Macroalgae exhibit diverse responses to human disturbances on coral reefs

Scientists and managers rely on indicator taxa such as coral and macroalgal cover to evaluate the effects of human disturbance on coral reefs, often assuming a universally positive relationship between local human disturbance and macroalgae. Despite evidence that macroalgae respond to local stressors in diverse ways, there have been few efforts to evaluate relationships between specific macroalgae taxa and local human-driven disturbance. Using genus-level monitoring data from 1205 sites in the Indian and Pacific Oceans, we assess whether macroalgae percent cover correlates with local human disturbance while accounting for factors that could obscure or confound relationships. Assessing macroalgae at genus level revealed that no genera were positively correlated with all human disturbance metrics. Instead, we found relationships between the division or genera of algae and specific human disturbances that were not detectable when pooling taxa into a single functional category, which is common to many analyses. The convention to use percent cover of macroalgae as an indication of local human disturbance therefore likely obscures signatures of local anthropogenic threats to reefs. Our limited understanding of relationships between human disturbance, macroalgae taxa, and their responses to human disturbances impedes the ability to diagnose and respond appropriately to these threats.

Gut microbial communities of hybridising pygmy angelfishes reflect species boundaries

Hybridisation and introgression of eukaryotic genomes can generate new species or subsume existing ones, with direct and indirect consequences for biodiversity. An understudied component of these evolutionary forces is their potentially rapid effect on host gut microbiomes, and whether these pliable microcosms may serve as early biological indicators of speciation. We address this hypothesis in a field study of angelfishes (genus Centropyge), which have one of the highest prevalence of hybridisation within coral reef fish. In our study region of the Eastern Indian Ocean, the parent fish species and their hybrids cohabit and display no differences in their diet, behaviour, and reproduction, often interbreeding in mixed harems. Despite this ecological overlap, we show that microbiomes of the parent species are significantly different from each other in form and function based on total community composition, supporting the division of parents into distinct species, despite the confounding effects of introgression acting to homogenize parent species identity at other molecular markers. The microbiome of hybrid individuals, on the other hand, are not significantly different to each of the parents, instead harbouring an intermediate community composition. These findings suggest that shifts in gut microbiomes may be an early indicator of speciation in hybridising species.

The use of environmental DNA to monitor impacted coastal estuaries

Environmental DNA (eDNA) metabarcoding is increasingly being used to assess community composition in coastal ecosystems. In this study, we chose to examine temporal and spatial changes in the aquatic community of Manly Lagoon - one of the most heavily developed and polluted estuaries in eastern Australia. Based on metabarcoding of the 16S mitochondrial gene (for fish) and the 18S nuclear gene (for macroinvertebrates), we identified seasonal differences in fish and macroinvertebrate community composition as well as species richness, which correlated, in some cases, with the environmental parameters of sea surface temperature and freshwater input. Moreover, given the greater taxonomic resolution of fish versus macroinvertebrate assignments, we identified several known migratory fish species of management importance that contributed significantly to the overall patterns observed. Overall, our data support the use of eDNA metabarcoding to track fish assemblages shifting in response to environmental drivers in polluted estuaries with increased sampling and consultation with historical data.

Regional versus latitudinal variation in the life-history traits and demographic rates of a reef fish, Centropyge bispinosa, in the Coral Sea and Great Barrier Reef Marine Parks, Australia

Environmental temperature is an important determinant of physiological processes and life histories in ectotherms. Over latitudinal scales, variation in temperature has been linked to changes in life-history traits and demographic rates, with growth and mortality rates generally being greatest at low latitudes, and longevity and maximum length being greater at higher latitudes. Using the two-spined angelfish, Centropyge bispinosa, as our focal species, we compared growth patterns, growth rates, longevity, mortality, asymptotic length and maximum length across 22 reefs that span 13° of latitude within the Great Barrier Reef Marine Park (GBRMP) and the Coral Sea Marine Park (CSMP), Australia. We found no predictable latitudinal variation in mortality rates, growth patterns, growth rates, asymptotic or maximum length of C. bispinosa at regional to biogeographic scales. However, C. bispinosa consistently exhibited reduced longevity at lower, warmer latitudes within the CSMP. The greatest differences in mean maximum length of C. bispinosa were between continental (GBRMP) and oceanic (central CSMP) reefs of similar latitude, with individuals being larger on average on continental versus oceanic reefs. The lack of predictable life-history and demographic variation in C. bispinosa across a 13° latitudinal gradient within the CSMP, coupled with differences in mean maximum length between continental and oceanic reefs at similar latitudes, suggest that local environmental conditions have a greater influence than environmental temperature on the demographic rates and life-history traits of C. bispinosa.

Angels in disguise: sympatric hybridization in the marine angelfishes is widespread and occurs between deeply divergent lineages

Hybridization events are not uncommon in marine environments where physical barriers are attenuated. Studies of coral reef taxa have suggested that hybridization predominantly occurs between parapatric species distributed along biogeographic suture zones. By contrast, little is known about the extent of sympatric hybridization on coral reefs, despite the large amount of biogeographic overlap shared by many coral reef species. Here, we investigate if the propensity for hybridization along suture zones represents a general phenomenon among coral reef fishes, by focusing on the marine angelfishes (family Pomacanthidae). Although hybridization has been reported for this family, it has not been thoroughly surveyed, with more recent hybridization studies focusing instead on closely related species from a population genetics perspective. We provide a comprehensive survey of hybridization among the Pomacanthidae, characterize the upper limits of genetic divergences between hybridizing species and investigate the occurrence of sympatric hybridization within this group. We report the occurrence of hybridization involving 42 species (48% of the family) from all but one genus of the Pomacanthidae. Our results indicate that the marine angelfishes are among the groups of coral reef fishes with the highest incidences of hybridization, not only between sympatric species, but also between deeply divergent lineages.

Biofluorescence as a survey tool for cryptic marine species

As ecosystems come under increasing anthropogenic pressure, rare species face the highest risk of extinction. Paradoxically, data necessary to evaluate the conservation status of rare species are often lacking because of the challenges of detecting species with low abundance. One group of fishes subject to this undersampling bias are those with cryptic body patterns. Twenty-one percent of cryptic fish species assessed for their extinction risk (International Union for Conservation of Nature [IUCN]) are data deficient. We developed a nondestructive method for surveying cryptically patterned marine fishes based on the presence of biofluorescence (underwater biofluorescence census, UBC). Blue LED torches were used to investigate how widespread biofluorescence was in cryptic reef fishes in the Coral Triangle region. The effectiveness of UBC to generate abundance data was tested on a data-deficient pygmy seahorse species (Hippocampus bargibanti) and compared with data obtained from standard underwater visual census (UVC) surveys. We recorded 95 reef fish species displaying biofluorescence, 73 of which had not been previously described as biofluorescent. Of those fish with cryptic patterns, 87% were biofluorescent compared with 9% for noncryptic fishes. The probability of species displaying biofluorescence was 70.9 times greater for cryptic species than for noncryptic species. Almost twice the number of H. bargibanti was counted using the UBC compared with UVC. For 2 triplefin species (Ucla xenogrammus, Enneapterygius tutuilae), the abundance detected with UBC was triple that detected with UVC. The UBC method was effective at finding cryptic species that would otherwise be difficult to detect and thus will reduce interobserver variability inherent to UVC surveys. Biofluorescence is ubiquitous in cryptic fishes, making this method applicable across a wide range of species. Data collected using UBC could be used with multiple IUCN criteria to assess the extinction risk of cryptic species. Adopting this technique will enhance researchers' ability to survey cryptic species and facilitate management and conservation of cryptic marine species.

Spatial and temporal patterns of mass bleaching of corals in the Anthropocene

Tropical reef systems are transitioning to a new era in which the interval between recurrent bouts of coral bleaching is too short for a full recovery of mature assemblages. We analyzed bleaching records at 100 globally distributed reef locations from 1980 to 2016. The median return time between pairs of severe bleaching events has diminished steadily since 1980 and is now only 6 years. As global warming has progressed, tropical sea surface temperatures are warmer now during current La Niña conditions than they were during El Niño events three decades ago. Consequently, as we transition to the Anthropocene, coral bleaching is occurring more frequently in all El Niño-Southern Oscillation phases, increasing the likelihood of annual bleaching in the coming decades.

Genomic signatures of local adaptation reveal source-sink dynamics in a high gene flow fish species

Understanding source-sink dynamics is important for conservation management, particularly when climatic events alter species’ distributions. Following a 2011 ‘marine heatwave’ in Western Australia, we observed high recruitment of the endemic fisheries target species Choerodon rubescens, towards the cooler (southern) end of its distribution. Here, we use a genome wide set of 14 559 single-nucleotide polymorphisms (SNPs) to identify the likely source population for this recruitment event. Most loci (76%) showed low genetic divergence across the species’ range, indicating high levels of gene flow and confirming previous findings using neutral microsatellite markers. However, a small proportion of loci showed strong patterns of differentiation and exhibited patterns of population structure consistent with local adaptation. Clustering analyses based on these outlier loci indicated that recruits at the southern end of C. rubescens’ range originated 400 km to the north, at the centre of the species’ range, where average temperatures are up to 3 °C warmer. Survival of these recruits may be low because they carry alleles adapted to an environment different to the one they now reside in, but their survival is key to establishing locally adapted populations at and beyond the range edge as water temperatures increase with climate change.

Naturally occurring hybrids of coral reef butterflyfishes have similar fitness compared to parental species

Hybridisation can produce evolutionary novelty by increasing fitness and adaptive capacity. Heterosis, or hybrid vigour, has been documented in many plant and animal taxa, and is a notable consequence of hybridisation that has been exploited for decades in agriculture and aquaculture. On the contrary, loss of fitness in naturally occurring hybrid taxa has been observed in many cases. This can have negative consequences for the parental species involved (wasted reproductive effort), and has raised concerns for species conservation. This study evaluates the relative fitness of previously documented butterflyfish hybrids of the genus Chaetodon from the Indo-Pacific suture zone at Christmas Island. Histological examination confirmed the reproductive viability of Chaetodon hybrids. Examination of liver lipid content showed that hybrid body condition was not significantly different from parent species body condition. Lastly, size at age data revealed no difference in growth rates and asymptotic length between hybrids and parent species. Based on the traits measured in this study, naturally occurring hybrids of Chaetodon butterflyfishes have similar fitness to their parental species, and are unlikely to supplant parental species under current environmental conditions at the suture zone. However, given sufficient fitness and ongoing genetic exchange between the respective parental species, hybrids are likely to persist within the suture zone.

Cohabitation promotes high diversity of clownfishes in the Coral Triangle

Global marine biodiversity peaks within the Coral Triangle, and understanding how such high diversity is maintained is a central question in marine ecology. We investigated broad-scale patterns in the diversity of clownfishes and their host sea anemones by conducting 981 belt-transects at 20 locations throughout the Indo-Pacific. Of the 1508 clownfishes encountered, 377 fish occurred in interspecific cohabiting groups and cohabitation was almost entirely restricted to the Coral Triangle. Neither the diversity nor density of host anemone or clownfish species alone influenced rates of interspecific cohabitation. Rather cohabitation occurred in areas where the number of clownfish species exceeds the number of host anemone species. In the Coral Triangle, cohabiting individuals were observed to finely partition their host anemone, with the subordinate species inhabiting the periphery. Furthermore, aggression did not increase in interspecific cohabiting groups, instead dominant species were accepting of subordinate species. Various combinations of clownfish species were observed cohabiting (independent of body size, phylogenetic relatedness, evolutionary age, dentition, level of specialization) in a range of anemone species, thereby ensuring that each clownfish species had dominant reproductive individuals in some cohabiting groups. Clownfishes are obligate commensals, thus cohabitation is an important process in maintaining biodiversity in high diversity systems because it supports the persistence of many species when host availability is limiting. Cohabitation is a likely explanation for high species richness in other obligate commensals within the Coral Triangle, and highlights the importance of protecting these habitats in order to conserve unique marine biodiversity.

Habitat Selectivity and Reliance on Live Corals for Indo-Pacific Hawkfishes (Family: Cirrhitidae)

Hawkfishes (family: Cirrhitidae) are small conspicuous reef predators that commonly perch on, or shelter within, the branches of coral colonies. This study examined habitat associations of hawkfishes, and explicitly tested whether hawkfishes associate with specific types of live coral. Live coral use and habitat selectivity of hawkfishes was explored at six locations from Chagos in the central Indian Ocean extending east to Fiji in the Pacific Ocean. A total of 529 hawkfishes from seven species were recorded across all locations with 63% of individuals observed perching on, or sheltering within, live coral colonies. Five species (all except Cirrhitus pinnulatus and Cirrhitichthys oxycephalus) associated with live coral habitats. Cirrhitichthys falco selected for species of Pocillopora while Paracirrhites arcatus and P. forsteri selected for both Pocillopora and Acropora, revealing that these habitats are used disproportionately more than expected based on the local cover of these coral genera. Habitat selection was consistent across geographic locations, and species of Pocillopora were the most frequently used and most consistently selected even though this coral genus never comprised more than 6% of the total coral cover at any of the locations. Across locations, Paracirrhites arcatus and P. forsteri were the most abundant species and variation in their abundance corresponded with local patterns of live coral cover and abundance of Pocilloporid corals, respectively. These findings demonstrate the link between small predatory fishes and live coral habitats adding to the growing body of literature highlighting that live corals (especially erect branching corals) are critically important for sustaining high abundance and diversity of fishes on coral reefs.

New precise dates for the ancient and sacred coral pyramidal tombs of Leluh (Kosrae, Micronesia)

Monumental tombs within ancient civilizations worldwide hold precious clues for deciphering the architectural skill, acumen, and industry of prehistoric cultures. Most tombs were constructed from abiotic materials-stone, soil, and/or clay, predominately-and were built to permanently inter royalty or high-status individuals. On the island of Kosrae in the central Pacific, monumental tombs were constructed with scleractinian coral and were confined to the prehistoric island capital of Leluh, where they served as temporary mortuary processing points. Like other prehistoric tombs, the Leluh tombs were dated by association-from the remnants of the temporarily interred. We present new dates for three sacred tombs using high-precision U-Th dates from 24 corals collected directly from the structural materials. The results suggest that the tombs were built about 700 years ago during the 14th century, about three centuries earlier than previously reported. The new dates redefine the peak occupation of Leluh and place its ruling paramountcy at the leading edge of the developing trans-oceanic political hierarchies, as well as the social and economic systems that dominated the civilizations in this part of the world.

Hybridisation on coral reefs and the conservation of evolutionary novelty

Hybridisation was traditionally considered rare on coral reefs. However, a rapid increase in hybrid studies over the last 20 years has revealed that hybridisation on coral reefs is common and widespread. In this review, we summarise the growing body of evidence arising from studies on stony corals and reef fishes to verify the occurrence of hybridisation, and we examine the influence hybridisation has had on the enormous level of biodiversity present on coral reefs. We discuss the challenges of distinguishing hybridisation from alternative hypotheses (e.g. incomplete lineage sorting). This review also explores the evolutionary consequences of hybridisation, which range from increasing genetic diversity and the production of novel lineages that may outperform the parent species, to reverse speciation and extinction by genetic swamping. Instances of hybridisation can be natural or occur as a result of human impacts (e.g. habitat degradation) and distinguishing between these two very different causal mechanisms is important for management. Currently, the legislative status of hybrids is unclear and hybrids are rarely protected in conservation programs. Failing to adequately manage hybridisation and hybrid lineages may lead to potential losses of evolutionary novelty, declines in phylogenetic diversity or species extinctions. To conserve existing coral reef biodiversity, and the processes that generate biodiversity, conservation policies must be re-defined and instances of hybridisation must be assessed and managed on a case-by-case basis.

Abundance, diversity, and feeding behavior of coral reef butterflyfishes at Lord Howe Island

Endemic species are assumed to have a high risk of extinction because their restricted geographic range is often associated with low abundance and high ecological specialization. This study examines the abundance of Chaetodon butterflyfishes at Lord Howe Island in the south-west Pacific, and compares interspecific differences in local abundance to the feeding behavior and geographic range of these species. Contrary to expected correlations between abundance and geographic range, the single most abundant species of butterflyfish was Chaetodon tricinctus, which is endemic to Lord Howe Island and adjacent reefs; densities of C. tricinctus (14.1 ± 2.1 SE fish per 200m²) were >3 times higher than the next most abundant butterflyfish (Chaetodon melannotus), and even more abundant than many other geographically widespread species. Dietary breadth for the five dominant butterflyfishes at Lord Howe Island was weakly and generally negative correlated with abundance. The endemic C. tricinctus was a distinct outlier in this relationship, though our extensive feeding observations suggest some issues with the measurements of dietary breadth for this species. Field observations revealed that all bites taken on benthic substrates by C. tricinctus were from scleractinian corals, but adults rarely, if ever, took bites from the benthos, suggesting that they may be feeding nocturnally and/or using mid-water prey, such as plankton. Alternatively, the energetic demands of C. tricinctus may be fundamentally different to other coral-feeding butterflyfishes. Neither dietary specialization nor geographic range accounts for interspecific variation in abundance of coral reef butterflyfishes at Lord Howe Island, while much more work on the foraging behavior and population dynamics of C. tricinctus will be required to understand its’ abundance at this location.

Does genetic distance between parental species influence outcomes of hybridization among coral reef butterflyfishes?

Christmas Island is located at the overlap of the Indian and Pacific Ocean marine provinces and is a hot spot for marine hybridization. Here, we evaluate the ecological framework and genetic consequences of hybridization between butterflyfishes Chaetodon guttatissimus and Chaetodon punctatofasciatus. Further, we compare our current findings to those from a previous study of hybridization between Chaetodon trifasciatus and Chaetodon lunulatus. For both species groups, habitat and dietary overlap between parental species facilitate frequent heterospecific encounters. Low abundance of potential mates promotes heterospecific pair formation and the breakdown of assortative mating. Despite similarities in ecological frameworks, the population genetic signatures of hybridization differ between the species groups. Mitochondrial and nuclear data from C. guttatissimus × C. punctatofasciatus (1% divergence at cyt b) show bidirectional maternal contributions and relatively high levels of introgression, both inside and outside the Christmas Island hybrid zone. In contrast, C. trifasciatus × C. lunulatus (5% cyt b divergence) exhibit unidirectional mitochondrial inheritance and almost no introgression. Back-crossing of hybrid C. guttatissimus × C. punctatofasciatus and parental genotypes may eventually confound species-specific signals within the hybrid zone. In contrast, hybrids of C. trifasciatus and C. lunulatus may coexist with and remain genetically distinct from the parents. Our results, and comparisons with hybridization studies in other reef fish families, indicate that genetic distance between hybridizing species may be a factor influencing outcomes of hybridization in reef fish, which is consistent with predictions from terrestrially derived hybridization theory.

Predicting coral species richness: the effect of input variables, diversity and scale

Coral reefs are facing a biodiversity crisis due to increasing human impacts, consequently, one third of reef-building corals have an elevated risk of extinction. Logistic challenges prevent broad-scale species-level monitoring of hard corals; hence it has become critical that effective proxy indicators of species richness are established. This study tests how accurately three potential proxy indicators (generic richness on belt transects, generic richness on point-intercept transects and percent live hard coral cover on point-intercept transects) predict coral species richness at three different locations and two analytical scales. Generic richness (measured on a belt transect) was found to be the most effective predictor variable, with significant positive linear relationships across locations and scales. Percent live hard coral cover consistently performed poorly as an indicator of coral species richness. This study advances the practical framework for optimizing coral reef monitoring programs and empirically demonstrates that generic richness offers an effective way to predict coral species richness with a moderate level of precision. While the accuracy of species richness estimates will decrease in communities dominated by species-rich genera (e.g. Acropora), generic richness provides a useful measure of phylogenetic diversity and incorporating this metric into monitoring programs will increase the likelihood that changes in coral species diversity can be detected.

Taxonomic, spatial and temporal patterns of bleaching in anemones inhabited by anemonefishes

BACKGROUND

Rising sea temperatures are causing significant destruction to coral reef ecosystems due to coral mortality from thermally-induced bleaching (loss of symbiotic algae and/or their photosynthetic pigments). Although bleaching has been intensively studied in corals, little is known about the causes and consequences of bleaching in other tropical symbiotic organisms.

METHODOLOGY/PRINCIPAL FINDINGS

This study used underwater visual surveys to investigate bleaching in the 10 species of anemones that host anemonefishes. Bleaching was confirmed in seven anemone species (with anecdotal reports of bleaching in the other three species) at 10 of 19 survey locations spanning the Indo-Pacific and Red Sea, indicating that anemone bleaching is taxonomically and geographically widespread. In total, bleaching was observed in 490 of the 13,896 surveyed anemones (3.5%); however, this percentage was much higher (19-100%) during five major bleaching events that were associated with periods of elevated water temperatures and coral bleaching. There was considerable spatial variation in anemone bleaching during most of these events, suggesting that certain sites and deeper waters might act as refuges. Susceptibility to bleaching varied between species, and in some species, bleaching caused reductions in size and abundance.

CONCLUSIONS/SIGNIFICANCE

Anemones are long-lived with low natural mortality, which makes them particularly vulnerable to predicted increases in severity and frequency of bleaching events. Population viability will be severely compromised if anemones and their symbionts cannot acclimate or adapt to rising sea temperatures. Anemone bleaching also has negative effects to other species, particularly those that have an obligate relationship with anemones. These effects include reductions in abundance and reproductive output of anemonefishes. Therefore, the future of these iconic and commercially valuable coral reef fishes is inextricably linked to the ability of host anemones to cope with rising sea temperatures associated with climate change.

Dynamic stability of coral reefs on the west Australian coast

Monitoring changes in coral cover and composition through space and time can provide insights to reef health and assist the focus of management and conservation efforts. We used a meta-analytical approach to assess coral cover data across latitudes 10-35°S along the west Australian coast, including 25 years of data from the Ningaloo region. Current estimates of coral cover ranged between 3 and 44% in coral habitats. Coral communities in the northern regions were dominated by corals from the families Acroporidae and Poritidae, which became less common at higher latitudes. At Ningaloo Reef coral cover has remained relatively stable through time (∼28%), although north-eastern and southern areas have experienced significant declines in overall cover. These declines are likely related to periodic disturbances such as cyclones and thermal anomalies, which were particularly noticeable around 1998/1999 and 2010/2011. Linear mixed effects models (LME) suggest latitude explains 10% of the deviance in coral cover through time at Ningaloo. Acroporidae has decreased in abundance relative to other common families at Ningaloo in the south, which might be related to persistence of more thermally and mechanically tolerant families. We identify regions where quantitative time-series data on coral cover and composition are lacking, particularly in north-western Australia. Standardising routine monitoring methods used by management and research agencies at these, and other locations, would allow a more robust assessment of coral condition and a better basis for conservation of coral reefs.

Limited contemporary gene flow and high self-replenishment drives peripheral isolation in an endemic coral reef fish

Extensive ongoing degradation of coral reef habitats worldwide has lead to declines in abundance of coral reef fishes and local extinction of some species. Those most vulnerable are ecological specialists and endemic species. Determining connectivity between locations is vital to understanding recovery and long-term persistence of these species following local extinction. This study explored population connectivity in the ecologically-specialized endemic three-striped butterflyfish (Chaetodon tricinctus) using mt and msatDNA (nuclear microsatellites) to distinguish evolutionary versus contemporary gene flow, estimate self-replenishment and measure genetic diversity among locations at the remote Australian offshore coral reefs of Middleton Reef (MR), Elizabeth Reef (ER), Lord Howe Island (LHI), and Norfolk Island (NI). Mt and msatDNA suggested genetic differentiation of the most peripheral location (NI) from the remaining three locations (MR, ER, LHI). Despite high levels of mtDNA gene flow, there is limited msatDNA gene flow with evidence of high levels of self-replenishment (≥76%) at all four locations. Taken together, this suggests prolonged population recovery times following population declines. The peripheral population (NI) is most vulnerable to local extinction due to its relative isolation, extreme levels of self-replenishment (95%), and low contemporary abundance.

Genetic connectivity among and self-replenishment within island populations of a restricted range subtropical reef fish

Marine protected areas (MPAs) are increasingly being advocated and implemented to protect biodiversity on coral reefs. Networks of appropriately sized and spaced reserves can capture a high proportion of species diversity, with gene flow among reserves presumed to promote long term resilience of populations to spatially variable threats. However, numerically rare small range species distributed among isolated locations appear to be at particular risk of extinction and the likely benefits of MPA networks are uncertain. Here we use mitochondrial and microsatellite data to infer evolutionary and contemporary gene flow among isolated locations as well as levels of self-replenishment within locations of the endemic anemonefish Amphiprion mccullochi, restricted to three MPA offshore reefs in subtropical East Australia. We infer high levels of gene flow and genetic diversity among locations over evolutionary time, but limited contemporary gene flow amongst locations and high levels of self-replenishment (68 to 84%) within locations over contemporary time. While long distance dispersal explained the species' integrity in the past, high levels of self-replenishment suggest locations are predominantly maintained by local replenishment. Should local extinction occur, contemporary rescue effects through large scale connectivity are unlikely. For isolated islands with large numbers of endemic species, and high local replenishment, there is a high premium on local species-specific management actions.

Reef fish hybridization: lessons learnt from butterflyfishes (genus Chaetodon)

Natural hybridization is widespread among coral reef fishes. However, the ecological promoters and evolutionary consequences of reef fish hybridization have not been thoroughly evaluated. Butterflyfishes form a high number of hybrids and represent an appropriate group to investigate hybridization in reef fishes. This study provides a rare test of terrestrially derived hybridization theory in the marine environment by examining hybridization between Chaetodon trifasciatus and C. lunulatus at Christmas Island. Overlapping spatial and dietary ecologies enable heterospecific encounters. Nonassortative mating and local rarity of both parent species appear to permit heterospecific breeding pair formation. Microsatellite loci and mtDNA confirmed the status of hybrids, which displayed the lowest genetic diversity in the sample and used a reduced suite of resources, suggesting decreased adaptability. Maternal contribution to hybridization was unidirectional, and no introgression was detected, suggesting limited, localized evolutionary consequences of hybridization.

Comparisons to other reef fish hybridization studies revealed that different evolutionary consequences emerge, despite being promoted by similar factors, possibly due to the magnitude of genetic distance between hybridizing species. This study highlights the need for further enquiry aimed at evaluating the importance and long-term consequences of reef fish hybridization.

Coral disease in the Indian Ocean: taxonomic susceptibility, spatial distribution and the role of host density on the prevalence of white syndrome

Coral diseases, such as white diseases and white syndrome (WS), have caused widespread damage to coral reefs throughout the Caribbean and are increasing in prevalence on Pacific Ocean reefs. The current study confirms that WS is also present on coral reefs in the Indian Ocean and tests whether patterns in taxonomic susceptibility and spatial variability conform to patterns of WS reported in the Pacific Ocean. Underwater surveys at 19 sites around Christmas and Cocos Islands revealed that WS primarily affects Acropora plate corals (A. clathrata, A. cytherea and A. hyacinthus), and prevalence of WS varied significantly across all 3 spatial scales investigated (island, exposure and depth). Approximately 13.0% (range = 0 to 43% per site) of plate corals at Christmas Island sites exhibited WS compared to <1% at the Cocos Islands. At Christmas Island, WS prevalence was greater in shallow (31.5%) than in deeper water (6.7%) and greatest on the northern (leeward) side of the island (31.5%) compared to the more exposed coastlines (0 to 1.5%). Importantly, the spatial distribution of WS was positively correlated with host density, but not with hard coral cover, suggesting a role of host density in WS outbreaks. Overall the present study has established that WS is impacting remote, near-pristine reefs in the Indian Ocean. However, the highly variable spatial distribution of WS illustrates that patterns in disease prevalence, and the subsequent impact on coral reefs, can be location- or region-specific.

Marine hybrid hotspot at Indo-Pacific biogeographic border

Studying hybridization is crucial to understanding speciation and almost all our knowledge comes from terrestrial and freshwater environments. Marine hybrids are considered rare, particularly on species-rich coral reefs. Here, we report a significant marine hybrid zone at Christmas and Cocos Islands (eastern Indian Ocean) with 11 hybrid coral reef fishes (across six families); the most recorded hybrids of any marine location. In most cases, at least one of the parent species is rare (less than three individuals per 3000 m(2)), suggesting that hybridization has occurred because individuals of the rare species have mated with another species owing to a scarcity of conspecific partners. These islands also represent a marine suture zone where many of the hybrids have arisen through interbreeding between Indian and Pacific Ocean species. For these species, it appears that past climate changes allowed species to diverge in allopatry, while recent conditions have facilitated contact and subsequent hybridization at this Indo-Pacific biogeographic border. The discovery of the Christmas-Cocos hybrid zone refutes the notion that hybridization is lacking on coral reefs and provides a natural laboratory for testing the generality of terrestrially derived hybridization theory in the marine environment.

Tribute to P. L. Lutz: respiratory ecophysiology of coral-reef teleosts

One of the most diverse vertebrate communities is found on tropical coral reefs. Coral-reef fishes are not only remarkable in color and shape, but also in several aspects of physiological performance. Early in life, at the end of the pelagic larval stage, coral-reef fishes are the fastest swimmers of all fishes in relation to body size, and show the highest specific rates of maximum oxygen uptake. Upon settling on the reef, coral-reef fishes have to adopt a demersal lifestyle, which involves coping with a habitat that can become severely hypoxic, and some fishes may even have to rely on air breathing when their coral homes become air exposed. Oxygen availability appears to be a major ambient selection pressure, making respiratory function a key factor for survival on coral reefs. Consequently, hypoxia tolerance is widespread among coral-reef fishes. Hypoxia can even be a factor to gamble with for those fishes that are mouthbrooders, or a factor that the coral inhabitants may actively seek to reduce by sleep-swimming at night. Here, we summarize the present knowledge of the respiratory ecophysiology of coral-reef teleosts. From an ecophysiological perspective, the coral reef is an exciting and largely unexplored system for testing existing hypotheses and making new discoveries.

Aromatase pathway mediates sex change in each direction

The enzyme aromatase controls the androgen/oestrogen ratio by catalysing the irreversible conversion of testosterone into oestradiol (E2). Therefore, the regulation of E2 synthesis by aromatase is thought to be critical in sexual development and differentiation. Here, we demonstrate for the first time that experimental manipulation of E2 levels via the aromatase pathway induces adult sex change in each direction in a hermaphroditic fish that naturally exhibits bidirectional sex change. Our results demonstrate that a single enzymatic pathway can regulate both female and male sexual differentiation, and that aromatase may be the key enzyme that transduces environmental, including social, cues to functional sex differentiation in species with environmental sex determination.

Social induction of maturation and sex determination in a coral reef fish

Labile maturation and sex determination should be advantageous where the probability of finding a mating partner is unpredictable. Here we tested the hypothesis that the presence of a potential mating partner induces maturation and sex determination in a coral-dwelling fish, Gobiodon erythrospilus. In natural populations at Lizard Island (Great Barrier Reef), single individuals were less likely to be mature than paired individuals and they matured at a larger size, indicating plasticity in the timing of maturation. By manipulating group structure we demonstrated that both the timing of maturation and the sex of maturing individuals are socially controlled. Single juveniles did not mature, but maturation was rapidly induced by the presence of an adult partner. In addition, sex determination was found to be labile, with juveniles maturing into the opposite sex of the partner encountered. To our knowledge, this is the first experimental demonstration of social induction of maturation in conjunction with labile sex determination at maturation in vertebrates. This flexibility enables individuals to maximize their reproductive success in an environment where the timing of mate acquisition and the sex of their future partner are unpredictable.