2073_C007.fm Page 119 Tuesday, April 18, 2006 11:14 AM If You Build It, Will They Come? Toward a Concrete Basis for Coral Reef Gardening Les S Kaufman CONTENTS 7.1 7.2 Introduction 119 Coral Reef Gardening in the Context of Reef Restoration 121 7.2.1 Task 1: Restore and Create Wave-Resistant Structures 121 7.2.2 Task 2: Induce Circumstances on and around the Structure That Are Conducive to the Establishment and Growth of Living Framework Builders .124 7.2.3 Task 3: Speed Succession toward a Community Dominated by Framework Builders That Will Continually Renew and Grow the Structure 132 7.2.4 Task 4: Craft Community Ontogeny So as to Maximize the Value and Sustainability of Goods and Services Produced by Restored or Created Reef 133 7.3 Conclusion 136 Acknowledgments 137 References 137 7.1 INTRODUCTION Coral reefs are in precipitous global decline before the eyes of a single generation of biologists The reason is simple and familiar: a confluence of desperate poverty and demanding affluence, of swelling hunger and rapacious consumption by a mobile, high-tech society Combined human impacts have corroded the quality of ecological services provided by coral reefs and compromised their capacity to heal after even the normal annoyances of hurricanes and predators These impacts include climate change, pollution, depressed aragonite saturation levels (due to elevated atmospheric CO2), coral disease, coral mining, the elimination of keystone species called “ecological engineers,” and destructive resource extraction.1–7 Coral reef decline often manifests itself as a dramatic shift in dominance from hard corals to fleshy algae — very dramatically so in the tropical Atlantic.2,8,9 Coral reefs are not the only ecosystems around the world that are shifting from desirable to undesirable states due to human activity, a process that has been in play for centuries.10 However, the drama unfolding on coral reefs is especially stunning because it is happening around us right now Coral reefs support a massive tourism industry, protect shorelines, and feed a large portion of the world’s population dependent upon subsistence fishing As the highly popular subjects of television nature specials, coral reefs enjoy a broad, if distant, constituency Even neophytes may 119 © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 120 Friday, April 7, 2006 4:46 PM 120 Coral Reef Restoration Handbook be shocked when the see a degraded reef, for they have built up quite reasonable expectations from all the nature films.11 Consequently, where reefs die a fierce enthusiasm for technological “solutions” brews Often, these are myopically conceived Wisdom would put prevention first: reduce coastal eutrophication, eliminate destructive fishing practices, drastically lower CO2 emissions, establish marine reserves, and enforce laws to protect coral reef species and habitats Nonetheless, when the next reef to go is the one within your village, resort, or national marine sanctuary, you are likely to take a sudden interest in coral reef restoration Reef restoration is expensive, whether measured in sweat or dollars or both, but it can work on a small scale.12 There have been quite successful projects to propagate corals, giant clams, sea urchins, and other reef framework builders and ecological facilitators Orphaned coral fragments and mass-produced propagules have been placed out in reef habitats and affixed to artificial reeflike structures, even electrically self-healing ones, to good effect (e.g., reference 13) The technologies have the potential to someday rescue some coral reef species and to heal some types of damage, such as the dynamite blast craters that spread like a pox across the Coral Triangle Restoration is a very tough job in the tropical west Atlantic, and it is definitively not the first priority in reef conservation anywhere, but neither is it the very last Restoration can be emotionally and politically therapeutic and can produce a beautiful small aquatic garden, just so long as this tiny balm does not quell the greater winds of change The primary driver for coral reef gardening will always be its value to the local economy It attracts tourists It can also attract and possibly boost the productivity of some fishery species Coral gardens can concentrate marine ornamental fishes for extraction14 and have been promoted as “arks” that could one day seed and rejuvenate neighboring natural reefs Isolated reef systems and possibly even most coral reefs are dependent largely upon self-supply of invertebrate and fish larvae.15,16 To the extent that this is true at any given site, the “ark” idea may not be so farfetched Coral gardens can alter the dynamic of a society’s relationship with its coral reefs; this is their most unsung value Cognitive dissonance (e.g., you love the things you suffer for) is writ large on the human psyche The intensive stewardship required to start and keep a coral garden going is bound to generate greater compassion for the ebb and flow of life on coral shores The Balinese, a distinctly land-oriented people, have been brought to glance lovingly and frequently out to sea by coral gardening projects in the tourist towns of Seraya, Tulamben, and Permuteran (personal observation) Diving at Tulamben supports tiny village women who famously walk scuba tanks to the beach, balanced on their heads, for overweight tourists Coral gardens maintained by local citizens have galvanized communities and awakened citizens to attend closely to human impacts on coral reef health Finally, by focusing human use and extraction on a managed, nearby corner of the sea, the coral reef garden can be a sort of a halfway house or easement between the crush of civilization and true wilderness reserves farther afield Coral gardens can act as diver aggregation devices or “DADs,” drawing divers away from fragile natural reefs into areas managed expressly for this purpose.17 Like all else in conservation, coral gardening and restoration are about people People are inspired by small but beautiful works to much grander and more practical things Coral gardening has its place So, how can we make it work really well? The challenge in coral gardening is to trigger a self-assembly process that supplants its human preparations and culminates in a functional reef community within a practical time frame — say, less than 10 years Epstein et al.18,19 have explored in detail the validity of the silvicultural analogy Success will be measured by what an initial, undesirable state — i.e., bare construction site or fleshy algal-shrouded reef surface — eventually turns into ecologically.20 This means the benthic community that jackets it and the motile fishes and invertebrates that recruit to it In this garden, corals and fishes are the wildflowers, shrubs, and trees, and we for the most part are but tilling and doing the initial rockwork … and then waiting We want to know: if we build it…will they come? And who, exactly, are “they”? And by the way, what creatures would we not want to see arriving in droves, and how might these be discouraged? We are gardeners in the truest sense of the word © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 121 Friday, April 7, 2006 4:46 PM If You Build It, Will They Come? Toward a Concrete Basis for Coral Reef Gardening 121 7.2 CORAL REEF GARDENING IN THE CONTEXT OF REEF RESTORATION Gardening can be a valuable adjunct to coral reef restoration when there is need: To accelerate healing on damaged patches of an otherwise healthy reef: e.g., dynamite blast fields and ship groundings To heal relatively isolated target patches in order to create or restore entire reef sections within a reef system For a scheme to produce coral reef goods and services on a sustainable basis in a place where they were absent before, such as in the middle of a sand plain or seagrass bed What these applications have in common is that all aim to deflect community ontogeny on a patch of sea bottom away from alternative states (such as seagrass, seaweed, or dancing rubble) and toward a physically stable, accretionary state dominated by crustose coralline algae, hard corals, and other reef builders along with their highly valued associated fauna and flora Reef restoration technologies range from very simple to highly sophisticated and expensive and include the following: Dump bulky solid waste (tires, cars, ships, preformed concrete) on the seabottom in one area Build a carefully designed and placed wave-resistant structure from mass-produced modules of natural and/or recycled materials Design modular deployments to kick-start succession toward hard coral dominance by modulating flow fields and recruitment Dope structure with chemical morphogens to promote the settlement of larvae of hard coral and other desired reef organisms Engineer structures for self-repair and self-renewal during the establishment phase for a hard coral community; corals eventually subsume these functions Manually plant structure with framework builders (corals, giant clams) removed from a natural reef Manually plant structures with framework builders produced in aquaculture Manipulate the community of motile invertebrates and fishes (facilitators) to enhance the robustness and growth rates of the framework builders Manipulate the community of motile invertebrates and fishes to maximize levels of sustainable production of extractive resources For the most part, these techniques have been employed individually; a comprehensive protocol for coral reef gardening using integrated best practices does not exist We can get there more quickly by parsing the problem and considering the ecological processes attendent to each critical step toward a self-sustaining patch of coral reef.21 Here are four candidate steps for such a clinical protocol: Task 1: Restore or create a wave-resistant structure Task 2: Induce circumstances on and around the structure toward a state conducive to the growth of living framework builders Task 3: Speed succession toward a community dominated by framework builders that will continually renew and regrow the structure Task 4: Craft community ontogeny so as to maximize the value of goods and services produced by this engineered patch of reef 7.2.1 TASK 1: RESTORE AND CREATE WAVE-RESISTANT STRUCTURES A coral reef begins with topographic relief on the seabottom that resists the destructive forces of waves and sea The engineering of a wave-resistant structure is what ultimately leads to a coral reef’s valued biological features, such as its high local productivity and high species diversity © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 122 Friday, April 7, 2006 4:46 PM 122 Coral Reef Restoration Handbook Unprepared to wait the thousands of years required for a reef to grow where desired or for a damaged reef to repair large sections of its basic architecture, coastal communities have lobbied hard for the creation of artificial reefs to enhance fishing and diving There is now an established lore on the best ways to go about that exercise Coral reef restoration projects usually focus less on massive structural architecture and more on providing an adequate foundation for coral growth Concrete is clearly better than tires, old cars, or even ships Simple piles of limestone boulders may be even better, and coating culverts with limestone rocks can improve the outcome as well Three standardized technologies for fabricating modular structures are now being marketed: ReefBalls (and the conceptually similar Grouper Ghettos), Ecoreefs, and Biorock ReefBalls and Grouper Ghettos are cast cement hemispheres (ReefBalls) or angular forms with holes and open spaces in them ReefBalls in particular can be mass produced and deployed by the hundreds or thousands to construct composite structures as large as breakwaters and sections of reef The largest such structure to date is an artificial fringing barrier reef in Antigua Modular concrete structures are comparable to a surgical implant; they replace coral reef framework in the short term, while coral and other invertebrate fragments that recruit or are secured to the balls have a chance to become established Over time, the implant is obscured and supplanted by desirable epifauna, including native framework-builders Concrete modules have been used to routinely create about to m of vertical relief Information on ReefBalls is consolidated on the company’s web site, but rigorous, long-term study comparing ReefBalls to alternative structures is still wanting Eventually, however, we shall discover how long it takes for the concrete hemispheres to be supplanted by a hard coral superstructure under varied conditions and in different parts of the world One other advantage of the modular concrete approach is that it may provide a scaffolding capable of surviving storm damage, bleaching events, and predator outbreaks In fairness, an artful pile of boulders can the much same thing The remnant, wave-resistant, rough-surfaced habitat of encrusted modules might regenerate hard coral cover more quickly than a low-lying rubble field EcoReefs (Figure 7.1a) are ceramic modules similar to a snowflake-shaped staghorn coral colony, which are arranged in clusters over reef rubble left behind from physical damage, such as dynamite explosion fields An EcoReef is only a few decimeters high at most, but the structure is meant to serve only as a physically stable and hydrodynamically attractive foundation for coral settlement EcoReef was designed for a very specific purpose: to provide physical stability and create a hydrodynamic current field conducive to hard coral recruitment (Figure 7.1b) Thus, it can jump-start the reestablishment of hard coral communities on low-relief rubble fields otherwise recalcitrant to hard coral regeneration for decades.22 Careful research is under way to assess the performance of EcoReef clusters in repairing rubble fields on reefs at Bunaken National Park, Indonesia; a new project is starting up in the Phillipines Electrochemical precipitation of mineral from seawater, based on a different kind of current, is a patented process invented by Wolf Hilbertz and perfected for use in artificial reef construction by Hilbertz and Thomas Goreau23–26 under the name “Biorock.” Biorock installations are purveyed through the Global Coral Reef Alliance, a small not-for-profit enterprise with a well-illustrated web site Biorock is one of several experiments in which an electric current has been run through submerged wire frames to build artificial reefs In Biorock, a hard carbonate material is deposited electrolytically over a form constructed principally from iron reinforcing bars (Figure 7.2) The material that accretes on properly electrified structures is a variable mixture (depending upon how well the array is working) of aragonite (calcium carbonate) and brucite (magnesium hydroxide) (Land, personal communication) Brucite is highly soluble in seawater, and a high proportion of brucite in the electrically deposited material would therefore be undesirable This author has seen Biorock installations in Jamaica, St Croix, and Bali (Seraya, Tulamben, and Permuteran) The mineral accreted to the structures in Bali was durable and strongly adherent to the embedded reinforcing bar, both on structures with live current and on those that had experienced a cessation of current within the preceding month or two I did not have the chance to examine a structure that had been without electricity for a very long time to see if the mineral coating was durable without © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 123 Friday, April 7, 2006 4:46 PM If You Build It, Will They Come? Toward a Concrete Basis for Coral Reef Gardening 123 (a) (b) FIGURE 7.1 (a) Demonstration project of EcoReefs® modules deployed on a coral rubble field at Bunaken National Park, Indonesia (b) Coral recruit Stylophora sp on a shaded lowering setting plate on an EcoReefs® array recent additional precipitation The accreted mineral can be deliberately sloughed off by reversing the charge, though to make this a useful feature would require some imagination Little is known about the submarine diagenesis of Biorock, or about its endolithic community or vulnerability to bioeroders However, a sliced Biorock sample with accretion of more than cm radius showed little or no boring at all (personal observation) Either it formed very rapidly and thus there had not been time for endoliths to accumulate, or Biorock is resistant to boring On the one hand, the material may remain quite solid and durable, unlike natural reef rock, which is usually highly tunnelled and friable Endoliths can play both positive and negative roles in reef development The endolithic community contains both primary producers and pathogens,27–29 as well as pore spaces that may be critical to nutrient cycling and the submarine diagenesis of reef rock Nonetheless, in coral reef restoration as opposed to underwater architecture, the primary function of the accreted mineral in Biorock is to attract coral recruits and serve as an anchoring point for explanted coral colonies For the former, it may perform more poorly than a rougher, natural surface would As a system for securing coral fragments, however, electrochemical accretion is quite good © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 124 Friday, April 7, 2006 4:46 PM 124 Coral Reef Restoration Handbook (a) (b) FIGURE 7.2 (a) Prolific coral growth and fish on a 3-year-old “Biorock” reef in Bali, Indonesia (b) Coral recruitment of acroporid coral on “Biorock” mesh All three of these methods result in a submarine structure that can resist currents and waves, setting up flow fields that are attractive to both juvenile and adult fishes, and may enhance coral settlement In all cases there is the possibility that the resulting structures can be large enough to also act as a measure to control beach erosion 7.2.2 TASK 2: INDUCE CIRCUMSTANCES ON AND AROUND THE STRUCTURE THAT ARE CONDUCIVE TO THE ESTABLISHMENT AND GROWTH OF LIVING FRAMEWORK BUILDERS Placing a hard substratum on the seabottom triggers the development of a fouling community of sessile organisms and an associated assemblage of motile organisms dependent upon the fouling © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 125 Friday, April 7, 2006 4:46 PM If You Build It, Will They Come? Toward a Concrete Basis for Coral Reef Gardening 125 (a) (b) FIGURE 7.3 (a) Mobile coral rubble produced by blast fishing in Indonesia (b) Coral recruitment on a pile of large boulder-sized rubble placed on dead reef in Indonesia community for food and habitat The hope in coral reef restoration is of course that this fouling community will, in its exposed portions, quickly become dominated by hard corals, and that these will in turn attract other creatures that help to foster the further development of a healthy coral reef community Whether or not events actually unfold in this manner depends upon numerous factors that are not fully understood.30 Frequently, the experiment fails, or at least the desired community takes so long to develop that the attendants give up A simple pile of limestone rocks in Indonesia may be obscured by lush growth of living corals in as little as years (Figure 7.3) The same pile of rocks off Negril, Jamaica, would likely be covered by unsightly macroalgae in a period of weeks and then retain this appearance for years Why such extreme variation in outcome? We know that ambient nutrient levels, the density of coral reef herbivores, and priority effects (i.e., the timing of bare substratum availability relative to larval © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 126 Friday, April 7, 2006 4:46 PM 126 Coral Reef Restoration Handbook supply) can be important determinants of fouling community structure Clearly it is ill-advised to place a coral gardening project just offshore from a sewer outfall or anywhere near a denuded watershed (though even this is context dependent: e.g., reference 31) Even under locally favorable conditions, however, artificial reef structures, and natural reef and hard bottoms as well, have transformed into fleshy algal reefs or pavements Once we understand fouling community dynamics well enough it may be possible to prevent artificial reef structures from being coated by intransigent sessile invertebrates or macroalgae, which can delay hard coral dominance indefinitely and greatly diminish the return on investment Coral planulae exhibit strong preferences as to where they will settle and establish themselves A good deal of research has been done on the suitability of various substrata to surface artificial reef structures and settlement tiles Throughout the world, early (and perhaps all) hard coral recruits settle preferentially on crustose coralline algae (CCA) Particular species of CCA exude chemical signatures that are particularly attractive These settlement-inducing substances can be used directly by doping substrata where coral settlement is desired.32–34 Many coral planulae exhibit a settling preference for the undersides of surfaces, and settlement can be strongly inhibited by the presence of macroalgae or anything else that physically obstructs the openings to crevices and overhangs The combination of a limestone or concrete foundation, CCA dominance, a rough surface texture, and many unobstructed overhangs is a killer combination for attracting coral spat Dominance of the sessile community by coralline algae is seen within a broad range of light and nutrient levels but often does not occur due to competition from fleshy algae Indeed, the interactions among macrophytes, crustose coralline algae, and hard corals are at the crux of reef conservation and restoration Macrophytes probably capitalize on readily accessible nutrient pools faster, or at least in a more competitive manner than corallines do, though it is not clear that oligotrophic conditions are particularly favorable to coralline growth except for their tendency to reduce spatial competition from macrophytes Corallines enjoy a competitive advantage over turf algae under grazing pressures that are high enough to exclude macrophytes but not so high as to compromise coralline growth and accretion Intuitively, corallines should best at the junction of intermediate light level and intermediate grazing pressure The higher the light and nutrients, the higher the grazing pressure necessary to achieve the same effect Thus, a second way of increasing the efficacy of coral gardening would be to terminate fishing for herbivorous fishes, particularly on exposed reefs in the Indo-Pacific where fishes are the dominant large algal grazers In the tropical west Atlantic and on inshore Pacific reefs urchins of the genera Diadema and Echinometra can be the primary grazers Their importance is heightened once overfishing has reached the point where large predators are rare and herbivorous fish populations, the next fishery resource in line, have been chewed down.2,8 It is worth keeping in mind that the relationship among corals, corallines, and fleshy algae has been broadly oversimplified Although a fundamental play-off between macrophytes and hermatypic corals or coralline algae (or both) does exist,35 there are complexities.36 Macrophyte appearance or disappearance can have cascading effects on the community.37,38 Fleshy algae can kill adjacent corals in a variety of ways besides overshading and abrasion They can trigger disease39 and harbor other organisms, such as fireworms, that can this themselves, as well as feed directly on the corals.40 Interactions among sessile benthos are mediated by a diverse host of inquilines and herbivores across a broad size range, as well as a complex web of inducible chemical weapons and herbivore associations.41 Circumstances may even exist under which fleshy algae are beneficial to hard corals One example of this arose in an experimental study of coral-algal-herbivore interactions led by the author in the ocean mesocosm of Biosphere II during its tenure as a Columbia University Earth Institute research facility Biosphere II was a nearly 4-acre, hermetically sealed greenhouse in Oracle, Arizona Harboring an assortment of ecosystem mesocosms including a 750,000-gallon tropical reef, Biosphere II was conceived initially as a test of technologies and psychology for the long-term human habitation of space and later as an experimental system for biospheric research Our experiment pitted hard corals against fleshy algae under various experimental treatments: grazed © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 127 Friday, April 7, 2006 4:46 PM 127 Ungrazed Ungrazed High light Grazed 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 −0.1 −0.2 −0.3 −0.4 −0.5 −0.6 Grazed Mean (residuals P3 growth rate) If You Build It, Will They Come? Toward a Concrete Basis for Coral Reef Gardening Low light Treatment within location Treatment Grazed Ungrazed FIGURE 7.4 Results of Biosphere II experiment Coral growth of Montipora capitata under high light and low light conditions Treatments are grazed and ungrazed versus ungrazed, high versus low light, and three different levels of aragonite saturation state, mimicking the effects of varied atmospheric carbon dioxide concentrations New data on CO2 effects are still being analyzed Independent of these effects, however, we noted an odd relationship between axial extension rates of the coral Montipora capitata and growths of a red alga, Polysiphonia sp (Figure 7.4) The best coral growth was expected at high light levels (ca 800 u3/m2/min), with regular removal of fleshy algal growth by a technician (the “grazer” in this portion of the experiment) due to direct algal−coral interactions.42 Growth was fastest at the higher light level, but in the ungrazed condition with its lush Polysiphonia mat, not the grazed treatments, free of the fleshy algae At lower light levels, grazed corals did grow faster than ungrazed Our candidate hypotheses are that so long as light was not a limiting factor, the algae had a positive effect on coral growth either due to reduced CO2 levels in the immediately adjacent water or to nutrient provisioning via algal exudates Just because the relationship between reef corals and fleshy algae is complex does not make it inaccessible to reason and experiment, but it does make it a lot more interesting, and coral reef gardening that much more challenging.43 It is important to consider how we might construct a reef surface so as to attract the settling juveniles of facilitator species that promote coralline algal growth Alternatively, for the impatient, it might be fruitful to cultivate and force-recruit crustose corallines where desired Who are the crustose coralline facilitators? The herbivore guild on coral reefs is a diverse, polyphyletic assemblage of species that vary in their mode of feeding, and hence in the degree to which they are likely to assist in achieving coralline dominance A full cast of reef herbivores is familiar to “living reef ” aquarists, who struggle mightily to achieve the same effects in their living room microcosms that others strive for on damaged coral reefs The players include microcrustaceans, browsing and grazing hermit crabs, majid brachyurans (e.g., Mithrax sp.), a variety of gastropods, regular echinoids, and herbivorous reef fishes, principally members of the families Scaridae and Acanthuridae, along with representatives of the Pomacentridae, Pomacanthidae, Blenniidae, Gobiidae, Kyphosidae, and others.44,45 Just as intermediate grazing pressures are best for coral settlement and © 2006 by Taylor & Francis Group, LLC 2073_C007.fm Page 128 Friday, April 7, 2006 4:46 PM 128 Coral Reef Restoration Handbook growth, a coral gardener’s favored actors in the reef herbivore guild should be species that graze at an intermediate level of intensity In other words, browsers on algal fronds are worse than useless, while monster mashers of coral rock (variously in search of live coral tissue, cryptic sponges, boring clams, and endolithic algae) may be equally unwelcome Aquarists lump the monsters, such as triggerfishes, under the term “not reef-safe.” Macrophytic algae are not the only potential competitors capable of displacing corallines and thus inhibiting settlement by hard corals Fouling communities may be composed of any number of sessile invertebrates, both colonial and solitary, plus diverse algae.46 The invertebrate assemblage is dominated by sponges, tunicates, bryozoans, and nonscleractinian Cnidaria Typically we think of invertebrate fouling assemblages as being limited to cryptic environments and undersides of things by spatial competition from photosynthetic scleractinians and macroalgae, but predation (often by supposed herbivores) can be a factor as well.47 Many sessile invertebrates other than corals host photosynthetic bacterial or protistan symbionts and abound on exposed hard surfaces in shallow waters in the tropics For example, the emerald-mouthed didemnid tunicate Didemnon molle and allied species, colored by their symbiotic strains of the cyanobacterium Prochloron, are extremely common recruits to bare limestone (and concrete) in the Indo-Pacific Even nonsymbiotic sponges and colonial tunicates can monopolize exposed surfaces of both natural and artificial structures, provided they are adequately defended.47 In addition to occupying space that might otherwise be available to coral recruits, many fouling organisms are capable of inhibiting coral settlement through allelochemical interactions, or killing and overgrowing young coral recruits Figure 7.3b shows an unidentified invertebrate that has recently become a pest on Biorock frameworks in Permuteran, Bali It covers surfaces, making them unavailable to coral settlement, and also attacks and overgrows hard corals already established on these structures Maida et al.48 have demonstrated that soft corals produce substances that can inhibit hard coral settlement These same soft corals are highly opportunistic space occupants on newly bared substratum For example, hard corals were formerly dominant over a large area of seabottom near Tulamben, Bali These were killed in November 2002 by sedimentation in an area close to a river mouth This bottom is currently occupied by a nearly continuous mat of soft corals (personal observation, 2004) The widespread dominance of soft corals on the tops of Indo-Pacific platform reefs — regarded by diving afficianados and scientists alike as the norm — may rather be a reflection of the differential vulnerability of hard corals to extreme low tides, foul storms, and bleaching events on these shallow (