single temperature optimum, which occurs at different times throughout the year from north to south. Furthermore, this hints towards a high phenotypic plasticity (acclimatization) rather than local genetic adaptation of the investigated coral species. A clear shift in the genetic population structure from north to south in another coral species, however, indicates local adaption. Adjusting mechanisms need to be further understood in order to provide indication for predicted climate change effects