Ecological and physiological differences between two colour morphs of the coral Pocillopora damicornis

Pocillopora damicornis (Linnaeus) is a ubiquitous branching coral found throughout the Indo-Pacific region. Like many other species of coral, P. damicornis displays a large range of morphologies. At One Tree Island, it occurs as two distinct morphs that are easily distinguished by the presence or absence of pink pigmentation. The two colour morphs of P. damicornis were found to differ in their distribution and abundance in the One Tree Island Lagoon. The brown morph was more abudant than the pink morph in the shallows (<1 m),whereas the pink morph was more abundant at deeper sites (>3 m). The two morphs also differed physiologically. The brown morph tended to have a greater calcification rate than the pink morph, regardless of environmental conditions. However, the difference in the calcification rate between the two morphs became non-significant under shaded conditions (5% full sunlight), indicating some degree of physiological plasticity of the morphs. The pink colour in P. damicornis was due to a hydrophilic pigment with a major peak absorbance at 560 nm. The expression of pink pigment had both genetic and phenotypic components. The brown morph has a reduced genetic capacity to express the pigment relative to the pink morph. On the other hand, pigment expression could be induced by light in the pink morph. Although genetic differences ultimately determine the differences between the two morphs of P. damicornis, the extent of pigment expression is under some degree of environmental influence.     

Genetic structure of dictyoceratid sponge populations on the western Coral Sea reefs

Allozyme variation at six polymorphic loci was examined in foliose dictyoceratid sponges from isolated reefs in the western Coral Sea. Four major genetic groups corresponding to the species Phyllospongia lamellosa, P. alcicornis, Carterospongia flabellifera and Collospongia auris were examined. A further two rare morphotypes from individual reefs formed genetic outliers to the P. lamellosa group, and may represent further taxa related to P. lamellosa. Gene frequencies in individual reef populations were largely in Hardy-Weinberg equilibrium, suggesting that random mating occurred in local populations of all four common species. Genetic variability was high and observed heterozygosities within populations ranged from 0.13 to 0.40. All four taxa showed significant genetic differentiation among populations (F _ST=0.05 to 0.36). Genetic distances (Nei's D) among populations within species ranged from 0 to 0.723 and increased with increasing geographical separation. There was evidence that genetic differentiation between populations to the north and to the south of the southern limit of the South Equatorial Current (SEC) divergence was greater than expected on the basis of their geographical separation. The SEC divergence may form a partial barrier to gene flow among populations of these ecologically important sponges on the submerged Queensland Plateau. Levels of migration among populations of three of the species was less than those required to prevent divergence of the populations through genetic drift (Nm<1). Restricted migration among populations may provide a mechanism to explain the occurrence of highly divergent populations of dictyoceratid sponges whose specific identity is not clear, and may allow them additionally to develop partial reproduction isolation from other populations.     

Genetic diversity,clonality and connectivity in the scleractinian coral Pocillopora damicornis: a multi-scale analysis in an insular,fragmented reef system

Clonality and genetic structure of the coral Pocillopora damicornis sensu lato were assessed using five microsatellites in 12 populations from four islands of the Society Archipelago (French Polynesia) sampled in June 2008. The 427 analysed specimens fell into 132 multilocus genotypes (MLGs), suggesting that asexual reproduction plays an important role in the maintenance of these populations. A haploweb analysis of ITS2 sequences of each MLG was consistent with all of them being conspecific. Genetic differentiation was detected both between and within islands, but when a single sample per MLG was included in the analyses, the populations turned out to be nearly panmictic. These observations provide further evidence of the marked variability in reproductive strategies and genetic structure of P. damicornis throughout its geographic range; comparison with results previously obtained for the congeneric species Pocillopora meandrina underlines the importance of life history traits in shaping the genetic structure of coral populations.     

Sexual reproduction in Acropora (Isopora) (Coelenterata: Scleractinia)

In contrast to the seasonal gamete and planula production of Acropora (Isopora) palifera on Heron Island reef (Lat. 23° S), populations on Lizard Island reef (Lat. 14° S), sampled in 1979, 1981 and 1983, and Salamaua and Busama reefs (Lat. 7° S), sampled from 1980 to 1983, planulated year-round. Intensive sampling of colonies at Salamaua and Busama showed that gametes ripened at two-month intervals and that up to six cycles of gametes and larvae could be produced by an individual colony. Gametes of only a portion of the population — usually close to 50% — ripened each month. The Salamaua population, on average, produced fewer and smaller planulae than the Heron Island population during each two-monthly reproductive cycle. Hypotheses correlating the annual periodicity of breeding in marine animals with latitudinal variation of temperature were tested. In general, the time of breeding in A. palifera at Heron Island reef is much more restricted than theories based on latitudinal variations of water temperature would predict. It is hypothesized that, in the near subtropical environment of Heron Island reef, this species has evolved a life-history strategy that limits the amount of energy allocated to reproduction and allocates more energy to growth.     

Effect of nutrient enrichment on the complementary (secondary) metabolite composition of the soft coral Sarcophyton ehrenbergi (Cnidaria: Octocorallia: Alcyonaceae) of the Great Barrier Reef

 A long-term study of the effects of nutrient enrichment on coral reefs (ENCORE Experiment) was carried out at One Tree Island (23°30′S; 152°96′E), Great Barrier Reef, between 1992 and 1996. The experiment involved the addition of water-soluble nutrients to 12 microatolls which contained a range of organisms and were situated within the larger lagoon. Three replicates of each of three nutrient treatments (nitrogen, phosphorus, nitrogen plus phosphorus) and an untreated set of three control atolls were prepared using the 12 selected microatolls. As part of the larger ENCORE experiment, changes in the chemical composition of the alcyonacean soft coral Sarcophyton ehrenbergi Marenzeller (Octocorallia: Alcyonacea), placed in the treatment microatolls, were monitored for a 1 yr period in an attempt to detect any responses attributable to nutrient enrichment. Multivariate analyses were performed to determine whether there were any patterns of response in the different nutrient treatments. At the level of individual metabolites, there were no clear treatment effects. However, the ratio of bioactive or stress metabolites (terpenes) to energy storage metabolites (lipids), referred to as the “physiological-change indicator”, revealed effects of nutrient enrichment. Nitrogen enrichment resulted in a trend towards higher physiological-change indicators than control or phosphorus treatments in the majority of cases, while phosphorus enrichment significantly decreased the ratio relative to controls. In most cases, the physiological-change indicator increased in soft soft corals relocated into contact with the scleractinian Pocillopora damicornis. The potential of soft corals to serve as indicators of a changed nutrient regime is discussed. Received: 23 November 1998 / Accepted: 23 September 1999     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Sexual reproduction in Acropora (Isopora) species (Coelenterata: Scleractinia)

Despite the wide range of morphological diversity among the Acropora (Isopora) colonies on Heron Island (Great Barrier Reef, Australia), only two reproductively isolated species were present from 1977 to 1982: A. cuneata (Dana, 1846) and A. palifera (Lamarck, 1816). Both species released planulae lacking zooxanthellae and were simultaneous hermaphrodites with testes and ovaries occurring on separate mesenteries within the same polyp. Oogenesis preceded spermatogenesis. Seasonal cycles of gametogenesis, embryogenesis and planulation occurred in the two species. Colonies of A. cuneata developed two cycles of gametes. One cycle matured near the first quarter moon in April and the other on the same lunar phase in June. Planulae release occurred from about September to December each year and was not correlated with lunar phase. Gametes of A. palifera ripened only once per annum, a few days after the last quarter moon in November, and planulation occurred from about January to March. Embryos were brooded in the coelenteron of the polyps in both species. Ova were fertilized in the mesenteries and embryos were retained within an envelope of mesoglea and gastrodermis, remaining attached to the mesentery by a stalk until the larvae matured and were released. A. palifera and A. cuneata were abundant in the unpredictable reef flat environment. However, their life-history traits, e.g. seasonal reproduction, delayed sexual maturity, large colony size and fairly long life span, were more specialised than had been predicted for this type of environment.     

Hidden genetic diversity in a key model species of coral

A total of 43 colonies of the scleractinian coral Pocillopora damicornis from lagoonal and reef slope sites in the western Indian Ocean (WIO) region were genetically characterised at one nuclear and two mitochondrial sequence markers and six microsatellite loci. Both mitochondrial and microsatellite data support the existence of two reciprocally monophyletic clusters (F- and NF-types) and provide evidence of the existence of two cryptic species of P. damicornis on reefs in WIO region and put current morphological delineation and geographical boundaries of P. damicornis and Pocillopora molokensis into question. The results add to ongoing studies on the phylogeny and phylogeography within the genus Pocillopora, which all point towards a range of unresolved morphological and molecular species boundaries. Nuclear phylogenies derived from the present and previously published sequences show evidence for incomplete lineage sorting and/or introgressive hybridisation between Pocillopora morphospecies. However, the two WIO types largely remain in separate clusters, further supporting the theory that these represent two different species.     

Size and structure of echinoderm populations associated with different coexisting coral species at Aldabra Atoll,Seychelles

The associated echinoderm fauna of 125 living heads of massive microatolls of Porites spp., branched heads of Porites nigrescens Dana and small, densely packed heads of the hydrocoral Millepora exacea Forskål of a shallow sublittoral lagoon flat at Aldabra Atoll, Seychelles was examined. The weight and number of echinoderms increased with coral size in all cases. The microatolls were the largest coral heads and sheltered the most diverse, underlying echinoderm cryptofauna comprising the largest bodied species, regardless of class or family. The infauna in heads of P. nigrescens was less diverse and smaller bodied than the echinoderms under microatolls. Only with P. nigrescens did species number increase with increasing coral size. Heads of M. exacea were the smallest and provided a habitat of compact interstitial spaces for the most depauperate, smallest bodied infauna which included juveniles of larger species. Ophiuroids dominated the coral-associated echinoderm faunas whereas larger bodied classes, such as echinoids and holothuroids, were less well represented. Large bodied ophiuroids such as certain ophiodermatids occurred only under microatolls, whereas the usually small bodied ophiotrichids dominated in the heads of M. exacea. The generally ubiquitous ophiocomids were well represented in, or under all corals. Heads of M. exacea could be nursery areas for recently metamorphosed juvenile ophiuroids and echinoids which, as they grow, seek shelter in corals with larger interstitial spaces such as P. nigrescens. Significant interspecific ophiuroid associations were related to space availability for larger species under microatolls and in heads of P. nigrescens. The possibility of microhabitat partitioning in corals among some ophiuroids is suggested.     

Comparative species composition and relative abundance of decapod crustaceans in marine habitats of Panamá

The community structure of the decapod crustacean fauna of 7 tropical, shallowwater, marine habitats (sandy beaches, mangrove swamps and rocky intertidal habitats on both the Pacific and Caribbean coasts of Panama, and Pocillopora damicornis coral habitat of the Bay of Panama) were examined and analyzed for species composition and relative abundances. Collections from the 7 habitats yielded 4361 individuals, representing 236 species. The number of species per habitat was (Pacific, Caribbean): sandy beach (16, 7); mangrove (20, 17); P. damicornis (53); rocky intertidal (78, 67). There were more species represented by more individuals in the Pacific habitats. An index of faunal similarity was calculated for each pair (Pacific-Caribbean) of habitats. This index is the number of ecologically similar congeneric species which occurred in both habitats expressed as a percentage of the total number of species present in the pair of habitats. For the sandy beach communities there were three Pacific species which were similar to three Caribbean species, a similarity of 6/23 or 26%. The index of similarity for the mangrove communities is 54% and for the rocky intertidal communities it is 37%. The P. damicornis community has affinities with the Pacific rocky intertidal community (18%), with that of the Caribbean rocky intertidal (16%) and with that of Indo-West Pacific pocilloporid corals (20%). A few specialized species dominated each of the communities. The habitats and the number of species accounting for over half of the individuals present are (Pacific, Caribbean): sandy beach (1, 1); mangrove (4, 4); P. damicornis (5); and rocky intertidal (3, 6). Most of the species in each community were represented by one or a few individuals.     

Genetic variation and clonal structure in the scleractinian coral Pocillopora damicornis in the Ryukyu Archipelago, southern Japan

Samples of the scleractinian coral Pocillopora damicornis were collected from six sites located around four islands in the Ryukyu Archipelago, southern Japan, and subjected to allozyme electrophoresis. Seven polymorphic loci were examined for their allelic patterns. The ratio of observed to expected genotypic diversity (0.30 < G _o :G _e  < 0.64), the ratio of the observed number of genotypes to the number of individuals (0.47 < N _g :N _i  < 0.75), and deviations from Hardy–Weinberg equilibrium indicated that asexual reproduction plays a major role in the maintenance of established populations. However, populations were not completely dominated by a single or a few clones, and most clones were represented by only a few individual samples. The high frequency of typhoons in the region suggests that, in P. damicornis, fragmentation caused through occasional exposure to powerful waves is a major mode of asexual reproduction, but asexual production of planulae may also be contributing to the maintenance of populations. A significant genetic differentiation (F ST) was found between the six populations examined (0.027 < F ST < 0.092, average F ST = 0.056). The moderate gene flow is discussed according to characteristics of the larval stage of the species, and to circulation patterns in the region. Received: 7 August 1998 / Accepted: 18 May 1999     

Nitrogen fixation on a coral reef

Acetylene reduction was used to assess nitrogen fixation on all major substrates at all major areas over a period of 1 to 6 yr (1980–1986) at One Tree Reef (southern Great Barrier Reef). Experiments using ¹⁵N₂ gave a ratio of 3.45:1.0 for C₂H₂ reduced:N₂ fixed. Acetylene reduction was largely light-dependent, saturated at 0.15 ml C₂H₂ per ml seawater, and linear over 6 h. High fixation was associated with two emergent cyanophyte associations, Calothrix crustacea and Scytonema hofmannii, of limited distribution. Subtidally, the major contribution to nitrogen fixation came from well-grazed limestone substrates with an epilithic algal community in the reef flat and patch reefs (3 to 15 nmol C₂H₄ cm^-2 h^-1). Similar substrates from the outer reef slope showed lower rates. Nitrogen fixation on beach rock, intertidal coral rubble, reef crest and lagoon sand was relatively small (0.3 to 1.0 nmol C₂H₄ cm^-2 h^-1). Seasonal changes in light-saturated rates were small, with slight reduction only in winter. Rates are also reported for experimental coral blocks (13 to 39 nmol cm^-2 h^-1) and for branching coral inside and outside territories of gardening damselfish (3 to 28 nmol cm^-2 h^-1). This work supports the hypothesis that the high nitrogen fixation on the reef flat and patch reefs of the lagoon (34 to 68 kg N ha^-1 yr^-1) is because these subtidal areas support highly disturbed communities with the greatest abundance of nitrogen-fixing cyanophyte algae. It is calculated from a budget of all areas that One Tree Reef has an annual nitrogen fixation rate of 8 to 16 kg N ha^-1 yr^-1.     

Availability of two forms of dissolved nitrogen to the coral Pocillopora damicornis and its symbiotic zooxanthellae

The relative contribution of dissolved nitrogen (ammonium and dissolved free amino acids DFAAs) to the nitrogen budget of the reef-building coral Pocillopora damicornis was assessed for colonies growing on control and ammonium-enriched reefs at One Tree Island (southern Great Barrier Reef) during the ENCORE (Enrichment of Nutrient on Coral Reef; 1993 to 1996) project. P. damicornis acquired ammonium at rates of between 5.1 and 91.8 nmol N cm⁻² h⁻¹ which were not affected by nutrient treatment except in the case of one morph. In this case, uptake rates decreased from 80.5 to 42.8 nmol cm⁻² h⁻¹ (P < 0.05) on exposure to elevated ammonium over 12 mo. The presence or absence of light during measurement did not influence the uptake of ammonium ions. Nitrogen budgets revealed that the uptake of ammonium from concentrations of 0.11 to 0.13 μM could completely satisfy the demand of growing P. damicornis for new nitrogen. P. damicornis also took up DFAAs at rates ranging from 4.9 to 9.8 nmol N cm⁻² h⁻¹. These rates were higher in the dark than in the light (9.0 vs 5.1 nmol m⁻² h⁻¹, P < 0.001). Uptake rates were highest for the amino acids serine, arginine and alanine, and lowest for tyrosine. DFAA concentrations within the ENCORE microatolls that received ammonium were undetectable, whereas they ranged up to 100 nM within the control microatolls. The contribution of DFAAs to the nitrogen budget of P. damicornis constituted only a small fraction of the nitrogen potentially contributed by ammonium under field conditions. Even at the highest field concentrations measured during this study, DFAAs could contribute only ≃11.3% of the nitrogen demand of P.␣damicornis. This contribution, however, may be an important source of nitrogen when other sources such as ammonium are scarce or during periods when high concentrations of DFAAs become sporadically available (e.g. cell breakage during fish-grazing). Received: 22 April 1998 / Accepted: 3 November 1998     

Mode and timing of reproduction in some common hermatypic corals of Hawaii and Enewetak

Common Hawaiian and Enewetak corals were examined to determine the method and mining of reproduction. Of the 7 Hawaiian species examined for the release of planulae, only 2 have planulated in captivity, Pocillopora damicornis and Cyphastrea ocellina. Both planulate year-round and both are characteristic of reef flats. Four of the 5 species which did not planulate were found to contain eggs, but not planulate, when polyps were examined microscopically. These 5 species do not usually occur on reef flats. Seven of the 12 Enewetak species examined in June, July, August and January planulated; 4 of these were pocilloporids, all of which are common in shallow water. Only 3 of the 8 species of Acropora planulated, and these 3 occur solely in shallow water. A greater proportion of the Pocillopora spp. colonies than Acropora spp. colonies planulated and they released more planulae per head. In previous studies and in this one, coral species which have released planulae are characteristic of shallow-water environments such as reef flats. Most of the 10 species reported on here which failed to planulate in captivity are not commonly found on reef flats. The failure to detect planulation in so many species, particularly those of deeper water, suggests that common hermatypic corals may not all reproduce in the same way, and that mode of reproduction may be related to habitat.     

Genetic structure across the GBR: evidence from short-lived gobies

The Great Barrier Reef (GBR) exhibits distinct cross-shelf zonation. These patterns are particularly well documented in reef fishes and have been attributed to either environmental gradients (e.g. wave energy, oceanography) or barriers to gene flow. This study examined the extent to which barriers to gene flow contribute to cross-shelf patterns by examining the mitochondrial DNA of gobies (genus Eviota). The genus Eviota was selected due to its extreme life history characteristics (shortest vertebrate lifespan) and cross-shelf distribution patterns (E. queenslandica, inner- and mid-shelf, and E. albolineata mid- and outer-shelf). Although cross-shelf barriers to gene flow were predicted, this study found no population structure between shelf locations. However, a genetically distinct population of E. queenslandica (the inner-shelf species) was observed at North Direction Island (Phi_st = 0.088, P = 0.004). As no comparable structure was observed in E. albolineata (the outer-shelf species) it may be that habitat type (E. queenslandica = reef lagoon, E. albolineata = reef crest) is a significant factor driving the structure observed in E. queenslandica. Larval behaviour, olfactory or auditory senses and reef selection at settlement could be assisting larvae to return to reefs similar to natal reefs. We suggest that ecological gradients are more important than barriers to gene flow in structuring cross-shelf distributions within Eviota.     

Influence of field-based nutrient enrichment on the photobiology of the giant clam Tridacna maxima

Nutrients were added to 12 microatolls in One Tree Island lagoon every low tide for 13 mo to an initial concentration of 10 μM (ammonium, N) and 2 M (phosphate, P). These concentrations remained above background for 2 to 3 h after addition. The addition of ammonium (N and N+P but not P alone) significantly increased P _g (gross photosynthesis) P _n (net photosynthesis) and R (respiration) per unit wet-tissue weight and α (photosynthetic efficiency) in Tridacna maxima after 3 mo nutrient enrichment. These responses to small and transient changes in ammonium concentrations suggest that symbiotic clams are not nutrient-replete, and that even subtle changes in nutrients can have a measurable effect on photosynthesis. The same clams did not show significant differences in photosynthetic parameters 6 mo after the beginning of nutrient enrichment, suggesting that their previous responses had either been seasonal or that symbiotic clams such as T. maxima are able to adjust their photophysiology following external changes in nutrient concentrations. Received: 26 August 1997 / Accepted: 11 December 1998     

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     

Extreme genetic diversity and temporal rather than spatial partitioning in a widely distributed coral reef fish

Mitochondrial control region (HVR-1) sequences were used to identify patterns of genetic structure and diversity in Naso vlamingii, a widespread coral reef fish with a long evolutionary history. We examined 113 individuals from eight locations across the Indo-Pacific Ocean. Our aims were to determine the spatial scale at which population partitioning occurred and then to evaluate the extent to which either vicariance and/or dispersal events have shaped the population structure of N. vlamingii. The analysis produced several unexpected findings. Firstly, the genetic structure of this species was temporal rather than spatial. Secondly, there was no evidence of a barrier to dispersal throughout the vast distribution range. Apparently larvae of this species traverse vicariance barriers that inhibit inter-oceanic migration of other widespread reef fish taxa. Thirdly, an unusual life history and long evolutionary history was associated with a population structure that was unique amongst coral reef fishes in terms of the magnitude and pattern of genetic diversity (haplotype diversity, h = 1.0 and nucleotide diversity π = 13.6%). In addition to these unique characteristics, there was no evidence of isolation by distance (r = 0.458, R ² = 0.210, P = 0.078) as has also been shown for some other widespread reef species. However, some reductions in gene flow were observed among and within Ocean basins [Indian–Pacific analysis of molecular variance (AMOVA), Φ _st = 0.0766, P < 0.05; West Indian–East Indian–Pacific AMOVA Φ _st = 0.079, P < 0.05]. These findings are contrasted with recent studies of coral reef fishes that imply a greater degree of spatial structuring in coral reef fish populations than would be expected from the dispersive nature of their life cycles. We conclude that increased taxon sampling of coral reef fishes for phylogeographic analysis will provide an extended view of the ecological and evolutionary processes shaping coral reef fish diversity at both ends of the life history spectrum.     

Protection of Genetic Diversity and Maintenance of Connectivity among Reef Corals within Marine Protected Areas

Abstract: High‐latitude coral reefs (HLRs) are potentially vulnerable marine ecosystems facing well‐documented threats to tropical reefs and exposure to suboptimal temperatures and insolation. In addition, because of their geographic isolation, HLRs may have poor or erratic larval connections to tropical reefs and a reduced genetic diversity and capacity to respond to environmental change. On Australia's east coast, a system of marine protected areas (MPAs) has been established with the aim of conserving HLRs in part by providing sources of colonizing larvae. To examine the effectiveness of existing MPAs as networks for dispersal, we compared genetic diversity within and among the HLRs in MPAs and between these HLRs and tropical reefs on the southern Great Barrier Reef (GBR). The 2 coral species best represented on Australian HLRs (the brooding Pocillopora damicornis and the broadcast‐spawning Goniastrea australensis) exhibited sharply contrasting patterns of diversity and connectedness. For P. damicornis, the 8‐locus genetic and genotypic diversity declined dramatically with increasing latitude (N_a= 3.6–1.2, H_e= 0.3–0.03, N_g:N = 0.87–0.06), although population structure was consistent with recruitment derived largely from sexual reproduction (G_o:G_e= 1.28–0.55). Genetic differentiation was high among the HLRs (F_ST[SD]= 0.32 [0.08], p < 0.05) and between the GBR and the HLRs (F_ST= 0.24 [0.06], p < 0.05), which indicates these temperate populations are effectively closed. In contrast for G. australensis, 9‐locus genetic diversity was more consistent across reefs (N_a= 4.2–3.9, H_e= 0.3–0.26, N_g:N = 1–0.61), and there was no differentiation among regions (F_ST= 0.00 [0.004], p > 0.05), which implies the HLRs and the southern GBR are strongly interconnected. Our results demonstrate that although the current MPAs appear to capture most of the genetic diversity present within the HLR systems for these 2 species, their sharply contrasting patterns of connectivity indicate some taxa, such as P. damicornis, will be more vulnerable than others, and this disparity will provide challenges for future management.     

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.