Reefs Worldwide Research Articles

Reefs under Siege—the Rise, Putative Drivers, and Consequences of Benthic Cyanobacterial Mats

Benthic cyanobacteria have commonly been a small but integral component of coral reef ecosystems, fulfilling the critical function of introducing bioavailable nitrogen to an inherently oligotrophic environment. Though surveys may have previously neglected benthic cyanobacteria, or grouped them with more conspicuous benthic groups, emerging evidence strongly indicates that they are becoming increasingly prevalent on reefs worldwide. Some species can form mats comprised by a diverse microbial consortium which allows them to exist across a wide range of environmental conditions. This review evaluates the putative driving factors of increasing benthic cyanobacterial mats, including climate change, declining coastal water quality, iron input, and overexploitation of key consumer and ecosystem engineer species. Ongoing global environmental change can increase growth rates and toxin production of physiologically plastic benthic cyanobacterial mats, placing them at a considerable competitive advantage against reef-building corals. Once established, strong ecological feedbacks [e.g., inhibition of coral recruitment, release of dissolved organic carbon (DOC)] reinforce reef degradation. The review also highlights previously overlooked implications of mat proliferation, which can extend beyond reef health and affect human health and welfare. Though identifying (opportunistic) consumers of mats remains a priority, their perceived low palatability implies that herbivore management alone may be insufficient to control their proliferation and must be accompanied by local measures to improve water quality and watershed management.

Emergence of serranid pigment abnormality syndrome (SPAS) in wire netting cod (Epinephelus quoyanus) from Heron Island on the southern Great Barrier Reef

Coral reefs worldwide are under increasing stress from anthropogenic impacts, but there are relatively few reports of increased rates of disease in coral reef fish. Herein we report the emergence of abnormal skin lesions in wild-caught wire netting cod (Epinephelus quoyanus) near Heron Island in the southern Great Barrier Reef. The lesion involves conspicuous darkening and disorganisation of the brown ‘wire netting’ colouration pattern typical of this species, most commonly on the lower jaw, premaxilla and head, with occasional involvement of the flanks and dorsal fin in some fish. The lesion was not present during research conducted in the mid-1990s; however, since it was first recorded in 2012, the prevalence of grossly visible lesions has increased to 16.9% in 2017, with fish >340mm long most affected (prevalence 64.7%). These data suggest emergence of the lesion is a recent phenomenon and that causative factors may be age related. Abnormal pigmentation lesions have only been observed to affect E. quoyanus and coral trout (Plectropomus leopardus; since 2010). Given the species affected and the currently unknown aetiology of these lesions, we name the condition serranid pigment abnormality syndrome (SPAS). Further research is required to determine its geographic distribution, establish causation and describe the course of disease in E. quoyanus.

Reversing a regime shift (or not)

Marine Conservation![Figure][1] Herbivores are needed to keep the macroalga Sargassum from replacing corals. CREDIT: WATERFRAME/ALAMY STOCK PHOTO Coral reefs worldwide face many challenges. One of these, brought on by declines in herbivore populations on reefs, is a tendency to shift from dominance by live corals to dominance by macroalgae. Loffler and Hoey, in a study of the leathery brown macroalga Sargassum on Australia's Great Barrier Reef, show how hard it is to reverse this regime shift. Physical removal of the Sargassum canopy, simulating the activity of storms, had a negligible effect, and even removal of the alga's “rooting” holdfasts was only a temporary setback to the Sargassum community. To improve the likelihood of restoring the coral community, physical removal needs to be accompanied by herbivore activity. J. Ecol. 10.1111/1365-2745.12875 (2017). [1]: pending:yes

Empirical data demonstrates risk-tradeoffs between landscapes for herbivorous fish may promote reef resilience

Herbivores balance resource requirements with predation risk, which can differ among landscapes; hence, landscape can shape these trade-offs, influencing herbivore distribution and behavior. While this paradigm has been well established on coral-dominated reefs, tropical reefs worldwide are shifting to algal dominance. If herbivores avoid algae due to higher risk and forage in coral, these algal states may be stabilized. However, if herbivores forage more in resource-rich algal states, this may promote coral recovery. We assessed the distribution and behavior of herbivorous fishes in Moorea, French Polynesia in coral and algal turf-dominated fringing reef sites. Acanthuridae were more abundant in coral states and Labridae, tribe Scarinae, in algal turf states, though total fish abundances were equivalent in the two states. Fish in both families spent more time feeding in algal states and hiding/swimming in coral states. Thus, behavior reflects the trade-off between resource acquisition and refuge in these two landscapes and may promote recovery to coral.

What Is Currently Known About the Effects of Climate Change on the Coral Immune Response

It is well documented that climate change has a negative effect on coral reefs worldwide. Recurrent warming events, ocean acidification, and nutrient pollution are some of the hallmarks of climate change; each affects the health of coral, and together, their effects are multiplied. It is hypothesized that a healthy coral will have a strong, highly active immune system when confronted with different stressors. However, there is very little that we understand about how the coral immune system reacts to different climate change stressors. In this review, we will examine what is known about the effects of heat stress, ocean acidification, and nutrient pollution on the coral immune system. We will identify gaps in our knowledge and briefly discuss a path forward to address these gaps.

Contact with turf algae alters the coral microbiome: contact versus systemic impacts

Coral reefs are degrading to algae-dominated reefs worldwide, with alterations of coral microbiomes commonly co-occurring with reef demise. The severe thermal anomaly during the 2016 El Nino event in the South Pacific killed many corals and stressed others. We examined the microbiome of turf algae and of the coral Porites sp. in contact with turf during this thermal event to investigate algal turf effects on the coral microbiome during a period of environmental stress. The microbial composition of turf did not differ between coral-contacted and non-contacted turfs. However, microbiomes of corals in direct contact with turf were similar to those of the turf microbiome, but differed significantly from coral portions 5 cm from the point of turf/coral contact and from portions of the coral that looked most healthy, regardless of location. Although the majority of significant differences occurred in coral samples at the point of contact, a small subset of microbial taxa was enriched in coral tissues taken 5 cm from turf contact compared to all other sample types, including samples from areas of the coral that appeared most healthy. These results suggest that the coral microbiome is susceptible to colonization by microbes from turf, but not vice versa. Results also suggest that algal contact elicits a subtle shift in the coral microbiome just beyond the contact site. The combination of turf microbiome stability and coral microbiome vulnerability at areas of contact may contribute to the continued decline in coral cover and increase in algal cover associated with coral–algae phase shifts.

Effects of Changing Weather, Oceanographic Conditions, and Land Uses on Spatio-Temporal Variation of Sedimentation Dynamics along Near-Shore Coral Reefs

Sedimentation is a critical threat to coral reefs worldwide. Major land use alteration at steep, highly erodible semi-arid islands accelerates the potential of soil erosion, runoff, and sedimentation stress to nearshore coral reefs during extreme rainfall events. The goal of this study was to assess spatio-temporal variation of sedimentation dynamics across nearshore coral reefs as a function of land use patterns, weather and oceanographic dynamics, to identify marine ecosystem conservation strategies. Sediment was collected at a distance gradient from shore at Bahia Tamarindo (BTA) and Punta Soldado (PSO) coral reefs at Culebra Island, Puerto Rico. Sediment texture and composition were analyzed by dry sieving and loss-on-ignition techniques, and were contrasted with environmental variables for the research period (February 2014 to April 2015). Rainfall and oceanographic data were analyzed to address their potential role on affecting sediment distribution with BEST BIO-ENV, RELATE correlation, and linear regression analysis. A significant difference in sedimentation rate was observed by time and distance from shore (PERMANOVA, p<0.0100), mostly attributed to higher sediment exposure at reef zones closer to shore due to strong relationships with coastal runoff. Sedimentation rate positively correlated with strong rainfall events (Rho= 0.301, p=0.0400) associated with storms and rainfall intensity exceeding 15 mm/hr. At BTA, sediment deposited were mostly composed of sand, suggesting a potential influence of resuspension produced by waves and swells. In contrast, PSO sediments were mostly composed of silt-clay and terrigenous material, mainly attributed to a deforestation event that occurred at adjacent steep sub-watershed during the study period. Spatial and temporal variation of sedimentation pulses and terrigenous sediment input implies that coral reefs exposure to sediment stress is determined by local land use patterns, weather, and oceanographic dynamics. Comprehensive understanding of sediment dynamics and coastal ecosystem interconnectivity is fundamental to implement integrated and adaptive management strategies aimed to promote sustainable development at watershed and island wide-scale to fully mitigate terrigenous sediment impact to marine ecosystems. Furthermore, decision-making processes and policy needs to address sedimentation stress in the context of future climate to reduce land-based threats and strengthen coral reef resilience.

Engineering Strategies to Decode and Enhance the Genomes of Coral Symbionts.

Elevated sea surface temperatures from a severe and prolonged El Niño event (2014–2016) fueled by climate change have resulted in mass coral bleaching (loss of dinoflagellate photosymbionts, Symbiodinium spp., from coral tissues) and subsequent coral mortality, devastating reefs worldwide. Genetic variation within and between Symbiodinium species strongly influences the bleaching tolerance of corals, thus recent papers have called for genetic engineering of Symbiodinium to elucidate the genetic basis of bleaching-relevant Symbiodinium traits. However, while Symbiodinium has been intensively studied for over 50 years, genetic transformation of Symbiodinium has seen little success likely due to the large evolutionary divergence between Symbiodinium and other model eukaryotes rendering standard transformation systems incompatible. Here, we integrate the growing wealth of Symbiodinium next-generation sequencing data to design tailored genetic engineering strategies. Specifically, we develop a testable expression construct model that incorporates endogenous Symbiodinium promoters, terminators, and genes of interest, as well as an internal ribosomal entry site from a Symbiodinium virus. Furthermore, we assess the potential for CRISPR/Cas9 genome editing through new analyses of the three currently available Symbiodinium genomes. Finally, we discuss how genetic engineering could be applied to enhance the stress tolerance of Symbiodinium, and in turn, coral reefs.

Facing a wicked problem with optimism: issues and priorities for coral conservation in Hong Kong

Economic development and environmental conservation are often seen as opposing forces in the arena of government policy-making. With more than 7 million people and a rich diversity of marine species and habitats, Hong Kong is an excellent case study to explore this dynamic. Despite anthropogenic impacts, Hong Kong still hosts more than 90 species of stony corals within a marine area of 1650 km2. This is remarkable in light of the global plight of coral reefs, which have been reduced by ~80% worldwide in recent decades. The Hong Kong Special Administrative Region has not been immune to this negative trend with an unfortunate track of marine environmental disasters and, as such, can be viewed a harbinger for the future trajectory of coral reefs worldwide. Yet, the story is not entirely negative. Hong Kong possesses key assets, including capable government environmental agencies and competitive research led by local universities, which can bring novel and promising approaches for coral biodiversity conservation in an urbanized context. To coordinate and assist conservation efforts in Hong Kong, we here identify and prioritize major management efforts and identify knowledge gaps for coral conservation based on updated coral biodiversity distribution and a current literature review. Specifically, we propose five priorities for the most positive impact on conservation efforts: (1) reconstruct environmental baselines and establish long-term monitoring of local coral communities, (2) enhance the management and protection of local coral habitats, (3) improve water quality, (4) gain an understanding of the genetic connectivity among local and distant coral communities, and finally, (5) establish an active restoration program for local coral species/communities. In order to build progressive and integrative management strategies for coral biodiversity conservation in Hong Kong, we suggest specific ways in which these priorities be addressed and encourage a fresh dialogue between the government, the public and academia.

Seriatopora Diversity Preserved in Upper Mesophotic Coral Ecosystems in Southern Japan

Coral reefs worldwide are facing increasing stress due to drastic changes in their environment. Mesophotic coral ecosystems (MCEs) have been considered as a potential refuge from several major stressors, such as warm-water bleaching events. However, their role as a subsequent source of larvae remains unclear for many species, particularly as genetic differentiation of corals over depth has frequently been observed. In 1998 and 2001, two severe bleaching events around Okinawa Island in Japan resulted in major changes to the shallow reefs, including the local “extinction” of species such as Seriatopora hystrix at Sesoko Island. However, recently this species was found to be present in abundance at mesophotic depths in the area, despite no clear signs of recovery being observed in the adjacent shallow waters. Here, we assessed the genetic diversity of Seriatopora from this deep population and provide a comparison with populations from shallow to mesophotic depths in other parts of the Ryukyu archipelago, to understand their depth specificity and their importance in genetic diversity conservation of affected shallow populations. High levels of genetic diversity were observed in both shallow and mesophotic Seriatopora populations for both the nuclear (internal transcribed spacer 2, ITS2) and the mitochondrial (hypervariable open reading frame, ORF) marker, with no clear partitioning of haplotypes over depth or across locations in the archipelago. Both ITS2 and ORF suggest the presence of potential cryptic species and the discrepancy between the markers could reflect hybridization or incomplete lineage sorting. Although associated endosymbionts (Symbiodinium spp.) all shared the same mitochondrial haplotype (cytochrome oxidase subunit 1, COI), the nuclear ribosomal ITS2 revealed slight potential habitat partitioning between the genotypes, with a small decrease of C59 Symbiodinium types below 10 m depth and a mirrored increase in C1/C78a-related types. The relative absence of depth-specific host lineages and the substantial overlap between shallow and deep Symbiodinium types indicate that Okinawan MCEs may act as a refuge preserving genotypic diversity of bleaching-sensitive coral such as Seriatopora, and on the long-term may have the potential to contribute to shallow-water recolonization of this species.

A coral-algal phase shift in Mesoamerica not driven by changes in herbivorous fish abundance.

Coral-algal phase shifts in which coral cover declines to low levels and is replaced by algae have often been documented on coral reefs worldwide. This has motivated coral reef management responses that include restriction and regulation of fishing, e.g. herbivorous fish species. However, there is evidence that eutrophication and sedimentation can be at least as important as a reduction in herbivory in causing phase shifts. These threats arise from coastal development leading to increased nutrient and sediment loads, which stimulate algal growth and negatively impact corals respectively. Here, we first present results of a dynamic process-based model demonstrating that in addition to overharvesting of herbivorous fish, bottom-up processes have the potential to precipitate coral-algal phase shifts on Mesoamerican reefs. We then provide an empirical example that exemplifies this on coral reefs off Mahahual in Mexico, where a shift from coral to algal dominance occurred over 14 years, during which there was little change in herbivore biomass but considerable development of tourist infrastructure. Our results indicate that coastal development can compromise the resilience of coral reefs and that watershed and coastal zone management together with the maintenance of functional levels of fish herbivory are critical for the persistence of coral reefs in Mesoamerica.

Metabolism of a tide‐dominated reef platform subject to extreme diel temperature and oxygen variations

AbstractBenthic dissolved oxygen fluxes were measured on the reef flat of Tallon Island, an intertidal reef platform in the Kimberley region of northwestern Australia, for periods of 2 weeks in the wet and dry seasons. This reef flat is strongly tidally forced by semidiurnal tides (spring range &gt; 8 m) and experiences highly asymmetric water level variability, with ebb durations lasting ∼10 h; this results in diel variations in water temperature and dissolved oxygen (DO) concentration (up to ∼11°C and 440 μM, respectively) that are among the most extreme recorded for reefs worldwide. Given the consistent tidal flow patterns, a one‐dimensional control volume approach was used to make continuous Eulerian measurements of net production and community respiration from observed changes in DO within two zones: an inner zone dominated by seagrass and an outer zone dominated by macroalgae. Community respiration (R) was controlled primarily by DO concentration; however, fluxes approached the limits of DO mass transfer at low flow speeds. Estimates of gross primary production (P) suggested that reef communities were able to fix carbon at rates comparable to other tropical seagrass and mixed reef flat communities despite short‐term (∼hours) extremes in light (up to 1800 μmol m−2 s−1) and temperature (&gt; 35°C). Daily net community production fluctuated between net autotrophy and heterotrophy over a ∼15 d period depending on the phase difference between the solar and tidal cycles but was nonetheless metabolically balanced on time scales greater than weeks (P : R = 1.0–1.1).

Tropical dead zones and mass mortalities on coral reefs.

Degradation of coastal water quality in the form of low dissolved oxygen levels (hypoxia) can harm biodiversity, ecosystem function, and human wellbeing. Extreme hypoxic conditions along the coast, leading to what are often referred to as "dead zones," are known primarily from temperate regions. However, little is known about the potential threat of hypoxia in the tropics, even though the known risk factors, including eutrophication and elevated temperatures, are common. Here we document an unprecedented hypoxic event on the Caribbean coast of Panama and assess the risk of dead zones to coral reefs worldwide. The event caused coral bleaching and massive mortality of corals and other reef-associated organisms, but observed shifts in community structure combined with laboratory experiments revealed that not all coral species are equally sensitive to hypoxia. Analyses of global databases showed that coral reefs are associated with more than half of the known tropical dead zones worldwide, with >10% of all coral reefs at elevated risk for hypoxia based on local and global risk factors. Hypoxic events in the tropics and associated mortality events have likely been underreported, perhaps by an order of magnitude, because of the lack of local scientific capacity for their detection. Monitoring and management plans for coral reef resilience should incorporate the growing threat of coastal hypoxia and include support for increased detection and research capacity.

Scientists Develop New Tool to Monitor Reef Health

A first-of-its-kind system could reveal short-term changes in threatened reefs worldwide

Coral Reef Ecosystems: Valuable and Critically Threatened

Coral reef ecosystems are beautiful and vibrant underwater communities inhabited by a spectacularly diverse and abundant array of organisms essential to human needs. Reefs are severely threatened worldwide—estimates are that one-fifth of coral reefs have already been lost or severely damaged, and the rest are expected to disappear before the end of this century. The intrinsic value of coral reefs is incalculable and immense, and this technical note seeks to educate students about that value, as well as the threats—global and local—to coral reefs. Excerpt UVA-ENT-0201 Mar. 12, 2014 CORAL REEF ECOSYSTEMS: VALUABLE AND CRITICALLY THREATENED The Value of Coral Reefs Coral reef ecosystems are beautiful and vibrant underwater communities inhabited by a spectacularly diverse and abundant array of organisms essential to human needs. Reefs are severely threatened worldwide—estimates are that one-fifth of coral reefs have already been lost or severely damaged, and the rest are expected to disappear before the end of this century. The intrinsic value of coral reefs is incalculable and immense. Figure 1 shows the distribution of coral reefs worldwide. Each red dot represents an image taken by the Landsat 7 spacecraft for the Millennium Coral Reef Mapping Project. Red dots located over smaller reefs may over-represent the area covered by those reefs. Figure 1. Worldwide coral reef distribution. Source: “NASA Coral Reef Images Key to New Global Survey,” http://www.nasa.gov/vision/earth/lookingatearth/coral_assessment.html (accessed March 29, 2013). . . .

Extensions of Island Biogeography Theory predict the scaling of functional trait composition with habitat area and isolation.

The Theory of Island Biogeography (TIB) predicts how area and isolation influence species richness equilibrium on insular habitats. However, the TIB remains silent about functional trait composition and provides no information on the scaling of functional diversity with area, an observation that is now documented in many systems. To fill this gap, we develop a probabilistic approach to predict the distribution of a trait as a function of habitat area and isolation, extending the TIB beyond the traditional species-area relationship. We compare model predictions to the body-size distribution of piscivorous and herbivorous fishes found on tropical reefs worldwide. We find that small and isolated reefs have a higher proportion of large-sized species than large and connected reefs. We also find that knowledge of species body-size and trophic position improves the predictions of fish occupancy on tropical reefs, supporting both the allometric and trophic theory of island biogeography. The integration of functional ecology to island biogeography is broadly applicable to any functional traits and provides a general probabilistic approach to study the scaling of trait distribution with habitat area and isolation.

Aspergillus sydowii and Other Potential Fungal Pathogens in Gorgonian Octocorals of the Ecuadorian Pacific.

Emerging fungal diseases are threatening ecosystems and have increased in recent decades. In corals, the prevalence and consequences of these infections have also increased in frequency and severity. Coral reefs are affected by an emerging fungal disease named aspergillosis, caused by Aspergillus sydowii. This disease and its pathogen have been reported along the Caribbean and Pacific coasts of Colombia. Despite this, an important number of coral reefs worldwide have not been investigated for the presence of this pathogen. In this work, we carried out the surveillance of the main coral reef of the Ecuadorian Pacific with a focus on the two most abundant and cosmopolitan species of this ecosystem, Leptogorgia sp. and Leptogorgia obscura. We collected 59 isolates and obtained the corresponding sequences of the Internal Transcribed Spacers (ITS) of the ribosomal DNA. These were phylogenetically analyzed using MrBayes, which indicated the presence of two isolates of the coral reef pathogen A. sydowii, as well as 16 additional species that are potentially pathogenic to corals. Although the analyzed gorgonian specimens appeared healthy, the presence of these pathogens, especially of A. sydowii, alert us to the potential risk to the health and future survival of the Pacific Ecuadorian coral ecosystem under the current scenario of increasing threats and stressors to coral reefs, such as habitat alterations by humans and global climate change.

Coral reef fish assemblages at Clipperton Atoll (Eastern Tropical Pacific) and their relationship with coral cover

Clipperton Atoll, one of the most isolated coral reefs worldwide, is of great scientific interest due to its geomorphology and high levels of endemism. This study explored the reef fish assemblage structure of Clipperton Atoll and its relationship with live coral cover. Nine stations were sampled at three sites and three depths (6, 12 and 20 m) around the reef, measuring fish species richness and biomass and hermatypic coral cover (at genus level). We evaluated variation in species richness, biomass and diversity of fish assemblages among sites and depths, as well as the relationship between the entire fish assemblage composition and live coral cover. The results showed that species richness and biomass were similar among sites, but differed across depths, increasing with depth. In contrast, diversity differed among sites but not among depths. Multivariate analyses indicated that fish assemblage composition differed among sites and depths in relation to changes in cover of coral of the genera Pocillopora, Porites, Pavona and Leptoseris, which dominate at different depths. The results showed that fish species richness and diversity were low at Clipperton Atoll and that, in isolated coral reefs with a low habitat heterogeneity and low human disturbance, live coral cover has a significant influence on the spatial variation of the reef fish assemblages. This study highlights the importance of coral habitat structure in shaping coral reef fish assemblages.

Elaborating an eco-engineering approach for stock enhanced sexually derived coral colonies

Despite all traditional conservation management efforts, coral reefs worldwide continue to face a future of degradation and destruction. Current studies call for the augmentation of coral reef management with restoration practices, particularly alternative reef management approaches, such as the ‘coral gardening’ tenet of active reef restoration. This includes the stock creation of sexually derived coral colonies and their mariculture in mid-water coral nurseries. In this study, single and aggregated (resulting in non-fusion and chimeric entities) spats of the branching coral Stylophora pistillata were studied. Planula larvae were settled and spat were reared ex situ for 50–75days before they were transplanted to the in situ mid-water nursery in Eilat, Israel, where they were followed for up to two years, showing a very low (<2%) detachment rate. Spats were cultured in horizontal and vertical orientations, in caged (for the first 9 nursery months) and noncaged scenarios, and on two nursery beddings (nets and ropes). Caging of horizontally situated young spats on the net substrate resulted in the highest survival (>80% after 2years). Corals farmed in a vertical orientation had the lowest survival rate of the caged experiments (36.7%) but showed that chimeric/aggregated entities performed significantly better than the single colonies. The uncaged experiments had low (32.7%) to zero surviving spats after two years in situ. The surviving colonies reared under the uncaged conditions were significantly smaller than the caged colonies after the 9months in situ ‘caging period’. Generally, the placement of the young spats in the mid-water nursery resulted in high growth rates: After two years in situ, coral colony diameter increased from 0.1cm to 8.16±1.58cm for the vertical caged scenario, and 7.08±1.72cm, 7.02±1.48cm for the two caged horizontal designs (HN and HR). This is nearly twice the growth rate observed in natal colonies. The midwater coral nursery is a much cheaper solution for growing corals compared to ex situ water tables, which require high maintenance and expensive facilities to mimic in situ conditions. Non-caged coral stocks showed reduced survival and growth rates, similar to previously published results. The culturing of caged spats in a horizontal position in a midwater nursery exponentially augment survival and growth rates, thus enhancing stock creation yields. This offers new possibilities for creating stocks of sexually derived spats from eco-engineering coral species such as S. pistillata.

Anti-predatory effects of organic extracts of 10 common reef sponges from Zanzibar

Predation is a key factor influencing the distribution and community composition of sponges and other benthic organisms. The ability to produce deterrent secondary metabolites may partially explain the high abundances of sponges on coral reefs worldwide. Nonetheless, studies investigating sponge abundances and chemical ecology are rare, particularly in the Western Indian Ocean. Therefore, this study assessed whether predation is a key factor in structuring the sponge community on reefs around Zanzibar by testing whether chemical defence potential correlates with sponge abundance. Sponge biodiversity and abundances (surface area) were determined, and the palatability of the most abundant sponge species was tested in laboratory feeding assays with the spongivorous pufferfish Canthigaster solandri. Sponges were abundant on the reefs on Zanzibar accounting for 4.8 ± 3.8 and 7.5 ± 1.7% of the benthic cover at 5 m and at 10 m depth, respectively. In the feeding assay, three sponges deterred feeding by C. solandri. However, the presence of feeding deterrent compounds in sponges did not correlate with their abundance on the reef. Low predatory fish abundances likely resulted in a high prevalence of chemically undefended species. Thus, chemically undefended sponges dominate the reef at Bawe Island, Zanzibar, subjecting reef-building corals to a higher competitive pressure.