The use of clear-water non-estuarine mangroves by reef fishes on the Great Barrier Reef

Within the tropics, mangroves and coral reefs represent highly productive biomes. Although these habitats are often within close proximity, the role and importance of mangrove habitats for reef fish species remains unclear. Throughout the Indo-Pacific, reef fish species appear to have few links with estuarine mangrove habitats. In contrast, clear-water non-estuarine mangrove habitats throughout the Caribbean support many reef fish species and may be fundamental for sustaining reef fish populations. But how important are clear-water non-estuarine mangroves for reef fishes within the Indo-Pacific? Using visual surveys during diurnal high tide, the fish assemblages inhabiting clear-water mangrove and adjacent reef habitats of Orpheus Island, Great Barrier Reef, were recorded. Of the 188 species of fishes that were recorded, only 38 were observed to inhabit both habitats. Of these, only eight were observed more than five times within each habitat. These observations provide little indication that the clear-water mangroves are an important habitat for reef fish species. In addition, although based on just a 3-month survey period, we found little evidence to suggest that these areas are important nurseries for reef fish species. The clear-water mangroves of Orpheus Island may, however, provide an additional foraging area for the few reef fish species that were observed to utilize these habitats during high tide. The difference in the importance of clear-water mangroves for reef fishes within this study compared with clear-water mangrove counterparts within the Caribbean is surprising. Although only preliminary, our observations would support suggestions that the patterns reflect the different hydrological characteristics and evolutionary histories of these two biogeographic regions.     

The distribution of herbivorous fishes on the Great Barrier Reef

The composition and functionality of ecologically important herbivorous fish assemblages were examined throughout much of Australia’s Great Barrier Reef (GBR). Diversity and abundance of surgeonfishes (Acanthuridae), parrotfishes (Labridae) and rabbitfishes (Siganidae) were strongly associated with position on the continental shelf, whilst effects of latitude were weaker and inconsistent. Species distributions varied considerably amongst taxonomic groups; parrotfishes were mostly widespread whilst distributions of surgeonfishes were often restricted. Most inshore environments supported depauperate herbivore assemblages dominated by different taxa and functional groups compared with assemblages in offshore environments. There were also strong cross-shelf transitions in the main taxa performing each functional role. Overall, this study show that the functional contributions of herbivorous fish assemblages to important ecosystem processes and the contributing taxa vary considerably amongst different GBR environments. Additionally, the two most numerically dominant herbivores actively select detritus, not algae, supporting increasing evidence for the importance of detritus in coral reef ecology.     

Significance of pelagic bacteria as a trophic resource in a coral reef lagoon,One Tree Island,Great Barrier Reef

Bacterial numbers and frequency of dividing cells (FDC) were estimated in surface waters at seven stations along a transeet across One Tree lagoon and reefs at 2 h intervals over one tidal cycle in May 1984 and once a day on 8 d in December 1984/January 1985. Cell numbers ranged between 1.24 and 13.10x10⁵ ml^-1, with an overall mean value of 4.81x10⁵. There was a progressive depletion in standing stock from windward to leeward stations across the lagoon, also reflected in the FDC, which generally showed similar dynamic patterns to cell numbers throughout the study. Predator-free seawater (filtered through 2 m pore-size filters) and mucus-enriched incubations were also used to establish the relationship between the growth rate () and FDC for coral-reef populations. Average growth rates predicted from FDC values ranged from 0.062 to 0.174 h^-1, which is equivalent to doubling times of 5.74 to 16.00 h, with an overall mean value of 11.5 h. These fast doubling times suggest that bacteria respond rapidly to pulses of enrichment as they float over the lagoon and reefs, characteristically achieving about one doubling per tidal cycle. This probably ensures that the bulk of labile organic matter and dissolved nutrients is conserved within the immediate coral-reef environment. Estimates of bacterial production also suggest that they may contribute up to 40% of total picoplankton production and about 25% of total microplankton production. Pelagic bacteria are therefore potentially a major food resource for benthic filter-feeders, especially for those mainly dependent on the smaller (pico) components of the plankton.     

Diel patterns of abundance of presettlement reef fishes and pelagic larvae on a coral reef

Most presettlement reef fish settled at night at One Tree Island, Great Barrier Reef. Fish were sampled day and night using channel nets located on the reef crest, and a plankton-mesh purse-seine net in the lagoon (1992–1994). Catches of fish at night were generally tens to hundreds of times greater than those taken during the day. Preflexion fish, as well as postflexion and pelagic juveniles, were taken in greater numbers at night. Preflexion forms were a combination of those that had hatched from demersal eggs and later stages that had been transported over the reef crest. Highest numbers of postflexion and pelagic juvenile forms of Apogonidae, Blenniidae, Gobiesocidae, Gobiidae, Labridae, Lutjanidae, Mugiloididae, Mullidae, Pomacentridae, Scaridae, Serranidae and Tripterygiidae were found at night. Observations, while SCUBA diving, and purse-seine samples in the lagoon indicated that the only resident larvae were of the genera Spratelloides and Hypoatherina; most of the fishes caught in nets, therefore, were immigrants. Patch reefs, sampled for new settlers early in the morning and late in the day, indicated that the majority of apogonids (Apogon doederleini, >95%) settled at night. Although greater numbers of pomacentrids were found in morning counts (e.g. Pomacentrus wardi), if data were converted to an hourly rate, many pomacentrids showed a similar hourly rate of settlement day and night. Depth-stratified sampling in waters near One Tree Island (to 20 m) indicated that some taxa rise to the surface at night. This behaviour, perhaps combined with avoidance of diurnal predators may explain on-reef movement of potential settlers soon after dark. Studies on settlement cues, therefore, need to focus on night-related phenomena. Received: 3 March 2000 / Accepted: 20 June 2000     

Ultraviolet photosensitivity and feeding in larval and juvenile coral reef fishes

The ability of young coral reef fishes to feed using solely ultraviolet-A (UV-A) radiation during ontogeny was examined using natural prey in experimental tanks. Larvae and juveniles of three coral reef fish species (Pomacentrus amboinensis, Premnas biaculeatus and Apogon compressus) are able to feed successfully using UV-A radiation alone during the later half of the pelagic larval phase. The minimum UV radiation intensities required for larval feeding occur in the field down to depths of 90–130 m in oceanic waters and 15–20 m in turbid inshore waters. There was no abrupt change in UV sensitivity after settlement, indicating that UV photosensitivity may continue to play a significant role in benthic juveniles on coral reefs. Tests of UV sensitivity in the field using light traps indicate that larval and juvenile stages of 16 coral reef fish families are able to detect and respond photopositively to UV wavelengths. These include representatives from families that are unlikely to possess UV sensitivity as adults due to the UV transmission characteristics of the ocular media. Functional UV sensitivity may be more widespread in young coral reef fishes than in the adults, and may play a significant role in detecting zooplanktonic prey.     

High genetic variability in a population of Tridacna maxima from the Great Barrier Reef

A population of the bivalve mollusk Tridacna maxima (Röding) from Hron Island, Great Barrier Reef, Australia, was studied by gel electrophoresis, and proved to be highly variable genetically, with an average heterozygosity of about 22%. This compares closely with a population of T. maxima from Enewetak (Eniwetok) Atoll, with an average heterozygosity of about 20%, very high for marine organisms. Enewetak Atoll was the site of a series of nuclear tests. The Heron Island study verifies that the high variability is natural, and supports the hypothesis that species from trophically stable environments tend to be highly variable genetically.     

Synchronous spawnings of 105 scleractinian coral species on the Great Barrier Reef

Following observations of mass spawning of hermatypic corals on the Great Barrier Reef in 1981 and 1982, spawning dates were successfully predicted and documented at five reefs on the Central and Northern Great Barrier Reef in 1983. During the predicted times, 105 species from 36 genera and 11 families were observed to spawn. Of these, 15 species were shown to have an annual gametogenic cycle. All but two of the species observed during mass spawnings shed gametes which underwent external fertilization and development. Synchronous spawning was observed both within and between the five reefs studied, which were separated by as much as 5° of latitude (500 km) or almost a quarter of the length of the Great Barrier Reef. The mass spawning of corals took place on only a few nights of the year, between the full and lastquarter moon in late spring. Maturation of gametes coincided with rapidly rising spring sea temperatures. Lunar and diel cycles may provide cues for the synchronization of gamete release in these species. The hour and night on which the greatest number of species and individuals spawned coincided with low-amplitude tides. Multispecific synchronous spawning, or mass spawning, of scleractinian and some alcyonacean corals represents a phenomenon which is, so far, unique in both marine and terrestrial communities.     

No apparent genetic basis to demographic differences in scarid fishes across continental shelf of the Great Barrier Reef

 Two species of parrot fish, Scarus frenatus and Chlorurus sordidus, are known to exhibit demographic and life-history differences across the continental shelf of the northern Great Barrier Reef (GBR). DNA sequences from the mitochondrial (mt) control region were analysed to determine whether there were genetic differences between the populations from the mid- and outer-shelf reefs. Analysis of molecular variance (AMOVA) indicated high levels of gene exchange for both species at a local scale between reefs on mid- and outer continental shelf positions (20 km apart) and at a broader scale along the length of the GBR province (>1000 km apart). There was no evidence to suggest that local differences in scarid life-history characteristics on the northern GBR have a genetic basis. Rather it appears more likely that phenotypically plastic responses to prevailing social and environmental conditions explain differences in the life-history characteristics of both taxa. However, analysis of genetic variation and historical demography revealed striking differences between the two species. S. frenatus haplotypes differed from one another at relatively few nucleotide sites (mean = 3.30), and the pairwise mismatch distribution suggested this species has undergone a population expansion within the limit of the resolution of the marker. C. sordidus haplotypes, however, differed from one another at a number of sites (mean = 7.67). Mismatch distribution analysis suggested that the population size of this species has remained at equilibrium over time. These patterns could also reflect differences in the metapopulation sizes or generation times between taxa. Some of the implications for fisheries management are discussed. Received: 28 January 2000 / Accepted: 9 July 2000     

Estimates of adult and juvenile mortality for labrid fishes at One Tree Reef,Great Barrier Reef

Few estimates of natural mortality have been reported for coral reef fishes, yet this information is essential for predicting the effects of recruitment fluctuations on adult populations. In this study, mortality of adult (10 species) and juvenile (11 species) labrid fishes resident on ten isolated patch reefs in One Tree Lagoon, southern Great Barrier Reef, was estimated by visual censuses covering 30 mo, from November 1981 to April 1984. Numbers of adults of all species declined linearly over this period. Mortality rates differed significantly among species, ranging from 4.9 to 69.5% per annum. Average life-span for fishes that had survived the first year of life varied from 1.6 to 11.5 yr, depending on species. Average juvenile mortality varied among species from 25 to 91.8% per annum and was highest for abundant schooling species. Different cohorts of the same species also experienced significantly different rates of mortality. No clear relationship emerged between the number of new recruits and the number of fishes surviving to one year of age. The influence of recruitment fluctuations on adult populations of labrid fishes may be difficult to predict.     

Spatial and temporal variation in coral recruitment on the Great Barrier Reef: Implications for dispersal hypotheses

Over 15 000 coral recruits were counted on settlement plates from three mid-shelf reefs and six fringing reefs in the northern section of the Great Barrier Reef during two summers (1986 and 1987) and one winter (1987). The density of coral recruits on some settlement plates from a fringing reef was up to 4.88 cm^–2, the highest value ever reported. Mean density of recruits was greater on fringing reefs (81.1 recruits/settlement plate) than on mid-shelf reefs (15.6 recruits/settlement plate), but there was greater spatial variation in abundance of recruits between the fringing reef sites. Other differences between the mid-shelf reefs and the fringing reefs were that different taxa were dominant, and that settlement orientation differed, with mid-shelf recruits settling preferentially on horizontally oriented surfaces and fringingreef recruits preferring vertical surfaces. Of the three midshelf reefs, Green Island reef recorded the highest recruitment rate for each of the two summers, despite having a depauperate adult coral population following predation by the asteroidAcanthaster planci. This suggests that coral larvae frequently travel between reefs. In contrast with an earlier study, there was no consistent difference in abundance of recruits between forereef and backreef locations. Overall, the results indicated great spatial variation in the availability of coral larvae, both on the scale of whole reefs and within-reef habitats.     

Among-habitat variation in herbivory on Sargassum spp. on a mid-shelf reef in the northern Great Barrier Reef

Herbivory is widely acknowledged as a key process determining the benthic community structure and resilience of coral reefs. Despite numerous studies that have examined herbivory across reef gradients in the Caribbean, few studies have directly quantified this process on Pacific reefs. Bioassays of two species of erect macroalgae (Sargassum swartzii and S. cristaefolium) were used to quantify variation in grazing intensity across seven habitats of varying depth and wave exposure on a mid-shelf reef in the northern Great Barrier Reef. Removal rates of Sargassum varied significantly among habitats, with both species displaying broadly similar patterns. The shallow habitats on the exposed aspect of the reef (i.e. reef crest, flat and back reef) experienced the highest reductions in mass (81.4–91.6% day⁻¹) for both S. swartzii and S. cristaefolium, while the deeper exposed habitats (reef slope and base) displayed the lowest reductions (3.8–13.4% day⁻¹) over a 24 h period. In contrast, the grazing intensity varied between the two species in the three habitats on the leeward aspect of the reef. Reductions in mass remained relatively high for S. swartzii on the patch reef and sheltered reef base and flat (62.7–76.5% day⁻¹) but were considerably lower for S. cristaefolium (37.9–63.5% day⁻¹) across the same habitats. Surprisingly, the rates of removal of Sargassum displayed no relationship with the density or biomass of roving herbivorous fishes or those species known to consume erect macroalgae, either collectively or independently. These results suggest that the relationship between browsing rates and herbivorous fish biomass is complex and may be driven by species that are underestimated in visual surveys. Direct quantification of browsing intensity using assays revealed a different pattern to inferences based on herbivore densities and highlights the potential difficulties of evaluating ecosystem processes based on visual census data alone.     

Environmental influences on larval duration,growth and magnitude of settlement of a coral reef fish

The influence of environmental variables on the planktonic growth, pelagic larval duration and settlement magnitude was examined for the coral reef surgeonfish Acanthurus chirurgus. Newly settled fish were collected daily from patch reefs in the San Blas Archipelago, Caribbean Panama for 3.5 years. Environmental influences on growth were examined at three different life history stages: from 0 to 6 days, 7 to 25 days and from 26 to 50 days after hatching. Larval growth was correlated, using multiple regression techniques, with a combination of factors including solar radiation, rainfall, and along-shore winds. Depending on the life history stage, these accounted for 13–38% of the variation in growth rates when all the months were included in the analyses. Correlations between environmental variables and growth also varied among seasons and were stronger in the dry than in the wet season. During the dry season solar radiation, rainfall and along-shore winds described 57%, 86% and 74% of the variability in growth between 0 and 6 days, 7 and 25 days and 26 and 50 days, respectively. During the wet season rainfall, along-shore winds and temperature only described 38% of the variability in early growth and 27% of growth just before settlement. No significant model was found to describe growth 7–25 days after hatching during the wet season. Rainfall, solar radiation and along-shore winds were negatively correlated with growth up to 25 days after hatching but positively correlated as larvae approached settlement at a mean age of 52 days. Over 65% of the variability in pelagic larval duration was accounted for by a regression model that included solar radiation and along-shore winds. When data sets from wet and dry seasons were analysed separately, along-shore winds accounted for 67% of the change in larval duration in the dry season, and solar radiation accounted for 23% of the variation in larval duration in the wet season. Only 22% of the variability in settlement intensity could be described by solar radiation and temperature, when all months of the year were included in the analysis. Solar radiation and rainfall were included in a regression model that accounted for 40% of the variation in numbers of fish settling during the dry season. This study suggests that the levels of solar radiation, along-shore winds and rainfall during the early larval life can have important effects on the growth, larval duration and consequently, the settlement magnitude of marine fishes. Results also highlight the need to account for seasonality and ontogeny in studies of environmental influences.Communicated by G.F. Humphrey, Sydney     

Rates of nitrogen fixation on coral reefs across the continental shelf of the central Great Barrier Reef

This study was undertaken in 1981 to determine whether there were major variations in potential rates of nitrogen fixation on apparently bare coralline substrate from reefs across the continental shelf of the central Great Barrier Reef. Nitrogen fixation, measured as rates of ethylene production (nmol cm^-2h^-1), was significantly lower on substrata from two inner-shelf reefs, (0.46 and 1.07) than on two middle-shelf reefs (2.10 and 2.97) and on two outer-shelf reefs (3.20 and 3.81). By contrast, algal biomass (mg cm^-2) on experimental substrate was significantly higher on inner-shelf reefs (80.8 and 59.4) than on middleshelf (27.1 and 23.8) and outer-shelf reefs (26.4 and 22.4). The rate of nitrogen fixation was positively correlated with the proportion of bare substratum and significantly higher concentrations of dissolved inorganic nitrogen were found in waters over the reefs than in water flowing onto those reefs. The abundance of algal-grazing fishes was reported previously to be significantly lower on inner-shelf reefs. It is suggested that this cross-shelf variation in the activity of algal-grazing fishes may be a determinant of the observed cross-shelf variations in potential nitrogen fixation.Contribution No. 233 from the Australian Institute of Marine Science     

Regional differences in duration of the planktonic larval stage of reef fishes in the eastern Pacific Ocean

Regional variation in the duration of the planktonic larval phase of three species of reef fishes, Thalassoma lucasanum (Labridae), Stegastes flavilatus, and Microspathodon dorsalis (Pomacentridae) was investigated between 1982 and 1991 at several sites in the tropical eastern Pacific over a distance of 3500 km, encompassing virtually their entire range of distribution. Durations of the larval phase, determined from counts of daily otolith increments, were significantly different (1.3 to 1.6 x) between sites. Populations of all three species had a consistently shorter larval life at the most northern site, Cabo San Lucas (Mexico) compared to Panamá and the offshore islands of Galápagos and Cocos. Analyses of otolith increment width over the precompetent period revealed that this disparity in larval duration primarily reflected differences in larval growth rates: faster growing fish spent less time in the plankton. In T. lucasanum, some of the variation in larval duration between Panamá and offshore sites (Galápagos Islands and Cocos Island) may be accounted for by a higher frequency of individuals delaying metamorphosis at the offshore sites. These data indicate that conditions in the planktonic environment are not homogeneous throughout the tropical eastern Pacific and may have a profound effect on aspects of the larval ecology of reef fishes in this region.     

Correlation between pelagic larval duration and realised dispersal: long-distance genetic connectivity between northern New Zealand and the Kermadec Islands archipelago

The extent to which marine populations are “open” (panmixia) or “closed” (self-recruitment) remains a matter of much debate, with recent reports of high levels of genetic differentiation and self-recruitment among populations of numerous species separated by short geographic. However, the interpretation of patterns of gene flow (connectivity) is often based on a stepping stone model of dispersal that can genetically homogenise even distant populations and blur genetic patterns that may better reflect realised dispersal. One way in which realised long-distance dispersal can be accurately determined is by examination of gene flow of taxa between isolated archipelagos and a mainland where there is no possible stepping stone dispersal across the open ocean. We investigated the genetic structuring of populations of the intertidal gastropod Nerita melanotragus from the subtropical Kermadec Islands and temperate New Zealand’s North Island (the mainland), separated by 750 km of open ocean and characterised by contrasting environmental conditions. Analyses of seven microsatellite markers revealed an absence of genetic structuring with low F _ST and Jost’s D values (from 0.000 to 0.007 and from 0.000 to 0.015, respectively) over large geographic distances and no evidence of isolation by distance among all populations. These results indicate that the realised dispersal of N. melanotragus is of at least 750 km, this species exhibits a very “open” form of connectivity and its larvae exhibit sufficient phenotypic plasticity to settle successfully in different environmental conditions, ranging from subtropical to cool temperate.     

Comparative population genetic structure of marine gastropods (Littorina spp.) with and without pelagic larval dispersal

 Population genetic theory predicts that marine animal species with planktonic larvae will have less genetic structure than those with direct development. We compared the genetic structure of four species of littorinid snails – two with planktonic egg capsules that hatch as planktonic larvae and two with benthic egg masses that hatch as crawl-away juveniles. We used DNA sequencing and single stranded conformational polymorphism (SSCP) to assess sequence variation in a 480 bp fragment of the mitochondrial cytochrome b gene and then used an analysis of molecular variance (AMOVA) to estimate Φ_st for populations from the northeastern Pacific coast. One of the two direct-developing species, Littorina subrotundata, had a moderate amount of population structure (Φ_st=0.209) as expected but the other direct-developing species, L. sitkana, was nearly fixed for a single haplotype that made it impossible to precisely estimate Φ_st. One of the two planktonic-developing species, L. scutulata, did not show any significant population structure (Φ_st=0.004). In contrast to our expectations, the other planktonic-developing species, L. plena, showed some weak but statistically significant population structure (Φ_st=0.052). We discuss how differences in population genetic structure between species with the same type of development may reflect differences in their historical demography. Received: 22 December 1999 / Accepted: 24 July 2000     

Symbiont specificity and bleaching susceptibility among soft corals in the 1998 Great Barrier Reef mass coral bleaching event

Considerable variability in bleaching was observed within and among soft coral taxa in the order Alcyonacea (Octocorallia: Cnidaria) on the central Great Barrier Reef (GBR, latitude 18.2°–19.0°S, longitude 146.4°–147.3°E) during the 1998 mass coral bleaching event. In April 1998, during a period of high sea surface temperatures, tissue samples were taken from bleached and unbleached colonies representative of 17 soft coral genera. The genetic identities of intracellular dinoflagellates (Symbiodinium spp.) in these samples were analyzed using PCR-denaturing gradient gel electrophoresis fingerprinting analysis of the internal transcribed spacer regions 1 and 2. Alcyonaceans from the GBR exhibited a high level of symbiont specificity for Symbiodinium types mostly in clade C. A rare clade D type (D3) was associated only with Clavularia koellikeri, while Nephthea sp. hosted symbionts in clade B (B1n and B36). Homogenous Symbiodinium clade populations were detected in all but one colony. Colonies that appeared bleached possessed symbiont types that were genetically indistinguishable from those in nonbleached conspecifics. These data suggest that parameters other than the resident endosymbionts such as host identity and colony acclimatization are important in determining bleaching susceptibility among soft corals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Diversity,abundance, and distribution of reef sharks on outer-shelf reefs of the Great Barrier Reef,Australia

Quantifying the distribution and habitat use of sharks is critical for understanding their ecological role and for establishing appropriate conservation and management regimes. On coral reefs, particularly the Great Barrier Reef (GBR), little is known regarding the distribution of sharks across major reef habitat types. In this study, we surveyed shark populations across outer-shelf reefs of the GBR in order to determine the diversity, abundance, and distribution of reef sharks across three major coral reef habitats: (1) the reef slope, (2) the back reef and (3) the reef flat. Model selection revealed that habitat was the principal factor influencing shark distribution and abundance. Specifically, overall shark abundance and diversity were significantly higher on the reef slope (and to a lesser degree, the back reef) than the reef flat. This confirms that shark populations are not homogeneously distributed across coral reefs. Thus, the results presented herein have important implications for shark population assessments. In addition, our results highlight the potential importance of the reef slope, with high levels of live coral cover and structural complexity, for sustaining reef shark populations. As this habitat is highly susceptible to disturbance events, this study provides a useful context for predicting and understanding how environmental degradation may influence reef shark populations in the future.     

Zooplankton community structure and water flow in the lee of Helix Reef (Great Barrier Reef, Australia)

To investigate which physical processes contribute most in moulding zooplankton community structure in the waters close to coral reefs, light traps moored in a grid pattern were used to collect zooplankton from the sea surface at 16 stations on the downstream side of Helix Reef during three time periods (2100-2200, 2400-0100, and 0300-0400 hours) over three consecutive nights covering the new moon period in January 1992. Two distinct zooplankton communities were present: a community composed primarily of reef-resident, demersal plankton immediately to the south of the reef in an area of reduced flushing, and a community containing coastal and shelf-seas taxa at the more exposed sites in the open flow field. The fauna composition at a number of exposed stations was as rich as that at sheltered stations both in terms of number of taxa and diversity indices but was almost an order of magnitude less abundant. The reef-resident, demersal plankton community was dominated by gammarid amphipods, mysids, and polychaetes, whereas only transient, meroplanktonic forms such as echinoderm and echinopluteus larvae and shelf-seas, holoplanktonic forms such as doliolids and larvaceans were significantly more abundant in the exposed community. Zooplankton associations were apparently formed by a combination of hydrodynamic processes, spatial and temporal distribution patterns of individual taxa, specific behaviours of certain taxa, and the interactions among taxa at different trophic levels.