The Adaptive Hypothesis of Bleaching

Despite the perception that corals and coral reefs are limited to stable habitats distinguished by very narrow environmental parameters, the coral–algal symbiosis is capable of surviving under a variety of extreme conditions. Through the process of photoadaptation, corals and their algal symbionts adjust algal densities and pigment concentrations to function over a wide range of light levels ranging from direct exposure to full sunlight in intertidal corals to virtual darkness at the extreme limits of the photic zone (>200 m) on reef slopes (Zahl and McLaughlin 1959; Schlichter et al. 1986). Corals and reef communities in some areas (such as the Arabian Gulf) tolerate salinity and temperature conditions that are lethal when imposed rapidly on the same species in less extreme environments (Coles 1988; Sheppard 1988; Coles and Fadlallah 1991; Chap. 23, Jokiel, this Vol.). There are abundant reports of reef corals occurring in turbid, high nutrient, nearshore habitats (Larcombe et al. 2001). Coral reefs exist at the inherently variable interface between the sea, air and land (Smith and Buddemeier 1992), and reef communities have persisted over geological time through significant climate and sea-level fluctuations. Despite this, rates of speciation and extinction in scleractinian corals have been relatively low over the last 220 million years (Veron 1995). Because the mechanisms by which reef corals adjust to these diverse environments are only poorly understood, we have little basis for estimating or predicting actual or potential rates of adaptive change. This issue is of particular interest because of the present high rates of coral mortality and reef degradation (e.g., Hoegh-Guldberg 1999; Wilkinson 2000) caused in substantial measure by the two large-scale threats of bleaching and other diseases.