Competition and coexistence of reef-corals

Coral reef mesopredator trophodynamics in response to reef condition

One of the major goals of ecology is to identify metrics of assemblage structure that are easy to obtain and that enable accurate predictions of how assemblages respond to disturbance and environmental change. One recent approach, termed Universal Adaptive Strategy Theory (UAST), has been hypothesised to apply to all creatures on the tree of life. However, previous attempts to classify reef-building corals according to UAST have been inconclusive, perhaps because they have not chosen species traits according to the principles set out in the theory. In addition, the utility of the approach for predicting the response of coral assemblages to disturbance has not been effectively tested. This aims of my thesis was to test whether UAST applies to reef-building corals and whether or not adaptive strategy grouping are useful for predicting the response of taxa to disturbance. In Chapter 2, I first classify coral species into groups using the principles of UAST and a comprehensive database of coral traits. Next, in Chapter 3, I test the ability of adaptive strategy groups to predict the response of coral taxa to disturbance using a long-term dataset from inshore reefs on the Great Barrier Reef (GBR). Finally, in Chapter 4, I test for variation in the relative abundance of adaptive strategy groups in coral assemblages along the 1600 km latitudinal or environmental gradient that is the GBR. I found that UAST does not apply to corals and the analyses suggests only two groups of species rather than the three predicted by the theory. I also found that adaptive strategy groups do not accurately predict how a taxa will respond to disturbance nor do these groups respond in a predictable way along an environmental gradient. In conclusion, it is much more tractable and informative to explore traits directly, rather than looking for groups based on traits.

The functional diversity and redundancy of corals

Ecological interactions affect species evolution and, acting in combination with environmental factors, determine the composition of an ecosystem. In the case of coral reefs, the interactions of species with the corals (Anthozoa) is essential in shaping the ecosystem. Competition is particularly intense in coral reef communities because of the limited availability of space where conditions are appropriate (e.g. depth, substrate, currents) for settlement and growth. Space limitation makes the interaction between corals an essential element determining coral assemblages. Competitive interactions are difficult to analyses due to the number and diversity of factors (e.g. environment, life history, genotype) affecting outcomes. In the case of corals, research on competitive interactions has mostly focused on visible signs of aggression, such as measuring the damaged tissue next to a competitor or reporting visual competitive behaviours (e.g. mesenteric filaments). However, competition (particularly non-contact competition) does not always lead to visible symptoms, which has led in some cases to the underestimation of the extent of competitive interactions. For example, many soft corals (Octocorallia) produce secondary metabolites that may be used to compete for space; the production of secondary metabolites is unlikely to be visually obvious, and their impact on competitors may be subtle or cryptic. The outcomes of competitive interactions between individual corals will also be affected by the health and history of those individuals. For example, individuals that are already immune-compromised are unlikely to be able to compete as efficiently as healthier individuals. The immune system is assumed to be a critical component of competitive mechanisms. Research on coral immunity has focused, with few exceptions, on hard corals (Scleractinia), very little information being available on soft corals immune systems. The lack of basic research on soft corals extends to many aspects of their biology, despite the importance and abundance of these organisms in reef ecosystems. More research on soft corals immunity is important in order to better understand how these organisms respond to environmental factors or competition and to better predict the future composition of coral reefs. In this thesis, I have attempted to advance the knowledge of soft coral biology and non-contact competition between soft and hard corals. I analysed, at a transcriptomic level, the response of the soft coral Lobophytum pauciflorum to challenge with the defined immunogen MDP and the effects on both L. pauciflorum and the hard coral Porites cylindrica (hard corals) when these were in noncontact competition. The response of the soft coral to MDP was variable and unexpectedly dominated by genes likely to have functions in the nervous system. Non-contact competition triggered general stress and immune responses in soft corals, as well as differential expression of genes likely to function in secondary metabolite production and others genes that may be involved in tissue remodelling. The transcriptomic response of the hard coral, Porites, on the other hand, suggested cellular stress combined with resistance and aggressive responses. This research also highlights the role of the coral nervous system and behaviour in the stress response, suggesting that neuro-related pathways are closely linked to the immune system. Similarities between the transcriptomic responses to non-contact competition identified here and previously reported responses to environmental stressors (e.g. ubiquitination, antioxidant production), is consistent with the recruitment of common gene repertoires; therefore climate change is likely to effects competitive interactions in complex ways. Finally, the research presented in this thesis demonstrates the extent of variation in the responses of individual corals to stress (immune challenge and competition) and the challenges that this poses particularly for the investigation of the molecular bases of competition. In the future, individual variation needs to be better accommodated for molecular investigations into coral research, which means increasing biological replication and stopping the practice of discarding outliers.

Manipulation of coral photosymbionts for enhancing resilience to environmental change

Effects of coral-dwelling damselfishes' abundances and diversity on host coral dynamics

Australia exports more coal by sea than any other nation. A series of new mines and port expansions are currently proposed that will lead to an estimated four-fold increase in coal exports through the Great Barrier Reef (GBR) World Heritage Area over the next decade. Increased shipping presents a greater risk of shipping incidents that can potentially release large quantities of coal into the environment. Despite local and international concern related to the shipment of coal through the GBR, there are currently large knowledge gaps pertaining to the risks associated with coal contamination, particularly in tropical marine environments. The overarching objective of this thesis was to quantify the levels at which coal particles become a threat to the health of tropical marine organisms. I placed specific focus on reef-building corals and seagrass because they provide essential ecosystem services. I also investigated juvenile-reef fish as they represent higher level taxa that inhabit coral reefs and seagrass ecosystems. I used an experimental approach to assess the threats posed by both acute and chronic coal particle contamination that is either currently taking place (i.e. fugitive losses from ports), or that could occur in the future (i.e., ship spill scenarios). Additionally, both the physical and chemical effects of coal contamination were considered. The maintenance of coral reef populations is dependent on the success of early life history processes, such as fertilisation, and larval recruitment onto the reef. A coal spill may contaminate the ocean surface through to the seabed, potentially posing a threat to the early life history stages of corals that develop in the water column. In Chapter 2 I established threshold values for short-term coal exposures to coal suspensions and leachate dilutions, for coral gametes, embryos and larvae. Moderate concentrations (≥ 50 mg l-1) of suspended coal particles significantly inhibited fertilisation and reduced embryo survivorship, which could lead to lower larval densities. Low levels of coal deposition (12.5 mg cm-2) onto surfaces significantly reduced larval settlement. While a large coal spill is unlikely to occur during a mass spawning event, this chapter highlighted that even low to moderate coal concentrations can impair coral recruitment. Another potential contamination scenario from a shipping incident involves the chronic release of coal suspensions. The hypothesis that chronic coal contamination will negatively affect key demographic rates (growth and mortality) of a reef-building coral (Acropora tenuis), seagrass (Halodule uninervis) and reef-fish (Acanthochromis polyacanthus) was tested in Chapter 3. The aim was to establish threshold values that elicit lethal and sub-lethal responses. This was investigated by employing a concentration-response experimental design that exposed the organisms to a range of coal concentrations (0 to 275 mg coal l-1) over 14 and 28 d. Low to moderate levels of suspended coal caused extreme light attenuation and coal particles were extremely sticky, adhering to the tissue surfaces of all test organisms. Coral survivorship and seagrass growth rates declined with increasing coal concentrations, and effects were larger after 28 compared with 14 d of exposure. Fish growth rates were similarly depressed in all coal treatment intensities but survivorship was high (98% survivorship). Reduced growth can have serious consequences on reproductive success and survivorship, which could alter population growth in reefs and seagrass meadows subjected to chronic coal contamination. The mechanisms underlying sub-lethal effects of coal suspensions on the reef fish, Acanthochromis polyacanthus, focusing on aerobic respiration and gill morphology, was investigated in Chapter 4. Acute exposure (5 d) to 38 or 73 mg coal l-1 temporarily reduced fish standard oxygen consumption rates (MO2) by 17 %; however, prolonged exposures resulted in significant elevations in MO2, by 30-47% compared with control fish. After 31 d exposure to suspended coal concentrations (0, 38, 73 and 275 mg coal l-1), coal particles had adhered to fish gills, most notably in the highest coal treatments. In response, the fish exposed to 275 mg coal l-1 shed parts of their filament epithelium, which increased their respiratory surface area, thus enhancing their oxygen uptake efficiency. This chapter emphasizes that low-moderate concentrations of chronic coal exposure can elicit energetically costly stress responses (increased metabolism, gill remodelling) in fish. However, changes in gill structure potentially mitigate negative effects of coal exposure and this may have contributed to the low fish mortality measured in Chapter 3. In Chapter 5 I investigated the effects of chronic coal deposition and acute coal suspensions on the physiological performance (photosynthesis, respiration and calcification) of three morphologically distinct coral species, and compared these responses with those elicited by carbonate sediments. Corals were exposed to chronic deposition (2 x 30 mg cm-2 exposures per week for 4 weeks) and acute exposure to a high concentration (1250 mg l-1) of suspended particles. Trace metal and polycyclic aromatic hydrocarbon (PAH) leachate was measured from each particle type. Corals were generally less efficient at clearing coal particles from their tissues compared to sediments, and coal exposure lead to greater reductions in physiological performance than sediment. Elevated dissolved concentrations of certain trace metals (Al, Mn, Co, Ni) in coal suspensions may have contributed to the negative impacts of coal. This chapter emphasizes the differences in the response of corals to coal and the need for management agencies to consider contamination by coal particles independently of- and in addition to sediment exposures. This thesis provides important evidence that tropical marine organisms can be directly and indirectly affected by a range of coal contamination scenarios; including decreased coral fertilisation and settlement, reduced physiological performance and survivorship of adult corals, and decreased seagrass and fish growth rates. The results underscore the importance of adequately assessing the risks posed by increased shipping through ecologically sensitive areas, such as coral reefs and seagrass meadows. The research identified threshold estimates for sub-lethal and lethal effects that can be implemented to improve environmental impact assessments and risk modelling. By addressing critical knowledge gaps that presently impede adequate assessment of the risks posed by coal contamination and spills to coral reef and coastal ecosystems, this thesis contributes to the management of sustainable coal transportation through the Great Barrier Reef Marine Park, and other coastal environments.

Understanding how co-occurring species within comparable trophic guilds (sympatry) partition resources provides fundamental information about their ecological roles within an ecosystem. Despite morphological and biological similarities, resources may be selected and exploited independently, leading to alternative interactions and influences within the ecosystem. Studying movement and dietary patterns directly relates to an animal's resource use, and is a valuable approach to characterise preferred prey and habitat within and between sympatric species. Expanding knowledge of resource use is essential to address how animals are affected by, and how they might respond to, an increasingly variable environment, and is necessary to implement ecosystem-based management practices. Coral trout or coralgrouper (Plectropomus spp.) are iconic and economically significant mesopredatory reef fishes within Australia's Great Barrier Reef (GBR) and throughout the Indo-Pacific region. Despite the importance of Plectropomus spp. in the Queensland Coral Reef Fin Fish Fishery, investigations focussed on their ecology are surprisingly limited. Much of the behavioural-based research has been conducted in scenarios of captivity, is biased by confounding sampling limitations, or only provides short-term, data-poor perspectives. Consequently, interpretations of findings are often applicable only to certain periods or locations, or are only based on patterns from a small number of individuals. This hinders the ability of managers to evaluate how fishing pressure, protection initiatives, and environmental fluctuations or disturbances might impact populations. Furthermore, research is overwhelmingly directed at P. leopardus (or grouped as Plectropomus spp.) which forms the majority of commercial catches on the GBR. Nevertheless, other species such as P. maculatus and P. laevis are readily captured by both recreational and commercial sectors, but their resource selection patterns and interactions with P. leopardus are unknown. The research in this thesis employed two methodological approaches – passive acoustic telemetry and stable isotope analysis, to study movement and dietary patterns, respectively, in three exploited species of coral trout – P. leopardus, P. maculatus, and P. laevis. The research was conducted at three primary locations – Orpheus Island, four mid-shelf/offshore reefs in the Townsville region (Townsville reefs), and the Marine and Aquaculture Research Facilities Unit (MARFU) at James Cook University. Samples and data were collected over the course of three years (2013-2015) providing extensive ecological and behavioural information from more than 300 individual Plectropomus. The overall aim of this research was to quantify, qualify, and compare long-term movement and dietary patterns of sympatric Plectropomus spp. By using multiple approaches, this thesis showed that broad resource selection trends differ between sympatric species, but interestingly, the way they differ is unique to each species pairing. At the Townsville reefs, P. laevis moved greater distances and had increased variability in depth use compared to P. leopardus. Movement patterns were correlated with distinct dietary niches between species, particularly when colour phases of P. laevis (footballer and blue-spot) were separated. The limited isotopic niche overlap between species was not correlated with fish size, indicating alternate prey selection, feeding styles, or energetic requirements engrained at a species level. Based on results from an aquarium-based stable isotope feeding trial, the trophic position of P. leopardus in the wild varied little between sampling locations and time periods. Similarly, the isotopic niches between species remained constant for several tissues (a proxy to feeding timeline) and at several reefs, suggesting feeding pressures exerted by each species is consistent within the region. Consequently, it is hypothesised that both P. laevis and P. leopardus will respond to environmental or humaninduced disturbances in similar ways within and across compatible reefs. At Orpheus Island, P. maculatus shared the same home range size as P. leopardus, however P. maculatus remained deeper in the water column throughout daily and monthly periods. These spatial patterns were correlated to overlapping isotopic niches - or similar prey selection. These trends indicate a high potential for competition that may be mediated by spatial or habitat partitioning. Overall, this research highlights the need for greater species-specific consideration relative to conservation and management initiatives since Plectropomus spp. readily demonstrate distinct behavioural patterns, and will likely respond to disturbances differently. Without fundamental knowledge of how co-occurring species select and partition resources, their interactions and impacts throughout the reef ecosystem remain unknown. Not only did this thesis provide new information about each species, it produced preliminary evidence that interactions between species may shape how resources are utilised on coral reefs.

Decades of research have defined the coral meta-organism as a complex microbial system due to the diversity, abundance and variability of the associated microorganisms. Bacteria are the most studied group of coral-associated prokaryotes, and they are located within the mucus, skeleton, tissues and cellular spaces. Abiotic factors including light irradiance, current, pH, and oxygen generate distinct micro-niches that differ between coral species, depths, reefs, and bioregions. This variety in microhabitats leads to enormous configurations of hundred of thousand bacteria, from tens of thousand phylotypes, associated with each coral species. The variability, diversity and richness of these bacterial communities have undermined the capacity to identify bacterial phylotypes in symbiosis with corals, to describe their functional roles and to establish the characteristics of a healthy bacterial community in corals. Herein, I dissect the variability, diversity and richness of bacterial communities in healthy corals. I aim to (1) define the characteristics of a healthy coral microbiome, (2) evaluate the presence of universal bacterial symbionts in coral-associated bacterial assemblages, and (3) identify factors generating variability in assessments of the coral-associated bacterial communities. In doing so, I developed a conceptual framework that extends on the core microbiome concept, attempting to structure and understand the high diversity observed in coral-associated microbial communities. Initially, I compared sample preservation and preparation methodologies with samples from the corals Goniastrea edwardsi and Isopora palifera collected from Heron Island (Southern Great Barrier Reef, Australia). I showed that preservation in DMSO and 4% paraformaldehyde solution generate comparable composition results to traditional snap freezing in liquid nitrogen for generating 16S microbiome datasets. Furthermore, I showed that homogenization with beat beating is the most reliable, reproducible and practical method for rapid sample preparation. I further evaluated the bacterial communities associated with the coral polyp and coenosarcs from the widely distributed coral Pocillopora damicornis. Although overall bacterial communities appeared similar between microhabitats, differences were evident when comparing diversity, dispersion and core, low-abundance bacteria. These results highlight the importance of considering rare bacteria in the coral microbiome, and the efficiency of core microbiome concept in detecting fine-scale differences. To address variability across broad geographic and ecological scales, I identified and quantified the bacterial community of three depth generalist corals, Pachyseris speciosa, Mycedium elephantotus and Acropora aculeus, at distinct depth intervals (10, 20, 40, and 60-80 m), across a broad latitudinal range in two distinct bioregions (the Great Barrier Reef and the western Coral Sea). I demonstrated that bacterial communities are comparable in richness, diversity and taxonomic structure. In the three coral species, the response of bacterial communities structure is reflective of differences in reef location and bioregion. I further identified ubiquitous bacterial phylotypes (core microbiome) for each species, and determined bacteria consistently associated with both shallow and mesophotic reefs. Coupling the core microbiome framework with an analysis of beta-diversity, taxonomic breadth, taxonomic redundancy and functional prediction on the databases of the three species, I further identified and quantified the variability associated to species, bioregions, reefs and individuals. I demonstrated that bacterial communities in corals show taxonomical, and potentially functional, redundancy in both the resident community and the core microbiome. Based on these results, I propose a conceptual framework defining bacterial communities in healthy corals as three layers: an environmentally responsive community (thousands of phylotypes, transient and variable), an individual microbiome (~500-600 OTUs, variable between reefs but with consistent taxonomy and function) and a core microbiome (few bacteria likely to be in symbiosis showing functional redundancy). This conceptual framework provides structure to the observed high levels of diversity and indicates that bacterial communities in corals are not as complex as previously considered. Given the ongoing degradation of reef environments and the increasing frequency and severity of anthropogenic stressors, future research should be directed towards identifying direct links of microbial contributions to coral resistance and resilience, including an understanding of their individual roles, functional redundancy, and their localization within coral niches.

Key factors influencing the occurrence and frequency of ciguatera

Temperature has a fundamental influence on the physiology, biology and ecology of all organisms, and varies over time and space. Organisms evolved different strategies to cope with this spatial and temporal thermal heterogeneity. For instance, organisms that inhabit thermally variable environments will function over a wider range of temperatures than organisms that live in relatively constant thermal environments. Reef-building corals including their algal symbionts generally live in warm, tropical environments close to their upper thermal maxima, however their performance at varying environmental temperatures remains poorly documented. The overarching aim of my thesis is to determine how temporal and spatial heterogeneity of the thermal environment influences coral and symbiont performance. Through a series of controlled thermal experiments in this thesis I quantify the rate of photosynthesis of reef-building corals and their algal symbionts (termed the holobiont) at various temperatures using coral colonies from different thermal environments and geographic regions. This study is the first to quantify and compare the thermal optima and performance breadth for holobiont and symbiont performance from different thermal environments using thermal performance curves and thereby providing new insights into the mechanisms underlying thermal acclimation. Acclimation to environmental change takes time and does not necessarily result in full compensation of an organism's performance. In Chapter 2 I identified the acclimation trajectory of massive Porites spp. for a set of host and symbiont physiological traits during exposure to heat (31 °C) and cold (21 °C) for 30 days. Cold acclimation took approximately two weeks and resulted in 'no' or 'inverse' compensation of the performance. In contrast, I found no evidence of heat acclimation holobiont and symbiont performance declined continuously instead of reaching a steady state. These results show that there is no rapid compensatory acclimation response when massive Porites spp. are exposed to a change in the thermal environment, and that compensation of the performance is unlikely to occur in response to short-term variations in temperature. I then investigated the between-season variation in performance of two coral species with contrasting life-history strategies (Chapter 3). Acclimation to seasonal variation was species-specific, with an increase of the thermal optimum in summer for a fast-growing and thermally sensitive species (A. valenciennesi) and a change of the thermal breadth for a slow-growing and thermally tolerant species (Porites cylindrica). Additionally, the symbiont performance was less plastic than the holobiont performance indicating that the reversible acclimation mostly occurs through the coral host. Comparisons of thermal performance of coral species living in different thermal environments along a latitudinal gradient in the Great Barrier Reef (Chapter 4) demonstrated significant geographic variation in the thermal performance among populations. Acclimation of the thermal optimum to the local environment was more accurate for the symbiont performance than for the holobiont. In general, the thermal optimum for holobiont performance was ~4 – 6 °C below the environmental temperature, which may result from an inherent time lag in the mechanisms of acclimation, or from constraints imposed during early ontogeny (i.e., developmental acclimation). In Chapter 5 I assessed whether the thermal performance of temperate corals is less sensitive to changes in temperature than that of tropical corals due to their history of exposure to more variable thermal environments. To do this I compared the thermal performance of corals sampled along the GBR latitudinal gradient, with the thermal performance of corals from the Mediterranean Sea. Interestingly, despite clear differences in thermal optima, no observable differences occurred between the performance breadths of temperate versus tropical corals at either the holobiont or symbiont level. This result is likely because all of the sampled coral species had a wide thermal tolerance, which fully encompassed the total local annual variation in temperature in each location. Overall, the results of this thesis demonstrate that reef-building corals may be more generalist than previously thought. However, a high degree of inter-colony variability in thermal performance was consistently observed for all of the sampled coral species, even between colonies from the same local population. These findings indicate that despite the mean thermal optima being consistently below the average environmental temperatures for all populations, some individual colonies maintain the capacity to perform well at very high and very low temperatures, which suggest that corals may cope with environmental variability through genetic variation rather than reversible plasticity. Hopefully, such high among-colony variation can contribute to the capacity of coral populations to persist in the face of rapid climate change.

Previous articleNext article No AccessLetters to the EditorsReversal of Dominance in Competing Species of DrosophilaFrancisco J. AyalaFrancisco J. Ayala Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 100, Number 910Jan. - Feb., 1966 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/282402 Views: 6Total views on this site Citations: 30Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Cyrill Hess, Jonathan M. Levine, Martin M. Turcotte, Simon P. 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Environmental influences on the epidemiology of fibropapillomatosis in green turtles (Chelonia mydas) and consequences for management of inshore areas of the Great Barrier Reef

Macroalgae are an important and diverse component of tropical inshore reefs, providing a range of ecological and functional services. Most of the research emphasis has focused on the negative effects of increased macroalgae on coral reefs to the growth and persistence of corals and fishes. However, on undisturbed reefs, macroalgal beds provide important habitats for newly settled fishes, many of which have ontogenetic changes to coral reefs and replenish adult populations that help control macroalgal growth. The few studies that have investigated macroalgal associations of juvenile fishes have treated macroalgae as a single entity. In doing so, any associations of fishes with taxonomic or functional groups cannot be discerned. The objective of this thesis was to undertake observational and experimental studies to investigate the patterns and mechanisms of habitat selection and preferences in algal-associated juvenile fishes on coastal reefs. In Chapter 2, I investigated the availability, use and selectivity of microhabitats on the reef flat of an inshore reef by recently settled coral reef fishes. Fish and benthic communities were quantified along three 50m transects at each of six reef flat sites along the leeward side of Orpheus Island, an inshore island on the central Great Barrier Reef (GBR). Benthic communities were quantified using the point-intercept method, recording the benthos directly underneath the transect every 0.5m. Recently settled fish were surveyed within a 2m belt along each transect. All recently settled fishes were identified to species, and the microhabitat they were associated with was recorded. A total of 22 species of recently settled fish were recorded across all transects, with the majority of the species being herbivorous as adults. Examination of the microhabitat preferences of the six most abundant species revealed that the two herbivorous rabbitfish species, Siganus lineatus and S. canaliculatus, appeared to associate with large fleshy macroalgae (namely Padina and Sargassum). In contrast, three herbivorous damselfishes, Pomacentrus wardi, P. adelus and P. chrysurus, and the invertebrate feeding cardinalfish Apogon cookii showed no apparent selection for any of the microhabitats examined. These findings suggest that macroalgae are important habitats for some newly settled coral reef fishes, and the availability and composition of macroalgae may shape juvenile populations. Both habitat selection at settlement and/or post-settlement processes may contribute to the establishment of these patterns. The objective of Chapter 3 was to investigate the specific habitat and olfactory preferences of two rabbitfishes (S. lineatus and S. canaliculatus) and two herbivorous damselfishes (P. wardi and P. adelus) at settlement. Naive settlement-stage fishes were collected in light traps and used in a series of aquarium-based settlement choice experiments in which an individual larva was introduced into the centre of a 500L aquarium and allowed to choose among four habitat options. The fishes were released overnight and their associations with the four habitats recorded the following morning. Four separate habitat-choice experiments were conducted, each with four different substratum options: (1) benthic habitats (live coral, dead coral, macroalgae, coral rubble), (2) macroalgal species (Sargassum, Padina, Galaxaura, coral rubble), (3) macroalgal density (8, 4, 1, 0 Sargassum thalli) and (4) macroalgal height (30, 20, 10, 0cm Sargassum). In addition, the role of olfaction was investigated using a 2 channel-choice flume chamber, pairing a benthic substrate water cue to the off-reef control cue (1 km from the nearest reef). The aquarium trials showed that both rabbitfish species preferred macroalgae in experiment 1 and the highest density Sargassum patch in experiment 3. The two rabbitfishes also differed in their habitat preferences as only S. canaliculatus preferred Sargassum in experiment 2 and S. lineatus preferred the tallest Sargassum patch in experiment 4. In contrast, neither damselfish species displayed any preferences among the habitats present. The olfactory trials revealed significant but weak attractions to various chemical cues from benthic microhabitats, however, these varied among species and are unlikely to be biologically significant. It is apparent that juvenile rabbitfishes have strong innate preferences for macroalgae at settlement, however, species-specific preferences for macroalgal species and habitat features may affect the distribution of juveniles on the reef. In summary, macroalgae appear to provide important settlement and juvenile habitats for at least the two rabbitfishes species examined herein, and given the prevalence of juvenile fishes in this habitat, it is likely that such preferences may be more widespread. It is unclear why these early life stages are using macroalgae, but they may be associated with the provision of refugia from predation, or enhanced food supply as these fishes transition from a carnivorous to herbivorous diet. Interestingly, many studies have focused on the role of herbivorous rabbitfishes in removing macroalgae from coral reefs, yet the results of this thesis highlight that these same fishes are dependent on macroalgae at a critical life stage. Clearly, further investigations are required to elucidate the importance of macroalgae to reef fishes at settlement, and potentially other life stages.

The increasing evidence of coexistence of cryptic species with no recognized niche differentiation has called attention to mechanisms reducing competition that are not based on niche-differentiation. Only sex-based mechanisms have been shown to create the negative feedback needed for stable coexistence of competitors with completely overlapping niches. Here we show that density-dependent sexual and diapause investment can mediate coexistence of facultative sexual species having identical niches. We modelled the dynamics of two competing cyclical parthenogens with species-specific density-dependent sexual and diapause investment and either equal or different competitive abilities. We show that investment in sexual reproduction creates an opportunity for other species to invade and become established. This may happen even if the invading species is an inferior competitor. Our results suggests a previously unnoticed mechanism for species coexistence and can be extended to other facultative sexual species and species investing in diapause where similar density-dependent life-history switches could act to promote coexistence.