Cleaner fish drives local fish diversity on coral reefs

Coral reefs are one of the most diverse habitats in the world, yet our understanding of the processes affecting their biodiversity is limited. At the local scale, cleaner fish are thought to have a disproportionate effect, in relation to their abundance and size, on the activity of many other fish species, but confirmation of this species' effect on local fish diversity has proved elusive. The cleaner fish Labroides dimidiatus has major effects on fish activity patterns and may indirectly affect fish demography through the removal of large numbers of parasites. Here we show that small reefs where L. dimidiatus had been experimentally excluded for 18 months had half the species diversity of fish and one-fourth the abundance of individuals. Only fish that move among reefs, however, were affected. These fish include large species that themselves can affect other reef organisms. In contrast, the distribution of resident fish was not affected by cleaner fish. Thus, many fish appear to choose reefs based on the presence of cleaner fish. Our findings indicate that a single small and not very abundant fish has a strong influence on the movement patterns, habitat choice, activity, and local diversity and abundance of a wide variety of reef fish species.     

Seascape-scale trophic links for fish on inshore coral reefs

It is increasingly accepted that coastal habitats such as inshore coral reefs do not function in isolation but rather as part of a larger habitat network. In the Caribbean, trophic subsidies from habitats adjacent to coral reefs support the diet of reef fishes, but it is not known whether similar trophic links occur on reefs in the Indo-Pacific. Here, we test whether reef fishes in inshore coral, mangrove, and seagrass habitats are supported by trophic links. We used carbon stable isotopes and mathematical mixing models to determine the minimum proportion of resources from mangrove or seagrass habitats in the diet of five fish species from coral reefs at varying distances (0–2,200 m) from these habitats in Moreton Bay, Queensland, eastern Australia. Of the fish species that are more abundant on reefs near to mangroves, Lutjanus russelli and Acanthopagrus australis showed no minimum use of diet sources from mangrove habitat. Siganus fuscescens utilized a minimum of 25–44 % mangrove sources and this contribution increased with the proximity of reefs to mangroves (R ² = 0.91). Seagrass or reef flat sources contributed a minimum of 14–78 % to the diet of Diagramma labiosum, a species found in higher abundance on reefs near seagrass beds, but variation in diet among reefs was unrelated to seascape structure. Seagrass or reef flat sources also contributed a minimum of 8–55 % to a fish species found only on reefs (Pseudolabrus guentheri), indicating that detrital subsidies from these habitats may subsidize fish diet on reefs. These results suggest that carbon sources from multiple habitats contribute to the functioning of inshore coral reef ecosystems and that trophic connectivity between reefs and mangroves may enhance production of a functionally important herbivore.     

Maintenance of fish diversity on disturbed coral reefs

Habitat perturbations play a major role in shaping community structure; however, the elements of disturbance-related habitat change that affect diversity are not always apparent. This study examined the effects of habitat disturbances on species richness of coral reef fish assemblages using annual surveys of habitat and 210 fish species from 10 reefs on the Great Barrier Reef (GBR). Over a period of 11 years, major disturbances, including localised outbreaks of crown-of-thorns sea star (Acanthaster planci), severe storms or coral bleaching, resulted in coral decline of 46–96% in all the 10 reefs. Despite declines in coral cover, structural complexity of the reef framework was retained on five and species richness of coral reef fishes maintained on nine of the disturbed reefs. Extensive loss of coral resulted in localised declines of highly specialised coral-dependent species, but this loss of diversity was more than compensated for by increases in the number of species that feed on the epilithic algal matrix (EAM). A unimodal relationship between areal coral cover and species richness indicated species richness was greatest at approximately 20% coral cover declining by 3–4 species (6–8% of average richness) at higher and lower coral cover. Results revealed that declines in coral cover on reefs may have limited short-term impact on the diversity of coral reef fishes, though there may be fundamental changes in the community structure of fishes.     

Parasite infestation increases on coral reefs without cleaner fish

Coral Reefs - Mutualisms are pivotal in shaping ecological communities. Iconic images of cleaner fish entering the mouths of predatory fish clients to remove ectoparasites epitomize their mutual...     

Precision and accuracy of visual census technique for fish assemblages on coral patch reefs

Synopsis A visual census technique is described in which the results of three separate enumerations of fish at a site are combined to produce a best estimate of the fish fauna present. Its precision and accuracy are examined, and compared to those of censuses obtained by modifications of the technique. Visual censuses can display high repeatability, but they seldom (if ever) completely sample the fish present at a site. Accuracy varies with technique used. In our tests, the preferred method yielded 82% of species and 75% of individuals known to be present and potentially censurable at the time the observations were made. Visual censuses are of comparable accuracy to ichthyocide collections of unenclosed sites, but the two methods sample different components of the total fish fauna. It is important when using visual censuses to remember that their accuracy is not 100%.     

Black reefs: iron-induced phase shifts on coral reefs

The Line Islands are calcium carbonate coral reef platforms located in iron-poor regions of the central Pacific. Natural terrestrial run-off of iron is non-existent and aerial deposition is extremely low. However, a number of ship groundings have occurred on these atolls. The reefs surrounding the shipwreck debris are characterized by high benthic cover of turf algae, macroalgae, cyanobacterial mats and corallimorphs, as well as particulate-laden, cloudy water. These sites also have very low coral and crustose coralline algal cover and are call black reefs because of the dark-colored benthic community and reduced clarity of the overlying water column. Here we use a combination of benthic surveys, chemistry, metagenomics and microcosms to investigate if and how shipwrecks initiate and maintain black reefs. Comparative surveys show that the live coral cover was reduced from 40 to 60% to <10% on black reefs on Millennium, Tabuaeran and Kingman. These three sites are relatively large (>0.75 km²). The phase shift occurs rapidly; the Kingman black reef formed within 3 years of the ship grounding. Iron concentrations in algae tissue from the Millennium black reef site were six times higher than in algae collected from reference sites. Metagenomic sequencing of the Millennium Atoll black reef-associated microbial community was enriched in iron-associated virulence genes and known pathogens. Microcosm experiments showed that corals were killed by black reef rubble through microbial activity. Together these results demonstrate that shipwrecks and their associated iron pose significant threats to coral reefs in iron-limited regions.     

Facilitation in Caribbean coral reefs: high densities of staghorn coral foster greater coral condition and reef fish composition

Recovery of the threatened staghorn coral (Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species.     

Large scale spatial and temporal variation in recruitment to fish populations on coral reefs

Summary Visual census was used to sample young of the year of fish species recruited to each of two habitats on seven lagoonal platform reefs of the Capricorn-Bunker Group, Great Barrier Reef. The reefs sampled span an area 70 km in extent. In 1983, 62 species from 13 families were detected as recruits on reef slope sites. The total number of cruits, and the number of each of 6 of 16 species tested, differed significantly among reefs, despite the fact that differences among sites within reefs did not exist, and that sampled slopes were chosen to be hydrographically, and physiographically as similar as possible. Lagoonal patch reefs were sampled in two years. In 1982, 76 species of 11 families occurred as recruits. In 1983, 86 species of 12 families were recorded. All of 22 species common enough to test showed some significant variation in abundance among reefs, years, or both. For 9 species, significant year x reef interactions occurred, demonstrating that relative recruitment success among reefs varied between years. Reasons for the substantial levels of variability are discussed, and implications for the organisation of reef fish communities are considered.     

Fifty million years of herbivory on coral reefs: fossils,fish and functional innovations

The evolution of ecological processes on coral reefs was examined based on Eocene fossil fishes from Monte Bolca, Italy and extant species from the Great Barrier Reef, Australia. Using ecologically relevant morphological metrics, we investigated the evolution of herbivory in surgeonfishes (Acanthuridae) and rabbitfishes (Siganidae). Eocene and Recent surgeonfishes showed remarkable similarities, with grazers, browsers and even specialized, long-snouted forms having Eocene analogues. These long-snouted Eocene species were probably pair-forming, crevice-feeding forms like their Recent counterparts. Although Eocene surgeonfishes likely played a critical role as herbivores during the origins of modern coral reefs, they lacked the novel morphologies seen in modern Acanthurus and Siganus (including eyes positioned high above their low-set mouths). Today, these forms dominate coral reefs in both abundance and species richness and are associated with feeding on shallow, exposed algal turfs. The radiation of these new forms, and their expansion into new habitats in the Oligocene–Miocene, reflects the second phase in the development of fish herbivory on coral reefs that is closely associated with the exploitation of highly productive short algal turfs.     

Mesopredator trophodynamics on thermally stressed coral reefs

Coral Reefs - Ecosystems are becoming vastly modified through disturbance. In coral reef ecosystems, the differential susceptibility of coral taxa to climate-driven bleaching is predicted to shift...     

Estimating three-dimensional surface areas on coral reefs

One of the main obstacles for biological assessments of coral reefs over large spatial scales is the ability to link data obtained at the laboratory scale to spatially large data sets. This is particularly the case when trying to assess the ecological function of microbial processes following dramatic large-scale events such as mass coral bleaching. To be able to infer ecological function of field corals from laboratory measurement standardised to surface area it is imperative to be able to measure the actual surface area of corals in-situ. There have been several approaches proposed to estimate the three-dimensional surface area of field corals. While these have been shown to be reliable for simple coral growth forms, large degrees of error are introduced when applying them to complex growth forms. This paper refines a technique for calculating the three-dimensional surface area based on the projected surface area, with errors associated with complex growth forms reduced to < 5%. Once developed, the simple mathematical relationship (called the surface index) can be used to estimate the three-dimensional surface area of field corals from photograph or video imagery, allowing physiological parameters of corals determined at the sub-colony scale to whole colony and spatially large data sets of coral reefs. The effectiveness of using laser scanning techniques to derive three-dimensional images of corals is also discussed.     

Biological destruction of coral reefs

The major agents of biological destruction of coral reefs can be divided into grazers, etchers and borers. Each of these groups is reviewed on a world wide basis, together with the mechanisms by which they destroy the coral substrate. Rates of bioerosion attributed to major agents of grazers, etchers and borers are given, together with limitations of some of the measurements. Recent work is highlighting the variability in rates of bioerosion both over time and space. Factors which may be responsible for this variability are discussed. Bioerosion is a major factor influencing reef morphology and the ways in which this is achieved is discussed in some detail. Although the review concentrates mainly on present day reefs, some attempt is made to consider the impact of bioerosion on older reefs.     

Marine pollution and coral reefs

Coral reefs are exposed to many anthropogenic stresses increasing in impact and range, both on local and regional scales. The main ones discussed here are nutrient enrichment, sewage disposal, sedimentation, oil-related pollution, metals and thermal pollution. The stress comprising the main topic of this article, eutrophication, is examined from the point of view of its physiological and ecological mechanisms of action, on a number of levels. Nutrient enrichment can introduce an imbalance in the exchange of nutrients between the zooxanthellae and the host coral, it reduces light penetration to the reef due to nutrient- stimulated phytoplankton growth, and, most harmful of all, may bring about proliferation of seaweeds. The latter rapidly outgrow, smother and eventually replace, the slow-growing coral reef, adapted to cope with the low nutrient concentrations typical in tropical seas.
Eutrophication seldom takes place by itself. Sewage disposal invariably results in nutrient enrichment, but it also enriches the water with organic matter which stimulates proliferation of oxygen-consuming microbes. These may kill corals and other reef organisms, either directly by anoxia, or by related hydrogen sulfide production. Increased sediment deposition is in many cases associated with other human activities leading to eutrophication, such as deforestation and topsoil erosion.
Realistically achievable goals to ensure conservation, and in some instances, rehabilitation of coral reefs are listed.     

Damselfish territories as a refuge for macroalgae on coral reefs

Herbivory is widely accepted as a key process determining the benthic community structure and resilience of coral reefs. Recent studies have mostly focused on the importance of roving herbivorous fishes in ecosystem processes. Here, we examine the role of territorial damselfish in shaping patterns of macroalgal distribution based on benthic surveys and macroalgal bioassays. The territory composition and effect of resident damselfish on the removal of Sargassum bioassays were quantified for six species of damselfish on Lizard Island, a mid-shelf reef in the northern Great Barrier Reef (GBR). The functional composition of algal communities within territories varied markedly among species. The territories of four species (Dischistodus perspicillatus, Dischistodus pseudochrysopoecilus, Plectroglyphidodon lacrymatus, and Stegastes nigricans) were characterized by algal turfs, while the territories of two species (Dischistodus prosopotaenia and Hemiglyphidodon plagiometopon) were characterized by foliose and leathery brown macroalgae. Sargassum, a generally rare alga on mid-shelf reefs, was a particularly common alga within D. prosopotaenia territories on the leeward side of the island but absent within their territories on the windward side of the island. D. prosopotaenia was the only species to retain the transplanted Sargassum, with only a minimal reduction in Sargassum biomass (1.1%) being recorded within their territories at both leeward and windward sites over a 24-h period. In contrast, reductions in Sargassum biomass were high in areas adjacent to D. prosopotaenia territories (83.8%), and within and adjacent to the territories of the five remaining damselfish species (76.2–92.5%). Overall, only one of the six damselfish species provided a refuge for leathery brown macroalgae and may facilitate the development of this macroalgae on mid-shelf reefs of the GBR.     

Coral bleaching, reef fish community phase shifts and the resilience of coral reefs

The 1998 global coral bleaching event was the largest recorded historical disturbance of coral reefs and resulted in extensive habitat loss. Annual censuses of reef fish community structure over a 12-year period spanning the bleaching event revealed a marked phase shift from a prebleach to postbleach assemblage. Surprisingly, we found that the bleaching event had no detectable effect on the abundance, diversity or species richness of a local cryptobenthic reef fish community. Furthermore, there is no evidence of regeneration even after 5–35 generations of these short-lived species. These results have significant implications for our understanding of the response of coral reef ecosystems to global warming and highlight the importance of selecting appropriate criteria for evaluating reef resilience.     

Aggression among sea urchins on Caribbean coral reefs

The existence and nature of intra- and interspecific aggression were examined for five species of sea urchins inhabiting Caribbean coral reefs. The studies took place in the San Blas Islands of Panama and involved the following species: Echinometra lucunter (Linnaeus), E. viridis Agassiz, Diadema antillarum Phillipi, Lytechinus williamsi Chesher, and Eucidaris tribuloides (Lamarck). An intruder was placed next to an undisturbed resident and the behaviors and responses of both individuals were followed. All pairwise combinations of resident species and intruder species were tested except for combinations involving D. antillarum as intruder.

For E. viridis and E. lucunter, agonistic interactions occurred commonly (46–79% of trials) between conspecifics and congeners. Almost all of the agonistic encounters involved pushing. Additionally, biting occurred in 8–25 % of the trials. Residents were most often the aggressors and usually succeeded in retaining their location. Intruders only succeeded in forcing residents out of their positions if the intruder was equal to or larger in size than the resident. Surveys of an undisturbed population of E. viridis during the daytime indicated that 16% of the individuals were engaged in intraspecific agonistic interactions at any one time.

D. antillarum exhibited biting behavior against both species of Echinometra in 23–24% of the trials. Biting attacks against L. williamsi and E. tribuloides occurred rarely. L. williamsi only once demonstrated pushing behavior and never was observed biting another sea urchin. E. tribuloides occasionally exhibited pushing and biting behaviors, both as residents and as intruders, and was twice observed biting Echinometra.

These studies suggest that two kinds of agonistic interactions may commonly occur among Caribbean reef-dwelling sea urchins: (1) intraspecific and interspecific aggression among Echinometra, and (2) predatory/aggressive attacks against Echinometra by D. antillarum. The former may result in greater dispersion of Echinometra relative to food and shelter resources and control the spatial distribution and concentration of Echinometra grazing pressure within an area. The attacks by D. antillarum may result in the restriction to, or higher densities of, Echinometra in crevices and rugose microhabitats that provide shelter from the larger-bodied D. antillarum.     

Effects of monsoon-driven wave action on coral reefs of Guam and implications for coral recruitment

Benthic cyanobacteria can respond rapidly to favorable environmental conditions, overgrow a variety of reef organisms, and dominate benthic marine communities; however, little is known about the dynamics and consequences of such cyanobacterial blooms in coral reef ecosystems. In this study, the benthic community was quantified at the time of coral spawnings in Guam to assess the substrate that coral larvae would encounter when attempting settlement. Transects at 9, 18, and 25-m depths were surveyed at two reef sites before and after heavy wave action driven by westerly monsoon winds. Communities differed significantly between sites and depths, but major changes in benthic community structure were associated with wave action driven by monsoon winds. A shift from cyanobacteria to crustose coralline algae (CCA) accounted for 44% of this change. Coral recruitment on Guam may be limited by substrate availability if cyanobacteria cover large areas of the reef at the time of settlement, and consequently recruitment may in part depend upon wave action from annual monsoon winds and tropical storms which remove cyanobacteria, thereby exposing underlying CCA and other substrate suitable for coral settlement.