Competitive strategies of soft corals (Coelenterata: Octocorallia). II. Variable defensive responses and susceptibility to scleractinian corals

Interactions involving competition for space between several species of alcyonacean and scleractinian corals were assessed experimentally on Britomart Reef, central region of the Great Barrier Reef, Australia. Colonies of three soft coral species, Sarcophyton ehrenbergi Marenzeller, Nephthea brassica Kukenthal, and Capnella lacertiliensis Macfayden Forskal (Coelenterata:Alcyonacea) were relocated within stands of two scleractinian corals, Parités andrewsi Vaughan (= P. cylindrica Dana) and Pavona cactus Förskal (Coelenterata:Scleractinia). Undisturbed scleractinian and relocated alcyonacean controls were also monitored.Alcyonacean corals induced necrosis of tissue in scleractinian corals. Necrosis was significantly more pronounced when colonies were in contact but was also observed in the absence of contact, implicating the presence of active allelopathic agents. Scleractinian coral species varied in their susceptibility to the ill effects of alcyonaceans, with Pontes andrewsi being more susceptible than Pavona cactus. Of the soft corals, Nephthea caused the highest degree of mortality in the two scleractinian corals examined and Sarcophyton the least. Some soft corals appear to retain their toxins while others release them, implying a combination of anti-predatory and anti-competitor roles for the secondary metabolites. Scleractinian corals were often overgrown by soft corals.Both species of scleractinian corals were found to cause approximately equal amounts of tissue necrosis in alcyonaceans. These effects were more pronounced when colonies were in direct contact. The necrotic effects among alcyonacean corals were species-specific. Alcyonaceans also overgrew scleractinian corals and secreted a protective polysaccharide layer in areas proximal to scleractinians. Secretion of this layer was stimulated differentially by the two scleractinian species and also varied in frequency of occurrence among the alcyonaceans.High levels of tissue necrosis were observed in both groups of organisms within 3 wk of initiation of the experiment. Necrosis increased with time in the scleractinian corals and decreased in the alcyonaceans. The development of a protective polysaccharide layer in the alcyonaceans increased with time.     

Loss of Symbiodinium from bleached soft corals Sarcophyton ehrenbergi, Sinularia sp. and Xenia sp.

The deleterious effects of temperature-induced coral bleaching, a process by which corals lose their endosymbiotic algae (zooxanthellae; genus Symbiodinium) primarily at temperatures above mean yearly maximums, has not been well described for alcyonacean soft corals (Coelenterata, Octocorallia). The study of Symbiodinium cells lost from Sarcophyton ehrenbergi, Sinularia sp., and Xenia sp., which have not been compared in bleaching studies, indicate that the soft coral S. ehrenbergi released the greatest number of symbiont cells, however, it was less susceptible to heat stress surviving temperatures of 34 °C for >39 h. Sinularia sp. showed intermediate levels of bleaching tolerance to elevated temperatures, surviving prolonged exposures at 32 °C, but dying within 24 h at 34 °C. Xenia sp., however, was the most vulnerable to high heat stress maximally releasing Symbiodinium at temperatures ≤30 °C. This evidence indicates that Xenia sp. is even more susceptible to elevated temperatures than Acropora spp., previously reported to be the most vulnerable coral species to elevated temperature-induced bleaching.

Molecular analysis showed that the more resistant soft coral species (S. ehrenbergi) had the same type of Symbiodinium (clade C) as less resistant soft corals (Xenia sp.). In comparison to scleractinian corals collected from the same region that show similar bleaching resistance to high temperatures (e.g. Porities solida—more robust; Favites complanata—moderate resistance; Acropora hyacinthus—less robust), all scleractinian corals were symbiotic with Symbiodinium from clade C. A. hyacinthus, however, was found to possess multiple symbionts (clades B and C), and this represents a first report of Clade B in any Acropora species.     

Soft body reconstructions of Palaeozoic corals: implications for the system of Anthozoa (Coelenterata)

The soft bodies and individual developmental sequences of the polyps of Palaeozoic corals are reconstructed on the basis of the functional, engineering and constructional morphology of Recent Anthozoa. Four types of body construction can be distinguished by the arrangement of mesenteries, the number of mesentery insertion zones and characteristic septal insertion patterns: (1) serial insertion of single mesenteries in four sectors, which has resulted in the serial insertion of septa, (2) symmetrical insertion of single mesenteries in four sectors, which has resulted in a symmetrical septal pattern or pseudo radial septal patterns, (3) exponential insertion of paired mesenteries in four sectors, which has resulted in septa bifurcation, and (4) exponential insertion of paired mesenteries in six sectors, which has resulted in a radial septal pattern. Suggestions for a constructional taxonomy are given on the basis of the arrangement of mesenteries and on individual development of the reconstructed polyps. Accordingly, new superorders are introduced: 'Seriales' for the first body construction and 'Symmetricales' for the second. The third body construction is that of the Heterocorallia and the fourth that of the Hexacorallia. Following a constructional taxonomy, new and stricter definitions are given for the new and already known taxonomic categories of Rugosa, Heterocorallia and Hexacorallia. A byproduct of this re-definition is that several Palaeozoic corals represent the body construction of the Hexacorallia and should therefore be revised as Hexacorallia; consequently, Hexacorallia must already have been present in the Palaeozoic.     

Patterns in the distribution of soft corals across the central Great Barrier Reef

Distribution patterns of soft coral genera were examined at 11 reefs situated in a broad transect from inshore to the Coral Sea in the central region of the Great Barrier Reef. Twenty-five genera representing the Orders Alcyonacea and Stolonifera were recorded, and the survey also included one genus of the Order Gorgonacea. Total living soft coral cover is greatest on outershelf reef slopes, and is often less than and inversely related to the cover by stony corals. Soft coral diversity is generally low on reef flats, where soft coral cover is low or nil except in protected, inshore areas. The most diverse assemblages occur on reef slopes in midshelf and outershelf areas, where Efflatounaria and nephtheid genera predominate, and widely distributed alcyoniid genera are common. These richer assemblages are less well represented in the Coral Sea, while innershelf reefs support a less diverse fauna of somewhat different generic composition. Distribution patterns of soft corals across the transect broadly match similar variations in the distributions of stony corals and fishes, inshore reefs being generally depauperate. Such variations across the continental shelf are closely associated with changes in prevailing environmental conditions, but further research will be required to elucidate the effects of environmental parameters on benthic community structure.     

In situ isolation of allelochemicals released from soft corals (Coelenterata : Octocorallia): A totally submersible sampling apparatus

A submersible apparatus has been developed which permits in situ sampling of allelochemical substances released from sessile marine organisms. Concentration of the allomones from sea water is achieved by adsorption on SEP-PAK C-18, reverse-phase cartridges. The procedure is performed under water and causes minimal disturbance to the organism while in its natural habitat. Design and application of this apparatus are detailed.

The isolation and identification of toxins released by two species of alcyonarian corals (order Alcyonacea) are described. This represents the first direct in situ isolation of water-borne allelochemicals released from a marine organism.     

Large-scale bleaching of corals on the Great Barrier Reef

 The Great Barrier Reef (GBR) experienced its most intensive and extensive coral bleaching event on record in early 1998. Mild bleaching commenced in late January and intensified by late February/early March 1998. Broad-scale aerial surveys conducted of 654 reefs (∼23% of reefs on the GBR) in March and April 1998, showed that 87% of inshore reefs were bleached at least to some extent (>1% of coral cover) compared to 28% of offshore (mid- and outer-shelf) reefs. Of inshore reefs 67% had high levels of bleaching (>10% of coral) and 25% of inshore reefs had extreme levels of bleaching (>60% of coral). Fewer offshore reefs (14%) showed high levels of bleaching while none showed extreme levels of bleaching. Ground-truth surveys of 23 reefs, which experienced bleaching in intensities ranging from none to extreme, showed that the aerial survey data are likely to be underestimates of the true extent and intensity of bleaching on the GBR. The primary cause of this bleaching event is likely to be elevated sea temperature and solar radiation, exacerbated by lowered salinity on inshore and some offshore reefs in the central GBR. Accepted: 30 July 1998     

Spatial study of juvenile corals in the Northern region of the Mesoamerican Barrier Reef System (MBRS)

Juvenile coral abundance and community composition depend on the spatial scale studied. To investigate this, an evaluation was made of juvenile coral density with hierarchical spatial analysis in the northern Mesoamerican Barrier Reef System (MBRS) at ~10 m depth. Study scope included semi-protected and unprotected areas located in this region. A total of 19 juvenile coral taxa were found, including 10 scleractinian species, 8 scleractinian coral genera not identified to species, and 1 Millepora species (Hydrozoa-Milleporidae). In terms of relative abundance, Agaricia spp., Siderastrea spp., and Porites spp. were the main juvenile taxa in the coral community at the surveyed sites, reefs, and regions levels. Greater variance was seen at smaller scales, at site level for taxa richness, and at the transect level for juvenile density, and lower variance was seen at larger scales (reefs and regions). The variance component contribution from each scale likely differed from other studies because of the different factors affecting the community and the different extensions of each scale used in each study. Densities (1–6.4 juvenile corals/m²) and dominant taxa found in this study agree with other studies from the Western Atlantic. Detected variability was explained by different causal agents, such as low grazing rates by herbivorous organisms, turbidity, and/or sediment suffocation and some nearby or distant localized disturbance (human settlement and a hurricane).     

Copepoda associated with the coral Gardineroseris planulata (Dana) on the Great Barrier Reef,northeastern Australia

Three new lichomolgid copepods associated with the scleractinian Gardineroseris planulata (Dana) are described: Sociellus torus, n. gen., n. sp., characterized by a 2-segmented endopod in leg 3 and the lack of an inner seta on the first segment of the endopod in legs 1–3; Odontomolgus pumilus, n. sp., of very small size and having a much elongated free segment in leg 5 of the female; and Paramolgus ampullaceus, n. sp., distinguished by the bottle-shaped genital segment of the female. In addition, the harpacticoid Alteuthellopsis corallina Humes, 1981, is reported from this coral. With the inclusion of 3 species of the poecilostomatoid genus Xarifia already described (Humes, 1985) there are now 7 species of copepods known to be associated with Gardineroseris planulata on the Great Barrier Reef.     

A new family of soft corals (Anthozoa,Octocorallia, Alcyonacea) from the aphotic tropical eastern Pacific waters revealed by integrative taxonomy

The new family Aquaumbridae is described based on distinct morphological characters and supported by a molecular phylogenetic analysis. Aquaumbridae is similar to Nidaliidae, Nepththeidae and Alcyoniidae in having arborescent colonies and lacking axis structure or stolons, but differs from them in having very different sclerite composition and having conspicuous transparent jelly-like lobes. Phylogenetic analysis of two mitochondrial genes, ND2 and mtMutS, strongly supports its placement in a separated clade. Herein we describe Aquaumbra klapferi sp. nov., gen. nov. in the new family. The organisms were obtained from the seamounts, ridges and canyons out of the insular shelf of Isla del Coco, Costa Rica, down to 400 m depth. The new species represents the first discovery of a soft coral in an eastern Pacific oceanic island, and provides hints of the biodiversity of the largely unexplored deep waters of the tropical eastern Pacific.     

Bleaching of corals on the Great Barrier Reef: differential susceptibilities among taxa

Large-scale coral bleaching episodes are potentially major disturbances to coral reef systems, yet a definitive picture of variation in assemblage response and species susceptibilities is still being compiled. Here, we provide a detailed analysis of the bleaching response of 4160 coral colonies, representing 45 genera and 15 families, from two depths at four sites on reefs fringing inshore islands on the Great Barrier Reef. Six weeks after the onset of large-scale bleaching in 1998, between 11 and 83% of colonies along replicate transects were affected by bleaching, and mortality was 1 to 16%. There were significant differences in bleaching response between sites, depths and taxa. Cyphastrea, Turbinaria and Galaxea were relatively unaffected by bleaching, while most acroporids and pocilloporids were highly susceptible. The hydrocorals (Millepora spp.) were the most susceptible taxa, with 85% mortality. Spatial variation in assemblage response was linked to the taxonomic composition of reef sites and their bleaching history. We suggest, therefore, that much of the spatial variation in bleaching response was due to assemblage composition and thermal acclimation. Accepted: 14 January 2000     

Enhanced particle-feeding capacity of corals on turbid reefs (Great Barrier Reef, Australia)

Reef corals occur across a wide range of habitats, from offshore clear waters to nearshore sediment-laden environments. This study tests the hypothesis that corals from turbid nearshore areas have greater capacity to utilise suspended sediment as a food source than conspecifics from less turbid and midshelf areas. The hypothesis was tested on two common and widespread coral species on the Great Barrier Reef (Pocillopora damicornis and Acropora millepora). The particle clearance rates of samples from more turbid reefs were two-fourfold those of conspecifics from less turbid and midshelf reefs. Rates of sediment ingestion were generally a linear function of sediment load indicating no significant saturation within the concentration range of 1–30 mg dry weight l⁻¹. Estimated assimilation efficiency of particulate ¹⁴C varied between 50 and 80%, and was maximised for midshelf A. millepora at the lowest sediment concentration, suggesting that heterotrophy is more efficient in oligotrophic habitats. Based on feeding-response curves, assimilation efficiencies, and published records of ambient particle concentrations, representatives of these species on turbid inshore reefs are 10–20 times more heterotrophic on suspended sediment than their conspecifics on less turbid and midshelf reefs. Accepted: 7 September 1999     

Diversity of Scleractinia and Octocorallia in the mesophotic zone of the Great Barrier Reef, Australia

Mesophotic coral reefs in the Indo-West Pacific, the most diverse coral reef region on earth, are among the least documented. This study provides the first detailed investigation of the diversity of Scleractinia and Octocorallia of the mesophotic Great Barrier Reef (GBR). Specimens were collected by 100-m rock dredge tows at 47–163 m depth on 23 sites in four regions (15.3°–19.7° latitude South). Twenty-nine hard coral species from 19 families were recorded, with the greatest diversity found at <60 m depth, and no specimen was found >102 m. Many of these species are also commonly observed at shallower depths, particularly in inshore areas. Twenty-seven octocoral genera were collected, 25 of which represented azooxanthellate genera. Generic richness of octocorals was highest at depths >60 m. Sixteen of the 25 azooxanthellate genera were either absent or very rare at <18 m, and only five azooxanthellate genera were common on both shallow and mesophotic reefs. Species-area models indicated that the total diversity of hard corals on the deep mesophotic reefs sampled during this study was ~84 species while octocorals were represented by ~37 genera; however, the wide 95% confidence limits indicates that more intensive sampling effort is required to improve the accuracy of these estimates. Nonetheless, these results show that the taxonomic richness, particularly of hard corals, on mesophotic reefs may be much higher than previously thought, a finding that has implications for the comprehensive and adequate protection of the full range of biodiversity of the GBR.     

Geographic and habitat partitioning of genetically distinct zooxanthellae (Symbiodinium) in Acropora corals on the Great Barrier Reef

Intra- and intercolony diversity and distribution of zooxanthellae in acroporid corals is largely uncharted. In this study, two molecular methods were applied to determine the distribution of zooxanthellae in the branching corals Acropora tenuis and A. valida at several reef locations in the central section of the Great Barrier Reef. Sun-exposed and shaded parts of all colonies were examined. Single-stranded conformational polymorphism analysis showed that individual colonies of A. tenuis at two locations harbour two strains of Symbiodinium belonging to clade C (C1 and C2), whereas conspecific colonies at two other reefs harboured a single zooxanthella strain. A. valida was found to simultaneously harbour strains belonging to two distinct phylogenetic clades (C and D) at all locations sampled. A novel method with improved sensitivity (quantitative polymerase chain reaction using Taqman fluorogenic probes) was used to map the relative abundance distribution of the two zooxanthella clades. At two of the five sampling locations both coral species were collected. At these two locations, composition of the zooxanthella communities showed the same pattern in both coral species, i.e. correlation with ambient light in Pioneer Bay and an absence thereof in Nelly Bay. The results show that the distribution of genetically distinct zooxanthellae is correlated with light regime and possibly temperature in some (but not all) colonies of A. tenuis and A. valida and at some reef locations, which we interpret as acclimation to local environmental conditions.     

Species richness and community structure of reef-building corals on the nearshore Great Barrier Reef

Species richness, cover and community structure of reef-building corals were assessed at 599 sites on 135 reefs along the Great Barrier Reef (GBR) between 1994 and 2001, with focus on the nearshore area. Communities were described hierarchically, with smaller regional communities forming part of higher level communities at increasing spatial scales. Site richness increased from the coast to the mid-continental shelf, declining on the outer shelf. Richness also increased with depth to 5 m, stabilizing thereafter. An anomaly was present in a 400 km section adjacent to the northern, ‘wet tropics’ coast, where site richness was 67 and 41% lower than the adjacent far northern and central GBR, respectively; this was probably due to the disturbance regime, with an apparent anthropogenic component. Site richness also declined in the Southern GBR, probably due to naturally marginal conditions. All indicator species had highest values in five small Far Northern and Central GBR communities. In the eight depauperate communities no indicator species had high values, indicating that these communities represent degraded, yet potentially transitional forms of the more diverse communities of the Far Northern and Central GBR. The study shows that on the GBR, disturbance results in the local removal of corals rather than a shift to suites of other coral species.     

Microbiological assessment of a disease outbreak on corals from Magnetic Island (Great Barrier Reef, Australia)

Unusual disease lesions were observed in Montipora corals on the fringing reef of Magnetic Island (Great Barrier Reef, Australia) following a period of high water temperature in early January 2002. Tissue death in Montipora spp. appeared as a black layer that spread rapidly across the colony surface, though this appeared as the final phase of disease progression (with three previous disease phases now identified, S. Anthony, unpublished). Culture and molecular-based microbial analysis of this layer did not identify a likely microbial pathogen. Despite this, DNA sequencing of microbial 16S rDNA indicated a shift in the bacterial population associated with affected coral tissue. A clone library of the healthy coral sample predominantly contained sequences within the -Proteobacteria. A disease coral sample representing the margin of the black lesion and healthy coral tissue was dominated by sequences, which demonstrated low sequence identity to a range of -Proteobacteria, -Proteobacteria and cyanobacteria. The microbes identified in the diseased Montipora spp. samples are likely to be opportunistic rather than the causative agent of the observed lesion. Studies are in progress to further characterise the ecology of this disease and describe the potential microbial pathogen(s).