Grazing dynamics on a Caribbean reef-building coral

Corals in certain Caribbean coral reef habitats are constantly grazed-on due to the territorial marking behavior of the stoplight parrotfish Sparisoma viride. We studied the grazing dynamics on the Caribbean reef-building coral Montastraea annularis. We transplanted colonies to algae-dominated reefs (Rosario Islands, Cartagena, Colombia), where they encountered higher grazing pressure. We counted grazed polyps every month throughout a year. Over the course of a year, 4,101 different grazed polyps were counted on lobe-like M. annularis transplants ( n =23). Grazing was evaluated on a monthly basis as the probabilities of all the possible transitions among four grazing categories (0%, >0–1%, >1–5%, >5% grazed tissue), uncovering a dynamic process. Higher transition probabilities were always between 0 and 1% (coral tissue grazed) grazing states, indicating that grazing did not usually exceed 1% per coral per month. The probability of remaining uninjured in a month was 0.19, 0.17 of a change from 0–1% grazing state, 0.31 of remaining at 1%, and of full recovery from 1% grazing was 0.16. More than one month was usually required for complete recovery ( P<<1) probably due to both steady grazing pressure and slow regeneration rates. Since the marking behavior of the parrotfish was not as common on other zones of the reef no comparison on the grazing among environments was possible. In spite of this, it is possible to have stable transplanted populations of corals such as M. annularis on algae-dominated Caribbean reef environments due to their tolerance to the natural grazing pressure.Communicated by: K. S. Sealey     

Coral reef rehabilitation through transplantation of staghorn corals: effects of artificial stabilization and mechanical damages

In order to develop and test a low-cost method of coral reef rehabilitation, the staghorn corals Acropora muricata and A. vaughani were transplanted to a shallow site with unstable substrate. To avoid abrasion, dislodgement and transport due to water movement, the transplanted corals were tied to string sections, which were connected at the seabed to form a grid. This created stability and improved the survival of the corals. The average increase in weight of live coral over 1 year was 56%, eight times more than the control treatment with unattached coral branches. This difference was mainly due to a reduced partial mortality among smaller coral fragments in the stabilized treatment. Survival was positively related to initial size among the loosely placed coral branches, whereas the attached treatment showed a negative relation between size and relative increase in weight of the surviving parts of the coral branches. Coral fragments were not significantly affected by severe physical damage simulating the effects of handling.     

Enhanced ultraviolet radiation can terminate sexual reproduction in the broadcasting coral species Acropora cervicornis Lamarck

The effects of enhanced ultraviolet radiation (280-400 nm: UVR) on the fecundity of Acropora cervicornis were measured in field-transplanted colonies from 20 m to 1 m depth and vice versa at La Parguera, Puerto Rico. Fecundity was estimated from histological sections made from tissue samples obtained at different time intervals during the experimental period (March - August 2003). All colonies transplanted from 20 m to 1 m showed a 100% reduction in gonads per mesentery per polyp one month after transplantation, while those transplanted from 1 m to 20 m did not show any significant reduction in fecundity throughout the experimental period. The latter colonies did show however, a delay in the spawning times by releasing their gamete bundles approximately two-three weeks after the controls at 1 m and two months after the controls at 20 m suggesting an induced response as a consequence of changes in their daily light cycle due to less radiation (PAR and UVR) available at 20 m compared to 1 m. Control colonies at 20 m spawned after the full moon of June 2003, while the controls at 1 m spawned 5-6 days after the full moon of July 2003. While a possible reabsorption of the gametes occurred in A. cervicornis colonies transplanted from 20 m to 1 m, the expulsion of these gametes due to the sudden stress caused by the transplantation is not discarded. The results suggest that sudden increases in UVR can completely stop sexual reproduction in ramose broadcasting coral species, which in turn can affect the dominance of the species and the composition and structure of shallow reef environments.     

Extrinsic control of species replacement on a Holocene reef in Belize: the role of coral disease

 Well-preserved, Holocene coral reefs provide the opportunity to discriminate between models of intrinsically driven succession and extrinsically driven species replacement, especially when paleontological patterns can be combined with ecological observations of the underlying mechanisms. Rhomboid shoals in the central shelf lagoon of the Belizean Barrier Reef experienced a recent and dramatic change in community composition. Agaricia tenuifolia replaced Acropora cervicornis as the dominant coral species at 3–15 m depth along the flanks of the reefs. We tested the hypothesis that shallowing upward caused this shift in dominance. A core extracted from 0.5 m water depth on one of the shoals, Channel Cay, revealed a shallowing-upward shift in dominance from Acropora to Porites divaricata. This successional sequence was quite different from the Acropora-to-Agaricia transition observed in four cores from 6–11 m water depth. Ecological observations showed that Agaricia became the dominant at ≥3 m depth after Acropora populations were decimated by a regional outbreak of white-band disease. The Acropora-to-Agaricia transition was clearly a case of extrinsically driven species replacement rather than an intrinsically driven, successional, shallowing-upward sequence. Accepted: 14 May 1998     

Densitometry from digitized images of X-radiographs: Methodology for measurement of coral skeletal density

Skeletons of massive coral colonies contain annual density bands that are revealed by X-radiography of slices cut along growth axes. These bands allow measurement of skeletal growth parameters such as annual extension rate and annual calcification rate. Such measurements have been important in understanding coral growth, in assessing environmental impacts and in recovering proxy environmental information. Measurements of coral calcification rate from annual density banding require measurements of skeletal density along tracks across skeletal slices and, until now, such density measurements have depended upon specialized and expensive equipment. Here, we describe a straightforward, inexpensive and accurate technique for measuring skeletal density from digitized images of X-radiographs of coral skeletal slices. An aragonitic step-wedge was included in each X-radiograph of a coral slice together with two aluminium bars positioned along the anode-cathode axis. Optical density was measured along tracks across the X-ray images of these different objects. The aragonite step-wedge provided a standard for converting optical density to skeletal density. The aluminium bars were used to correct for the heel effect—a variation in the intensity of the X-ray beam along the anode-cathode axis that would, otherwise, introduce large errors into measurements of skeletal density. Exposure was found to vary from X-radiographs to X-radiograph, necessitating the inclusion of the calibration standards in each X-radiograph of a coral slice. Results obtained using this technique compared well with results obtained by direct gamma densitometry of skeletal slices.     

Growth of reef fishes in response to live coral cover

Although the global decline in coral reef health is likely to have profound effects on reef associated fishes, these effects are poorly understood. While declining coral cover can reduce the abundance of reef fishes through direct effects on recruitment and/or mortality, recent evidence suggests that individuals may survive in disturbed habitats, but may experience sublethal reductions in their condition. This study examined the response of 2 coral associated damselfishes (Pomacentridae), Chrysiptera parasema and Dascyllus melanurus, to varying levels of live coral cover. Growth, persistence, and the condition of individuals were quantified on replicate coral colonies in 3 coral treatments: 100% live coral (control), 50% live coral (partial) and 0% live coral (dead). The growth rates of both species were directly related to the percentage live coral cover, with individuals associated with dead corals exhibiting the slowest growth, and highest growth on control corals. Such differences in individual growth between treatments were apparent after 29 d. There was no significant difference in the numbers of fishes persisting or the physiological condition of individuals between different treatments on this time-scale. Slower growth in disturbed habitats will delay the onset of maturity, reduce lifetime fecundity and increase individual's vulnerability to gape-limited predation. Hence, immediate effects on recruitment and survival may underestimate the longer-term impacts of declining coral on the structure and diversity of coral-associated reef fish communities.     

Remote video bioassays reveal the potential feeding impact of the rabbitfish Siganus canaliculatus (f: Siganidae) on an inner-shelf reef of the Great Barrier Reef

Herbivores are widely acknowledged as key elements maintaining the health and resilience of terrestrial and aquatic ecosystems. Understanding and quantifying the impact of herbivores in ecosystems are fundamental to our ability to manage these systems. The traditional method of quantifying the impact of herbivorous fishes on coral reefs has been to use transplanted pieces of seagrass or algae as “bioassays”. However, these experiments leave a key question unanswered, namely: Which species are responsible for the impact being quantified? This study revisits the use of bioassays and tested the assumption that the visual abundance of species reflects their role in the removal of assay material. Using remote video cameras to film removal of assay material on an inner-shelf reef of the Great Barrier Reef, the species responsible for assay-based herbivory were identified. The video footage revealed that Siganus canaliculatus, a species not previously recorded at the study site, was primarily responsible for removal of macroalgal biomass. The average percentage decrease in thallus length of whole plants of Sargassum at the reef crest was 54 ± 8.9% (mean ± SE), and 50.4 ± 9.8% for individually presented Sargassum strands (for a 4.5-h deployment). Of the 14,656 bites taken from Sargassum plants and strands across all reef zones, nearly half (6,784 bites or 46%) were taken by S. canaliculatus, with the majority of the remainder attributable to Siganus doliatus. However, multiple regression analysis demonstrated that only the bites of S. canaliculatus were removing macroalgal biomass. The results indicate that, even with detailed observations, the species of herbivore that may be responsible for maintaining benthic community structure can go unnoticed. Some of our fundamental ideas of the relative importance of individual species in ecosystem processes may be in need of re-evaluation.     

Engineering of coral reef larval supply through transplantation of nursery-farmed gravid colonies

The continuous worldwide degradation of coral reefs raises an urgent need for novel active restoration techniques as traditional conservation practices have failed to impede the incessant reefs' decline. While applying the “gardening coral reefs” methodology in Eilat (Red Sea, Israel), we examined reproductive outputs of naturally-grown and outplanted, nursery-farmed Stylophora pistillata colonies from three coral-transplantation trials (November 2005, May 2007, and September 2008), along three reproductive seasons. Surprisingly, transplanted colonies showed better reproductive capacities than the natal Stylophora colonies during > 4 post-transplantation years. A higher percentage of nursery-farmed colonies released planula larvae as compared to naturally-grown colonies. Gravid transplants also shed more planulae per colony, yielding significantly augmented numbers of total planulae over naturally developed S. pistillata colonies. Our results indicate that nursery-grown corals may be used to enhance reef resilience by contributing to the larval pool, forming an engineered larval dispersal instrument for reef rehabilitation.     

Experimental biology of coral reef ecosystems

Coral reef ecosystems are at the crossroads. While significant gaps still exist in our understanding of how “normal” reefs work, unprecedented changes in coral reef systems have forced the research community to change its focus from basic research to understand how one of the most diverse ecosystems in the world works to basic research with strong applied implications to alleviate damage, save, or restore coral reef ecosystems. A wide range of stressors on local, regional, and global spatial scales including over fishing, diseases, large-scale disturbance events, global climate change (e.g., ozone depletion, global warming), and over population have all contributed to declines in coral cover or phase shifts in community structure on time scales never observed before. Many of these changes are directly or indirectly related to anthropogenically induced changes in the global support network that affects all ecosystems. This review focuses on some recent advances in the experimental biology of coral reef ecosystems, and in particular scleractinian corals, at all levels of biological organization. Many of the areas of interest and techniques discussed reflect a progression of technological advances in biology and ecology but have found unique and timely application in the field of experimental coral reef biology. The review, by nature, will not be exhaustive and reflects the author's interests to a large degree. Because of the voluminous literature available, an attempt has been made to capture the essential elements and references for each topic discussed.     

Recurrent disturbances, recovery trajectories, and resilience of coral assemblages on a South Central Pacific reef

Coral reefs are increasingly threatened by various disturbances, and a critical challenge is to determine their ability for resistance and resilience. Coral assemblages in Moorea, French Polynesia, have been impacted by multiple disturbances (one cyclone and four bleaching events between 1991 and 2006). The 1991 disturbances caused large declines in coral cover (~51% to ~22%), and subsequent colonization by turf algae (~16% to ~49%), but this phase-shift from coral to algal dominance has not persisted. Instead, the composition of the coral community changed following the disturbances, notably favoring an increased cover of Porites, reduced cover of Montipora and Pocillopora, and a full return of Acropora; in this form, the reef returned to pre-disturbance coral cover within a decade. Thus, this coral assemblage is characterized by resilience in terms of coral cover, but plasticity in terms of community composition.     

Effects of nutrient enhancement on the fecundity of a coral reef macroalga

On coral reefs, there is concern that increased nutrient supply (e.g. due to eutrophication) causes increased algal growth and hence increased algal abundance, in turn causing colonisation and invasions of coral populations, resulting in reef degradation, or a coral-algal phase shift. For example, species of Sargassum, a highly seasonal, large, brown seaweed, are suggested to be colonising corals on inshore coral reefs of the Great Barrier Reef, as a result of anthropogenic increases in terrestrial runoff of sediments and nutrients. However, implicit in this argument is the assumption that nutrient-related increases in growth will lead to increased fecundity (and/or propagule success), since without such changes, increased abundance can only occur by vegetative means. Whilst plausible, there is no experimental evidence for this assumption in coral reef algae.We here present an initial study in which experimental increases in nutrient supply apparently did not lead to increased fecundity of Sargassum siliquosum; rather, density and biomass of receptacles were reduced in nutrient-enhanced algae. There was little effect of nutrient treatments on the proportional allocation of biomass to reproductive and vegetative structures: nutrient enhancement led to similar decreases in biomass of reproductive and vegetative tissue. Tissue nutrient levels indicated effective enhancement of nitrogen supply, although phosphorus levels were not different across nutrient treatments.The reduced fecundity with increased nutrients may reflect either a genuine inhibition by higher nutrient levels, as found in previous studies, or accelerated maturation, causing increased tissue losses due to more advanced seasonal senescence. Either way, this exploratory experiment provides initial evidence that nutrient effects on tropical coral reef macroalgae may be complex, and does not support the assumption that increased nutrient supply will result in a numeric increase in populations of Sargassum spp. Our results should not be taken as an unequivocal demonstration that nutrients inhibit fecundity overall, but illustrate the need to distinguish between effects on different life-history processes (e.g. growth and reproduction). For increased growth of pre-existing individuals to contribute to algal invasions and phase shifts, that growth must result in either increased fecundity, or increased propagule success.     

珊瑚礁生态保护与管理研究

珊瑚礁以其极高的生物多样性和生物生产力以及优美的自然景观 ,为人们提供了生活需要和游乐的资源 ,但同时也受到过度利用的破坏 ,尤其是近年来显得更为严重 ,因而珊瑚礁的生态保护与管理成为近 2 0年来倍受关注的问题。本文回顾了国内外珊瑚礁生态保护和管理的一些研究成果 ,通过自然和社会经济调查 ,并根据保护、研究和可持续利用的原则 ,将雷州半岛灯楼角珊瑚礁保护区划分为野生区 ,保护区、季节性封闭区和一般使用区 ,并强调公众参与、社区组织和领导组成、教育和培训、资源管理等为保护和管理中的措施     

Habitat choice,recruitment and the response of coral reef fishes to coral degradation

The global degradation of coral reefs is having profound effects on the structure and species richness of associated reef fish assemblages. Historically, variation in the composition of fish communities has largely been attributed to factors affecting settlement of reef fish larvae. However, the mechanisms that determine how fish settlers respond to different stages of coral stress and the extent of coral loss on fish settlement are poorly understood. Here, we examined the effects of habitat degradation on fish settlement using a two-stage experimental approach. First, we employed laboratory choice experiments to test how settlers responded to early and terminal stages of coral degradation. We then quantified the settlement response of the whole reef fish assemblage in a field perturbation experiment. The laboratory choice experiments tested how juveniles from nine common Indo-Pacific fishes chose among live colonies, partially degraded colonies, and dead colonies with recent algal growth. Many species did not distinguish between live and partially degraded colonies, suggesting settlement patterns are resilient to the early stages of declining coral health. Several species preferred live or degraded corals, and none preferred to associate with dead, algal-covered colonies. In the field experiment, fish recruitment to coral colonies was monitored before and after the introduction of a coral predator (the crown-of-thorns starfish) and compared with undisturbed control colonies. Starfish reduced live coral cover by 95–100%, causing persistent negative effects on the recruitment of coral-associated fishes. Rapid reductions in new recruit abundance, greater numbers of unoccupied colonies and a shift in the recruit community structure from one dominated by coral-associated fishes before degradation to one predominantly composed of algal-associated fish species were observed. Our results suggest that while resistant to coral stress, coral death alters the process of replenishment of coral reef fish communities.     

Varying responses of herbivorous and invertebrate-feeding fishes to macroalgal reduction on a coral reef

 The consequences of macroalgal overgrowth on reef fishes and means to reverse this condition have been little explored. An experimental reduction of macroalgae was conducted at a site in the Watamu Marine National Park in Kenya, where a documented increase in macroalgal cover has occurred over the last nine years. In four experimental 10 m by 10 m plots, macroalgae were greatly reduced (fleshy algal cover reduced by 84%) by scrubbing and shearing, while four similar plots acted as controls. The numerical abundance in all fish groups except wrasses and macroalgal-feeding parrotfishes (species in the genera Calotomus and Leptoscarus) increased in experimental algal reduction plots. Algal (Sargassum) and seagrass (Thalassia) assays, susceptible to scraping and excavating parrotfishes, were bitten more frequently in the algal reduction plots one month after the manipulation. Further, surgeonfish (Acanthurus leucosternon and A. nigrofuscus) foraging intensity increased in these algal reduction plots. The abundance of triggerfishes increased significantly in experimental plots relative to control plots, but densities remained low, and an index of sea urchin predation using tethered juvenile and adult Echinometra mathaei showed no differences between treatments following macroalgal reduction. Dominance of reefs by macrofleshy algae appears to reduce the abundance of fishes, mostly herbivores and their rates of herbivory, but also other groups such as predators of invertebrates (triggerfishes, butterflyfishes and angelfishes). Accepted: 2 February 1999     

Ecological structure of assemblages of coral reef fishes on isolated patch reefs

A 9-year study of the structure of assemblages of fish on 20 coral patch reefs, based on 20 non-manipulative censuses, revealed a total of 141 species from 34 families, although 40 species accounted for over 95% of sightings of fish. The average patch reef was 8.5 m² in surface area, and supported 125 fish of 20 species at a census. All reefs showed at least a two-fold variation among censuses in total numbers of fish present, and 12 showed ten-fold variations. There was also substantial variation in the composition and relative abundances of species present on each patch reef, such that censuses of a single patch reef were on average about 50% different from each other in percent similarity of species composition (Czekanowski's index). Species differed substantially in the degree to which their numbers varied from census to census, and in the degree to which their dispersion among patch reefs was modified from census to census. We characterize the 40 most common species with respect to these attributes. The variations in assemblage structure cannot be attributed to responses of fish to a changing physical structure of patch reefs, nor to the comings and goings of numerous rare species. Our results support and extend earlier reports on this study, which have stressed the lack of persistant structure for assemblages on these patch reefs. While reef fishes clearly have microhabitat preferences which are expressed at settlement, the variations in microhabitat offered by the patch reefs are insufficient to segregate many species of fish by patch reef. Instead, at the scale of single patch reefs, and, to a degree, at the larger scale of the 20 patch reefs, most of the 141 species of fish are distributed without regard to differences in habitat structure among reefs, and patterns of distribution change over time. Implications for general understanding of assemblage dynamics for fish over more extensive patches of reef habitat are considered.     

Grazing pressure of herbivorous coral reef fishes on low coral-cover reefs

The impact of grazing by herbivorous fishes (Acanthuridae, Scaridae, and Pomacentridae) on low coral-cover reefs was assessed by measuring rates of benthic algal production and consumption on inshore and offshore reefs in the upper Florida Keys. Algal production rates, determined in situ with caged and uncaged experimental plates, were low (mean 1.05 g C m⁻² day⁻¹) and similar among reef types. Algal consumption rates were estimated using two different models, a detailed model incorporating fish bite rates and algal yield-per-bite for one species extrapolated to a guild-wide value, and a general regression relating fish biomass to algal consumption. Algal consumption differed among reef types: a majority of algal production was consumed on offshore reefs (55–100%), whereas consumption on inshore patch reefs was 31–51%. Spatial variation in algal consumption was driven by differences in herbivorous fish species composition, density, and size-structure among reef types. Algal consumption rates also varied temporally due to seasonal declines in bite rates and intermittent presence of large-bodied, vagile, schooling species. Spatial coherence of benthic community structure and temporal stability of algal turf over 3 years suggests that grazing intensity is currently sufficient to limit further spread of macroalgal cover on these low coral-cover reefs, but not to exclude it from the system.     

Impact of micropredatory gnathiid isopods on young coral reef fishes

The ecological role of parasites in the early life-history stages of coral reef fish, and whether this varies between fish with and without a pelagic phase, was investigated. The susceptibility to, and effect of reef-based micropredatory gnathiid isopods on larval, recently settled, and juvenile fishes was tested using two damselfishes (Pomacentridae): Neopomacentrus azysron, which has pelagic larvae, and Acanthochromis polyacanthus, which does not. When larval and recently settled stages of N. azysron and very young A. polyacanthus juveniles (smaller than larval N. azysron) were exposed to one or three gnathiids, the proportion of infections did not vary significantly among the three host types or between the number of gnathiids to which the fish were exposed. The overall infection was 35%. Mortality, however, differed among the three gnathiid-exposed host types with most deaths occurring in larval N. azysron; no mortalities occurred for recently settled N. azysron exposed to one or three gnathiids, and A. polyacanthus exposed to one gnathiid. Mortality did not differ significantly between larval N. azysron and A. polyacanthus juveniles, failing to provide support for the hypothesis that reef-based A. polyacanthus juveniles are better adapted to gnathiid attack than fish with a pelagic phase. The study suggests that settling on the reef exposes young fish to potentially deadly micropredators. This supports the idea that the pelagic phase may allow young fish to avoid reef-based parasites.     

Evidence of density-independent mortality in a settling coral reef damselfish, Chromis viridis

To know if the variation in the number of settling fish larvae can be dampened by density-dependent postsettlement mortality, we investigated the relationship between settler density and predator-induced mortality of a coral reef damselfish, Chromis viridis. Totals of 2, 3, 5, 8, 10, 12, 14, 16, 18, and 20 fish of 10 or 20 mm total length were released in experimental cages enclosing a coral head of Porites rus (to provide settlement habitat) and five predators. The results showed that the mortality rate of both 10- and 20-mm fish was density independent.     

Habitat complexity modifies the impact of piscivores on a coral reef fish population

Patterns in juvenile mortality rates can have a profound affect on the distribution and abundance of adult individuals, and may be the result of a number of interacting factors. Field observations at Lizard Island (Great Barrier Reef, Australia) showed that for a coral reef damselfish, Pomacentrus moluccensis, juvenile mortality (over 1 year) varied between 20 and almost 100% among sites. Correlative data showed that juvenile mortality increased as a function of initial densities (recruitment), predator densities and the availability of preferred coral substrata. A multiple regression showed that these three variables together did not explain significantly more variation in mortality than the single factor showing the strongest relationship. This appeared to be because recruitment, predator densities and preferred coral substrata were all highly correlated, suggesting that one, two or all of these factors may be influencing juvenile mortality rates. One hypothesis was that density-dependent mortality in juveniles was the result of an interaction between predators (which appear to aggregate at high-recruitment sites) and the availability of preferred substrata (predator refuges). We tested this hypothesis by using both laboratory and field experiments to see whether fish predation could significantly alter survivorship of this damselfish, and whether this impact was dependent upon the coral substratum. The laboratory experiment was designed to test the effects of three common predators (Pseudochromis fuscus, Cephalopholis boenak and Thalassoma lunare) and three different coral substrata that varied in their complexity (Pocillopora damicornis, Acropora nasuta and A. nobilis) on the survival of juvenile Pomacentrus moluccensis. There was a significant interaction between predator species and microhabitat in determining survival. Pseudochromis fuscus and C. boenak were both significantly better at capturing juvenile damselfish than T. lunare. Juvenile survivorship was significantly better when they were given the more complex corals, Pocillopora damicornis and A. nasuta, compared with those given the open-structured species A. nobilis. This pattern reflects habitat selection in the field. Predators differed in their strike rates and the proportion of strikes that were successful, but all exhibited greater success at prey capture where A. nobilis was provided as shelter. The interaction between the effect of predator species and microhabitat structure on damselfish survival was tested in the field for a cohort of juvenile Pomacentrus moluccensis. We examined juvenile survival in the presence and absence of two predators that co-occur on natural patch reefs (C. boenak and Pseudochromis fuscus). The experimental patch reefs we used for this purpose were constructed from both high complexity (Pocillopora damicornis) and low complexity (A. nobilis) coral substrata. Both juveniles and predators were translocated to reefs at natural densities. The effects of predation were clearly dependent upon the microhabitat. Reefs of the high-complexity coral with predators supported the same high numbers of Pomacentrus moluccensis as the reefs with no resident predators. However, damselfish abundance was significantly lower on low-complexity reefs with resident predators, relative to the other treatments. Background rates of loss were high, even on preferred coral in the absence of the manipulated predator, suggesting that transient predators may be even more important than the residents. We suggest that adult abundances in this species were strongly influenced by the densities of different predators and the availability of preferred refuges. Received: 3 April 1997 / Accepted: 26 August 1997     

Coexistence in a sea urchin guild and its implications to coral reef diversity and degradation

Summary Coexistence between the coral reef inhabiting sea urchins Echinometra mathaei, Diadema savignyi and D. setosum was studied by comparing differences in body morphology, distribution, diet, susceptibility to predators, intra- and interspecific competition and settlement. The three species share similar diets and broad within-habitat distributions but differ in their microspatial preferences. E. mathaei is the smallest species, has the highest settlement rates and lives territorially within small burrows or crevices. D. savignyi is intermediate in size and lives frequently in intermediate size crevices or occassionally in social groups. D. setosum is the largest species and occassionally lives in large crevices or more frequently in social groups. Both Diadema have similarily low settlement rates. Competition experiments showed that E. mathaei was consistently the top competitor for crevice space. Diadema species shared larger crevices but competition occured within smaller crevices and was frequently won by the largest individual, regardless of species. D. savignyi may be the top competitor for crevice space between the Diadema species due to a reduced spine length/test size ratio which gives it a larger test for the same crevice size requirement. Predation rates were high for E. mathaei and low for both Diadema species. Coexistence is mediated by predation on the competitive-dominant while predation coupled with different body morphologies and behavior allows spatial resource partitioning of the reef's variable topography. Consequently, the three variables of predation, topographic complexity and differing body shapes create the observed species diversity. A reduction in predators due to stochastic fluctuations or from fishing pressure can lead to E. mathaei population increases and competitive exclusion of Diadema.