Effects of eutrophication on reef-building corals

Seven fringing reef complexes were chosen along the leeward coast (west) of Barbados to study the effects of eutrophication processes upon the scleractinian coral assemblages. The structure of scleractinian coral communities was studied along an eutrophication gradient with a quantitative sampling method (line transect) in terms of species composition, zonation and diversity patterns. On the basis of these data the fringing reefs were divided into three ecological zones: back reef, reef flat, and spur and groove. Statistically discernible and biologically significant differences in scleractinian coral community structure, benthic algal cover and Diadema antillarum Philippi densities were recorded among the seven fringing reefs. High correlations between environmental variables and biotic patterns indicate that the effects of eutrophication processes (nutrient enrichment, sedimentation, turbidity, toxicity and bacterial activity) were directly and/or indirectly affecting the community structure of scleractinian coral assemblages. In general, species diversity was most sensitive in delineating among-reef, and among-zone, differences, which were attributed to intensification of eutrophication processes. Porites astreoides Lamarck, P. porites (Pallas), Siderastrea radians (Pallas), and Agaricia agaricites (Linnaeus) were the most abundant coral species in the polluted southern reefs. The absence and/or low abundance of coral species previously characterized as well adapted to high turbidity and sedimentation [i.e. Montastrea cavernosa Linnaeus, Meandrina meandrites (Linnaeus)] indicate that eutrophication processes may adversely affect these species. It is suggested that sediment rejection abilities, combined with feeding and reproductive strategies, are the primary biological processes of scleractinian corals through which eutrophication processes directly and/or indirectly affect the structure of coral communities.     

Effects of eutrophication on reef-building corals

Fourteen environmental variables were monitored at seven locations along the west coast of Barbados on a weekly basis over a one-year period, 1981 to 1982. The physicochemical and biological data indicate that an environmental gradient exists as a result of increased eutrophication of coastal waters. Growth rates (linear extension) of Montastrea annularis (Ellis and Solander), measured along the environmental gradient, exhibit high correlation with a number of water quality variables. Concentration of suspended particulate matter is the best univariate estimator of M. annularis skeletal extension rates (r ²=0.79, P<0.0001). The results suggest that suspended particulate matter may be an energy source for reef corals, increasing growth up to a certain maximum concentration. After this, reduction of growth occurs due to smothering, reduced light levels and reduced zooxanthellae photosynthesis.     

Simulating light-saturation curves for photosynthesis and calcification by reef-building corals

Light-saturation curves for photosynthesis by reef-building corals have previously been simulated by three functions: the right rectangular hyperbola, a simple exponential function, and the hyperbolic tangent function. Studies of photosynthesis by other organisms have also frequently considered the application of a rectilinear function. This communication analyzes lightsaturation curves for photosynthesis by the Atlantic rose coral Manicina aerolata, the Atlantic staghorn coral Acropora cervicornis, and the Pacific staghorn coral A. formosa. It also analyzes light-saturation curves for calcification by A. cervicornis and A. formosa. This communication demonstrates that the two most accurate functions (as measured by coefficients of determination) are the simple exponential function and the hyperbolic tangent function. The hyperbolic tangent function is preferred because parameter estimates obtained with this function have narrower confidence intervals than those obtained through the application of the simple exponential function. The hyperbolic tangent function can also be used successfully to simulate light-saturation curves for light-enhanced calcification.     

Long-distance dispersal and the reef-building corals of the Eastern Pacific

It is currently widely accepted that the hermatypic coral fauna in the Eastern Pacific Ocean underwent massive extinction during the mid-Tertiary, with subsequent transoceanic colonizatiion by planulae from the Indo-West Pacific region during periods of favorable conditions. We suggest that the available evidence does not strongly support this biogeographic hypothesis; moreover, we contend that it is untestable in its present form. In its place we propose an alternative hypothesis based upon modification of a previously widespread, pan-Tethyan coral biota which has since been modified by tectonic events, speciations, and extinctions.Order of authorship was determined by the toss of a coin.     

Growth trajectories of corallites and ages of polyps in massive colonies of reef-building corals of the genus Porites

Massive colonies of the reef-building coral genus Porites were collected at inshore, midshelf and shelf-edge reefs in the central section of the Great Barrier Reef in November 1987. These colonies were comprised of 4 species: P. lobata, P. lutea, P. solida and P. mayeri. X-radiographs made of skeletal slices cut from the skeletons displayed the annual density-banding pattern characteristic of massive corals, and appeared to show corallites within each slice. The average age of the 36 colonies was 41±12 yr (mean±SD). The images of corallites displayed by the X-radiographs were not images of actual corallites, but approximated the position and size of actual corallites. Consequently, X-radiographs provide information about the formation and growth trajectories of corallites, and about the history of the polyps which deposited the corallites. Individual corallites were always normal to the growth surface. The growth surface of the colonies became bumpy when they reached 50 to 80 mm in height and, as a result, corallites took on a fan-shaped arrangement within a bump. New corallites were initiated at the summit of each bump and grew upwards and outwards. Thus, growth of colonies resulted in corallites becoming increasingly displaced from the summit of a bump. The X-radiographs showed that corallite growth becomes occluded at the bottom of valleys between adjacent bumps. Corallite growth then stops and the associated polyps are probably resorbed. Annual density banding showed that the average age of polyps in these colonies was 2 to 3 yr, average life expectancy 5 yr, and that no polyp was likely to be older than 8 yr. Small but significant variations in polyp longevity between corals from different reefs were probably associated with significant differences in bumpiness of growth surfaces. Even in Porites colonies which have been growing for several centuries, polyp longevity is likely to be 5 yr.     

The relative contribution of dinoflagellate photosynthesis and stored lipids to the survivorship of symbiotic larvae of the reef-building corals

The long-distance dispersal of larvae provides important linkages between populations of reef-building corals and is a critical part of coral biology. Some coral planulae have symbiotic dinoflagellates (Symbiodinium spp.) that probably provide energy in addition to the lipids provisioned within the egg. However, our understanding of the influence of symbionts on the energy metabolism and survivorship of planulae remains limited. This study examines the relative roles of symbiotic dinoflagellate photosynthesis and stored lipid content in the survivorship of the developing stages of the corals Pocillopora damicornis and Montipora digitata. We found that survivorship decreased under dark conditions (i.e. no photosynthetic activity) for P. damicornis and M. digitata at 31 and 22 days after release/spawning, respectively. The lipid content of P. damicornis and M. digitata planulae showed a significant decrease, at a higher rate, under dark conditions, when compared with light conditions. When converted to energy equivalents, the available energy provided by the depletion of lipids could account for 41.9 and 84.7% of larval metabolism for P. damicornis (by day 31) and 38.4 and 90.1% for M. digitata (by day 21) under light and dark conditions, respectively. This finding indicates that not all energy requirements of the larvae are met by lipids: energy is also sourced from the photosynthetic activities of the symbiotic dinoflagellates within these larvae, especially under light conditions. In addition, the amounts of three main lipid classes (wax esters, triglycerides, and phospholipids) decreased throughout the experiment in the planulae of both species, with the wax ester content decreasing more rapidly under dark conditions than under light conditions. The observations that the planulae of both species derive considerable amounts of energy from wax esters, and that symbiotic dinoflagellates enable larvae to use their stores at lower rates, suggested that symbiotic dinoflagellates have the potential to extend larval life under light conditions.     

Proximity to competitors changes secondary metabolites of non-indigenous cup corals, Tubastraea spp., in the southwest Atlantic

Competition for space changes species’ distributions and community organization on tropical rocky shores, and the presence of secondary metabolites in the tissues of non-indigenous species may aid them in establishing and expanding their range through negative competitive interactions. The aim of this study was to describe the range of chemical substances produced by the non-indigenous cup corals Tubastraea coccinea and T. tagusensis and to test whether they varied in the field when the corals were placed in proximity to two local competitors. Cholest-5-en-3β-ol and 9-octadecanoic acid were two common secondary metabolites found in the tissues of Tubastraea. In the competition interaction experiment, necrosis was detected on the tissues of the coral Mussismilia hispida, and this species induced variation in sterol, alkaloid, and fatty acid production in Tubastraea tissues. In contrast, a sponge overgrew Tubastraea colonies. These results indicate that chemical defense may contribute to the ability of these non-indigenous corals to invade native communities.     

Spatial variance in abundance and occupancy of corals across broad geographic scales

Species assemblages vary in structure due to a wide variety of processes operating at ecological and much broader biogeographical scales. Cross-scale studies of assemblage structure are necessary to fully understand this variability. Here, we evaluate the abundance and occupancy patterns of hierarchically sampled coral assemblages in three habitats (reef flat, crest, and slope) and five regions (Indonesia, Papua New Guinea, the Solomon Islands, American Samoa, and the Society Islands) across the west-central Pacific Ocean. Specifically, we compare two alternative models that unify spatial variance and occupancy via the negative binomial distribution. The first assumes a power-law scaling between the mean and variance of abundance; the second assumes a quadratic variance-mean relationship and a constant abundance-invariant aggregation parameter. Surprisingly, the well-established power-law model performs worse than the model assuming abundance-invariant aggregation, for both variance-mean and occupancy-abundance relationships. We also find strong evidence for regional and habitat variation in these relationships and in the levels of aggregation estimated by the abundance-invariant aggregation model. Among habitats, corals on reef flats exhibited lower occupancy and higher levels of aggregation compared to reef crests and slopes. Among regions, low occupancy and high aggregation were most pronounced across all habitats in American Samoa. These patterns may be related to habitat and regional differences in disturbance and recovery processes. Our results suggest that the spatial scaling of abundance and occupancy is sensitive to processes operating among these habitats and at regional scales. However, the consistency of these relationships across species within assemblages suggests that a theoretical unification of spatial variance and occupancy patterns is indeed possible.     

Spatial heterogeneity influences native and nonnative plant species richness

Spatial heterogeneity may have differential effects on the distribution of native and nonnative plant species richness. We examined the effects of spatial heterogeneity on native and nonnative plant species richness distributions in the central part of Rocky Mountain National Park, Colorado, USA. Spatial heterogeneity around vegetation plots was characterized using landscape metrics, environmental/topographic variables (slope, aspect, elevation, and distance from stream or river), and soil variables (nitrogen, clay, and sand). The landscape metrics represented five components of landscape heterogeneity and were measured at four spatial extents (within varying radii of 120, 240, 480, and 960 m) using the FRAGSTATS landscape pattern analysis program. Akaike's Information Criterion adjusted for small sample size (AICc) was used to select the best models from a set of multiple linear regression models developed for native and nonnative plant species richness at four spatial extents and three levels of ecological hierarchy (i.e., landscape, land cover, and community). Both native and nonnative plant species richness were positively correlated with edge density, Simpson's diversity index and interspersion/juxtaposition index, and were negatively correlated with mean patch size. The amount of variation explained at four spatial extents and three hierarchical levels ranged from 30% to 70%. At the landscape level, the best models explained 43% of the variation in native plant species richness and 70% of the variation in nonnative plant species richness (240-m extent). In general, the amount of variation explained was always higher for nonnative plant species richness, and the inclusion of landscape metrics always significantly improved the models. The best models explained 66% of the variation in nonnative plant species richness for both the conifer land cover type and lodgepole pine community. The relative influence of the components of spatial heterogeneity differed for native and nonnative plant species richness and varied with the spatial extent of analysis and levels of ecological hierarchy. The study offers an approach to quantify spatial heterogeneity to improve models of plant biodiversity. The results demonstrate that ecologists must recognize the importance of spatial heterogeneity in managing native and nonnative plant species.     

Spatial heterogeneity of photosynthesis and the effect of temperature-induced bleaching conditions in three species of corals

Heterogeneity in photosynthetic performance between polyp and coenosarc tissue in corals was shown using a new variable fluorescence imaging system (Imaging-PAM) with three species of coral, Acropora nobilis, Cyphastrea serailia and Pocillopora damicornis. In comparison to earlier studies with fibre-optic microprobes for fluorescence analysis, the Imaging-PAM enables greater accuracy by allowing different tissues to be better defined and by providing many more data points within a given time. Spatial variability of photosynthetic performance from the tip to the distal parts was revealed in one species of branching coral, A. nobilis. The effect of bleaching conditions (33°C vs. 27°C) was studied over a period of 8 h. Marked changes in fluorescence parameters were observed for all three species. Although a decline in _PSII (effective quantum yield) and Yi (the first effective quantum yield obtained from a rapid light curve) were observed, P. damicornis showed no visual signs of bleaching on the Imaging-PAM after this time. In A. nobilis and C. serailia, visual signs of bleaching over the 8 h period were accompanied by marked changes in F (light-adapted fluorescence yield), NPQ (non-photochemical quenching) and E _k (minimum saturating irradiance), as well as _PSII and Yi. These changes were most marked over the first 5 h. The most sensitive species was A. nobilis, which after 8 h at 33°C had reached a _PSII value of almost zero across its whole surface. Differential bleaching responses between polyps and coenosarc tissue were found in P. damicornis, but not in A. nobilis and C. serailia. NPQ increased with exposure time to 33°C in both the latter species, accompanied by a decreasing E _k, suggesting that the xanthophyll cycle is entrained as a mechanism for reducing the effects of the bleaching conditions.Communicated by L. Hagerman, Helsingør     

Antimicrobial activity of Red Sea corals

Scleractinian corals and alcyonacean soft corals are the two most dominant groups of benthic marine organisms inhabiting the coral reefs of the Gulf of Eilat, northern Red Sea. Antimicrobial assays performed with extracts of six dominant Red Sea stony corals and six dominant soft corals against marine bacteria isolated from the seawater surrounding the corals revealed considerable variability in antimicrobial activity. The results demonstrated that, while the majority (83%) of Red Sea alcyonacean soft corals exhibited appreciable antimicrobial activity against marine bacteria isolated from the seawater surrounding the corals, the stony corals had little or no antimicrobial activity. From the active soft coral species examined, Xenia macrospiculata exhibited the highest and most potent antimicrobial activity. Bioassay-directed fractionation indicated that the antimicrobial activity was due to the presence of a range of compounds of different polarities. One of these antibiotic compounds was isolated and identified as desoxyhavannahine, with a minimum inhibitory concentration (MIC) of 48 μg ml⁻¹ against a marine bacterium. The results of the current study suggest that soft and hard corals have developed different means to combat potential microbial infections. Alcyonacean soft corals use chemical defense through the production of antibiotic compounds to combat microbial attack, whereas stony corals seem to rely on other means.     

Algal overgrowth of alcyonacean soft corals

Colonies of the soft coral Lobophytum pauciflorum (Ehrenberg, 1834) (Coelenterata: Octocorallia: Alcyonacea: Alcyoniidae), some of which were heavily overgrown by the algae Ceranium flaccidum (Kuetzing) Ardissone and Enteromorpha sp., and other minor epizoites, were collected at Taylor Reef (17°50S; 146°35E) in the Great Barrier Reef. Overgrown colonies contained the diterpene 2-epi-sarcophytoxide as the major secondary metabolite, while conspecific colonies with clean polyparies contained two diterpenes in approximately equal amounts: 14-hydroxycembra-1,3,7,11-tetraene and 15-hydroxycembra-1,3,7,11-tetraene. By contrast, twenty conspecific pairs of overgrown and clean colonies of other alcyoniid soft corals collected from Pelorus Channel, Palm Island Group (18°34S; 146°29E), showed no chemical differences within in the pairs. Cultures of a common species of Ceramium [C. codii (Richards) Mazoyer] were incubated with different concentrations of nine soft-coral-derived diterpenes and significant algal growth inhibition was observed in many cases. It appears that terpenoids from soft coral may contribute to the lack of epizoic organisms on soft-coral polyparies.     

Patterns of Oil-Sediment rejection in corals

The patterns of oil-sediment rejection of 19 Caribbean hermatypic corals are identical to theri patterns of rejection of clean sediments. The rejection pattern is typical for coral species, and displays maximum and minimum rates dependent on the size and density of the oil-sediment particles. The viscosity of the oil determines the size of the oil-sediment particles. A coral's efficiency of rejection of sediment depends on the size and amount of the sediment particles. Oil drops 0.06 mm are removed by the coral's tissues. Physical contact with oil-sediment particles appears to be less harmful to corals than the toxic effects of oils.     

On the evidence for species coexistence: a critique of the coexistence program

A major challenge in ecology is to understand how the millions of species on Earth are organized into biological communities. Mechanisms promoting coexistence are one such class of organizing processes, which allow multiple species to persist in the same trophic level of a given web of species interactions. If some mechanism promotes the coexistence of two or more species, each species must be able to increase when it is rare and the others are at their typical abundances; this invasibility criterion is fundamental evidence for species coexistence regardless of the mechanism. In an attempt to evaluate the level of empirical support for coexistence mechanisms in nature, we surveyed the literature for empirical studies of coexistence at a local scale (i.e., species found living together in one place) to determine whether these studies satisfied the invasibility criterion. In our survey, only seven of 323 studies that drew conclusions about species coexistence evaluated invasibility in some way in either observational or experimental studies. In addition, only three other studies evaluated necessary but not sufficient conditions for invasibility (i.e., negative density dependence and a trade-off in performance that influences population regulation). These results indicate that, while species coexistence is a prevalent assumption for why species are able to live together in one place, critical empirical tests of this fundamental assumption of community structure are rarely performed. These tests are central to developing a more robust understanding of the relative contributions of both deterministic and stochastic processes structuring biological communities.     

Predominance of clade D <Emphasis Type="Italic">Symbiodinium</Emphasis> in shallow-water reef-building corals off Kish and Larak Islands (Persian Gulf,Iran)

Scleractinian coral species harbour communities of photosynthetic taxa of the genus Symbiodinium. As many as eight genetic clades (A, B, C, D, E, F, G and H) of Symbiodinium have been discovered using molecular biology. These clades may differ from each other in their physiology, and thus influence the ecological distribution and resilience of their host corals to environmental stresses. Corals of the Persian Gulf are normally subject to extreme environmental conditions including high salinity and seasonal variation in temperature. This study is the first to use molecular techniques to identify the Symbiodinium of the Iranian coral reefs to the level of phylogenetic clades. Samples of eight coral species were collected at two different depths from the eastern part of Kish Island in the northern Persian Gulf, and Larak Island in the Strait of Hormuz. Partial 28S nuclear ribosomal (nr) DNA of Symbiodinium (D1/D2 domains) were amplified by polymerase chain reaction (PCR). PCR products were analyzed using single stranded conformational polymorphism and phylogenetic analyses of the LSU DNA sequences from a subset of the samples. The results showed that Symbiodinium populations were generally uniform among and within the populations of eight coral species studied, and there are at least two clades of Symbiodinium from Kish and Larak islands. Clade D was detected from eight of the coral species while clade C was found in two of species only (one species hosted two clades simultaneously). The dominance of clade D might be explained by high temperatures or the extreme temperature variation, typical of the Persian Gulf. Publication of this article was held up owing to technical problems. The publisher apologizes sincerely for this lengthy delay.     

A test of the ideal free distribution with unequal competitors

Summary The distribution of goldfish differing in competitive status approximately mimicked that expected for the ideal free distribution but with slightly too many fish in the poorer site. As predicted by phenotype-limited game theory models of dispersion, the mean rank of fish in a site varied inversely with the number of fish, both in the high and low input sites, and there was no correlation between competitive rank and time spent in the better site. The intake was higher for each individual in the high input site than in the low input site, showing that the distribution was not an exact evolutionarily stable strategy. We suggest that the deviation is due to sampling or perceptual constraints. Analysis of other studies with continous input shows that the discrepancy from the theoretical expectation increases with input ratio.     

Microbial consortia increase thermal tolerance of corals

This study examined the response of a coral holobiont to thermal stress when the bacterial community was treated with antibiotics. Colonies of Pocillopora damicornis were exposed to broad and narrow-spectrum antibiotics targeting coral-associated α and γ-Proteobacteria. Corals were gradually heated from the control temperature of 26 to 31 °C, and measurements were made of host, zooxanthellar and microbial condition. Antibiotics artificially reduced the abundance and activity of bacteria, but had minimal effect on zooxanthellae photosynthetic efficiency or host tissue protein content. Heated corals without antibiotics showed significant declines in F _V /F _M , typical of thermal stress. However, heated corals treated with antibiotics showed severe tissue loss in addition to a decline in F _V /F _M . This study demonstrated that a disruption to the microbial consortium diminished the resilience of the holobiont. Corals exposed to antibiotics under control temperature did not bleach, suggesting that temperature may be an important factor influencing the activity, diversity and ecological function of the holobiont bacterial community.     

Antimicrobial activity of extracts of Caribbean gorgonian corals

Extracts of 39 species of Caribbean gorgonians were tested for antimicrobial activity against 15 strains of marine bacteria. The bacteria consisted of three opportunistic pathogens, Vibrio parahaemolyticus, Leucothrix mucor, and Aerococcus viridans, and 12 strains isolated from either healthy or decayed gorgonians. Overall, only 15% (79 out of 544) of the tests resulted in antibacterial activity with 33% (13 out of 39) of the gorgonians inhibiting only one bacterial strain and 23% (9 out of 39) showing no activity. The extracts of four Pseudopterogorgia species showed relatively high levels of activity, inhibiting 43 to 86% of the bacterial strains. The potency of the active Pseudopterogorgia species was variable, however, and three additional Pseudopterogorgia species were inactive against all bacterial strains. With the exception of one sensitive strain, Vibrio species were resistant to gorgonian metabolites. Our results indicate that organic extracts of most Caribbean gorgonians do not possess potent, broad-spectrum antibacterial activity inhibitory to the growth of opportunistic marine pathogens and bacteria associated with healthy and decayed gorgonian surfaces. These findings suggest that the inhibition of bacterial growth is not the primary ecological function of gorgonian secondary metabolites and that bacteria may not be important selective agents in the evolution of gorgonian secondary chemistry.     

Chemical resistance of gorgonian corals against fungal infections

The frequency and impact of diseases affecting corals throughout the Caribbean have been increasing but little is known about the factors promoting the emergence and outbreak of disease. A disease caused by a fungal pathogen [Aspergillus sydowii (Thom et Church)] which affects Caribbean sea fan corals provided an opportunity to examine the efficacy of coral crude extracts in disease resistance. Minimum inhibitory concentration (MIC) assays showed that of the 20 common gorgonian species in the Florida Keys, extracts from 15 species had MICs < 15 mg ml⁻¹ against A. sydowii pathogenic to sea fans. Extracts from several species in two gorgonian genera (Pseudoplexaura and Pseudopterogorgia) were among the most active, with MICs < 10 mg ml⁻¹. Gorgonia ventalina L., one of two sea fan species known to be hosts to A. sydowii in the field, had an MIC < 10 mg ml⁻¹, suggesting that complete disease resistance requires more active extracts. For the antifungal compounds to be effective in situ, they must also occur in sufficiently high concentrations in living coral tissue. For example, Pseudopterogorgia americana (Gmelin) had comparatively potent extracts but did not have sufficient concentrations in the tissue to be effective. Conversely, Plexaura homomalla Esper extracts were less potent but occurred in high enough concentrations in the tissue to be effective against A. sydowii. When potency and extract concentration are considered together (i.e. potency × concentration), several other gorgonian corals emerge as likely hosts to A. sydowii. Crude extracts from the most active gorgonian species were also effective against two geographic variants of A. sydowii pathogenic to sea fans, a non-pathogenic terrestrial strain of A. sydowii, and three strains of A. flavus Link known to be human, plant, and insect pathogens (MIC range, 7.5 to > 15 mg ml⁻¹). Although the potency in these assays did not attain a clinically significant level, the potency is comparable to a known antifungal agent, hygromycin B, which had an MIC ≤7.5 mg ml⁻¹ in our assays, highlighting the potential of these gorgonian corals for bioprospecting. Received: 29 May 1999 / Accepted: 22 May 2000