ReviewCoral bleaching––how and why?
Introduction
Everyone knows about coral bleaching. It has been discussed in the soap opera Neighbors (or so I am told) and, more seriously, it is recognised widely as a factor damaging the already fragile economies of various developing countries and livelihood of their peoples. Marine biologists have played a crucial role in achieving this awareness, by well-organised and publicised monitoring of bleaching events. Bleaching events are increasing in frequency and severity (Wilkinson, 1999), and the ecological collapse of the world’s reefs is predicted by simulation models using current data on the incidence of bleaching and projected climate change (Hoegh-Guldberg, 1999; Risk, 1999).
What should be done? Should we attempt to influence the incidence and scale of bleaching events as part of reef management strategy and, if so, what interventions should be made? Or are the causes of coral bleaching and the actions required to reduce its incidence entirely at a scale far larger than the remit of the coral biologist? Answers to these questions will require extended dialogue between multiple disciplines, including those with expertise in reef management, coral ecology and the fundamentals of the Symbiodinium symbiosis.
The purpose of this article is to provide some input to the dialogue from the perspective of the symbiosis. The key issues from this perspective are (1) the causes of bleaching; (2) the reasons for variation in susceptibility to bleaching; and (3) why, at the evolutionary level, corals bleach. They are addressed in turn, after a brief introduction to the defining features of bleaching. For general reviews of coral bleaching, including comprehensive surveys of the burgeoning literature, the reader is referred to Glynn (1993), Brown (1997) and Hoegh-Guldberg (1999).
Section snippets
What is bleaching?
Coral bleaching is a misnomer. Bleaching is not restricted to corals, but displayed by all animals in symbiosis with dinoflagellate algae of the genus Symbiodinium, also known as zooxanthellae because of their yellow–brown colour. This symbiosis is restricted largely to benthic animals of the phylum Cnidaria (e.g. sea anemones, zoanthids, scleractinian corals and octocorals) in the photic zone at low latitudes, but it is also borne, for example, by some sponges and the tridacnid clams.
Bleaching
The causes of bleaching
There is no coherent, simple explanation of the causes of bleaching, despite the considerable research on this topic. An explanatory framework of bleaching proposed in Fig. 1 may contribute to the organisation of available information and to a more precise definition of unresolved problems. Causation can be considered as three elements: the external factors which trigger bleaching; the symptoms which give rise to the observed bleaching; and, at the core of the causes of bleaching, the
Variation in susceptibility to bleaching
Symbioses with Symbiodinium species are not uniformly susceptible to bleaching. Monitoring of natural bleaching events has revealed marked interspecific and intraspecific variation in the degree of bleaching at one site. For example, among corals, branching forms, e.g. Acropora and Pocillopora species, generally bleach more strongly than massive corals (e.g. Jiménez, 2001; McClanahan et al., 2001). The determinants of this variation in bleaching susceptibility have been investigated from two
Why Symbiodinium-symbioses bleach
Animals and plants enter into symbioses with a wide diversity of micro-organisms, including micro-algae, and these associations are generally remarkably tolerant of variation in abiotic conditions (Douglas, 1994). Symbioses with Symbiodinium species are exceptional in that they commonly live in habitats at 1–2 °C below the temperature which triggers collapse of the symbiosis (i.e. bleaching) with negative consequences for the growth, reproduction and survival of the animal host (e.g. Szmant and
Conclusions
The experience of the last two decades suggests that bleaching is generally deleterious to corals and other Symbiodinium symbioses and to reef communities. Bleaching can, therefore, be considered as a linked group of ailments or diseases in the broadest sense of deleterious physiological responses (i.e. not restricted to pathogen-induced disease). It is probably unhelpful to describe bleaching as a single ailment, or to expect the explanation of the causes of bleaching to be simple. As Section 3
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