Water flow controls distribution and feeding behavior of two co-occurring coral reef fishes: I. Field measurements

The interaction of flowing water with reef topography creates a continuum of flow microhabitats that can alter species distributions directly via transport of organisms or propagules, or indirectly by modulating the availability of critical resources. To examine how water flow affects the distribution and feeding performance of two species of planktivorous tube blennies (Chaenopsidae), flow speed and turbulence were measured within the feeding areas of Acanthemblemaria spinosa and A. aspera at three sites within Glover’s Reef, Belize. Although co-occurring, A. spinosa occupies topographically high locations (e.g., upright coral skeletons) while A. aspera occupies topographically low shelters in the coral pavement. Boundary layer theory predicts that A. spinosa should experience higher flow (and a higher flux of planktonic food) relative to A. aspera; however, complex topography and oscillatory flow require that this prediction is tested directly in the field. Within each site, the flow experienced by A. spinosa was, indeed, faster and more turbulent than that experienced by A. aspera at site-specific intermediate wave heights. When waves were small, gentle velocity gradients produced similar flows for the two species. When waves were high, flow was uniformly fast through the water column due to thinning of the benthic boundary layer. Plankton availability was similar for the species, with the exception of a greater abundance of harpacticoid copepods at the shelters of A. aspera. Quantitative behavioral observations suggest that the foraging strategies employed by the two fishes exploit the prevailing hydrodynamic conditions. For example, A. spinosa, the stronger swimmer of the two, attacks nearly 100% of the time in the water column where it can exploit the higher flux of plankton associated with faster flows, while A. aspera attacks primarily toward the reef surface where currents are likely to be slower and it can exploit more abundant benthic prey. Communicated by Biology Editor Dr. Mark McCormick     

Avoiding the flow: refuges expand the swimming potential of coral reef fishes

While many coral reef fishes utilise substratum refuges, the direct influence of water flow and swimming ability on such refuging patterns is yet to be established. This study examined the swimming ability and refuging behaviour of a labrid (Halichoeres margaritaceus) and a pomacentrid (Pomacentrus chrysurus) that inhabit high flow, wave-swept coral reef flats. Field observations of refuging patterns were combined with experimental evaluations in a flow tank using a replica of a substratum hole frequently used by these species. Under a range of flow speeds commonly found on the reef flat (0–60 cm s⁻¹), flow within the substratum refuge was reduced to speeds of 0–12 cm s⁻¹, representing a 75–100% flow reduction. Swimming ability of each species was then tested at 60 cm s⁻¹ with and without access to this flow refuge. Both species were able to maintain activity within the 60 cm s⁻¹ flow for considerably longer when provided with a refuge, with increases from approximately 39 min to 36 h for H. margaritaceus and 8 min to 88 h for P. chrysurus. Despite H. margaritaceus having the strongest swimming ability without access to a refuge, P. chrysurus was able to maintain swimming activity more than twice as long as H. margaritaceus when provided with a refuge. These increases in activity are probably due to energetic savings, with this type of refuge providing an estimated 95% energy saving over swimming directly into a unidirectional flow of 60 cm s⁻¹. These results highlight the major advantages provided by refuging behaviour and emphasise the importance of habitat refuges in shaping patterns of habitat use in reef fishes.     

Phylogeography and the conservation of coral reef fishes

Here we present a review of how the study of the geographic distribution of genetic lineages (phylogeography) has helped identify management units, evolutionary significant units, cryptic species, and areas of endemism, and how this information can help efforts to achieve effective conservation of coral reefs. These studies have confirmed the major biogeographic barriers that were originally identified by tropical species distributions. Ancient separations, identified primarily with mtDNA sequence comparisons, became apparent between populations on each side of the barriers. The general lack of correlation between pelagic larval duration and genetic connectivity across barriers indicates that life history and ecology can be as influential as oceanography and geography in shaping evolutionary partitions within ocean basins. Hence, conservation strategies require a recognition of ecological hotspots, those areas where habitat heterogeneity promotes speciation, in addition to more traditional approaches based on biogeography. Finally, the emerging field of genomics will add a new dimension to phylogeography, allowing the study of genes that are pertinent to recent and ongoing differentiation, and ultimately providing higher resolution to detect evolutionary significant units that have diverged in an ecological time scale.     

Composition and sources of near reef zooplankton on a Jamaican forereef along with implications for coral feeding

Nocturnal near-reef zooplankton from the forereef of Discovery Bay, Jamaica, were sampled during winter and summer 1994 using a diver-operated plankton pump with an intake head positioned within centimeters of benthic zooplanktivores. The pump collected zooplankton not effectively sampled by conventional net tows or demersal traps. We found consistently greater densities of zooplankton than did earlier studies that used other sampling methods in similar locations. There was no significant difference between winter (3491±578 m^–3) and summer (2853±293 m^–3) zooplankton densities. Both oceanic- and reef-associated forms were found at temporal and spatial scales relevant to benthic suspension feeders. Copepods were always the most abundant group, averaging 89% of the total zooplankton, and most were not of demersal origin. The cyclopoids, Oithona spp., were the numerically dominant organisms, with an average density of 1684±260 m^–3. Other zooplankton (e.g., shrimp larvae, crab larvae, polychaetes, chaetognaths, amphipods, and isopods) were highly variable and much less abundant. Near-reef zooplankton abundances were high throughout the night sampling period, not just after sunset and before sunrise as previously described. Mean biomass was 4.5 mg C m^–3, with values ranging from 1.0 to 15.6 mg C m^–3. This work has important implications for evaluating which zooplankton types are available to benthic suspension feeders, including corals.     

Assessing the magnitude of intra- and interspecific competition in two coral reef fishes

Spatial associations of seven herbivore species in the Kruger National Park, South Africa, are analyzed using a new technique, Correlative Coherence Analysis (CoCA). CoCA is a generalization of the concept of correlation to more than two sequences of numbers. Prior information on the feeding ecology and metabolic requirements of these species is used to contrast spatial scales at which hypothesized guild aggregation or competition occurs. These hypotheses are tested using 13 years of aerial census data collected during the dry season. Our results are consistent with the hypothesis that distributions of large and small species of the same feeding type (i.e., grazers and browsers) overlap in potentially resource-rich areas, but have lower similarity values across all areas because the higher tolerance of large species for low quality foods results in a more even spatial distribution of large species compared to small species.     

Stochastic determinants of assemblage patterns in coral reef fishes: a quantification by means of two models

Synopsis One perspective emphasizing the importance of stochastic processes in determining coral reef fish assemblages implies that there is little organization in species richness, abundance structure, and spatial distribution. We examine the degree to which this perspective is correct by analyzing distribution of fishes on a collection of patch reefs (Discovery Bay, Jamaica). We ask the question whether these patches accumulate species and individuals in a manner consistent with stochastic expectations. To address this question we use two conceptual models, each permitting a different insight. One assumes that fish are distributed stochastically on patches while the other assumes presence of restrictions on fish distribution due to habitat structure. For each conceptual model we use two types of benchmark: we compare observed patterns to those predicted by theoretical models, and we also compare observed patterns to those obtained from a random reallocation of fish individuals to patches. We found that the conceptual model assuming stochastic processes appeared to provide weaker explanation of patterns than the conceptual model that includes restrictions due to habitat structure. Further, and more importantly, we found that (i) the community is shaped by a mixture of stochastic and non-stochastic mechanisms, and (ii) the stochastic assembly processes decrease in importance for species restricted to fewer microhabitat types and sites. Our study therefore indicates that patches do accumulate individuals and species in a manner consistent with stochastic expectations, however, this applies primarily to the habitat generalists (unrestricted species). By the same token, increased habitat specialization by some species imposes constraints on the stochastic model such that it eventually fails.     

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.     

Behavioral thermoregulatory abilities of tropical coral reef fishes: a comparison with temperate freshwater and marine fishes

Synopsis Eight species in six different families of tropical marine reef fishes from the Indo-West Pacific region (Naso lituratus, Zebrasoma.flavescens, Balistes fuscus, B. vidua, Forcipiger longirostris, Echidna zebra, Cromileptes altivelis, Canthigaster jactator) were tested for ability to thermoregulate behaviorally in electronic shuttleboxes. All of these species preferred mean temperatures between 20 and 30°C, but differed considerably in thermoregulatory precision. All species avoided lethal high or low temperatures (i.e., they did not die during the tests), and some species thermoregulated as precisely as temperate species. Some temperate species prefer higher temperatures (above 30°C) than do these tropical reef species.     

Comparison of remote video and diver's direct observations to quantify reef fishes feeding on benthos in coral and rocky reefs

This study compared remote underwater video and traditional direct diver observations to assess reef fish feeding impact on benthos across multiple functional groups within different trophic categories (e.g. herbivores, zoobenthivores and omnivores) and in two distinct reef systems: a subtropical rocky reef and a tropical coral reef. The two techniques were roughly equivalent, both detecting the species with higher feeding impact and recording similar bite rates, suggesting that reef fish feeding behaviour at the study areas are not strongly affected by the diver's presence.     

Introgression and selection shaped the evolutionary history of sympatric sister‐species of coral reef fishes (genus: Haemulon)

Closely related marine species with large overlapping ranges provide opportunities to study mechanisms of speciation, particularly when there is evidence of gene flow between such lineages. Here, we focus on a case of hybridization between the sympatric sister‐species Haemulon maculicauda and H. flaviguttatum, using Sanger sequencing of mitochondrial and nuclear loci, as well as 2422 single nucleotide polymorphisms (SNPs) obtained via restriction site‐associated DNA sequencing (RADSeq). Mitochondrial markers revealed a shared haplotype for COI and low divergence for CytB and CR between the sister‐species. On the other hand, complete lineage sorting was observed at the nuclear loci and most of the SNPs. Under neutral expectations, the smaller effective population size of mtDNA should lead to fixation of mutations faster than nDNA. Thus, these results suggest that hybridization in the recent past (0.174–0.263 Ma) led to introgression of the mtDNA, with little effect on the nuclear genome. Analyses of the SNP data revealed 28 loci potentially under divergent selection between the two species. The combination of mtDNA introgression and limited nuclear DNA introgression provides a mechanism for the evolution of independent lineages despite recurrent hybridization events. This study adds to the growing body of research that exemplifies how genetic divergence can be maintained in the presence of gene flow between closely related species.     

Effects of ultraviolet radiation and water motion on the reef coral, Porites compressa Dana: a transplantation experiment

Scleractinian corals have adapted to live in habitats were the level of ultraviolet radiation (UVR, 280–400 nm) is extremely high. The putative photoprotective molecules called mycosporine-like amino acids (MAAs) contained in the corals' tissues absorb UVR and release it harmlessly as heat. MAA concentration in corals is quite plastic and correlates well with UVR dose, but other ecological factors such as water motion may influence MAA production as well. In this study, the effects of ambient UVR and water motion on MAA concentration and several physiological parameters of the reef coral Porites compressa Dana were investigated in a two by two factorial transplantation experiment. Replicate branches from nine morphologically distinct colonies were transplanted from the windward side of Coconut Island (Kaneohe Bay, HI) to a control area on the windward side (ambient water motion) and to an area on the leeward side (low water motion). The transplanted corals were placed under UV-opaque (UVO) or UV-transparent (UVT) filters fixed to the reef. Initially and at 3 and 6 weeks, coral branches were weighed to determine calcification rate and tissues were extracted in methanol for photosynthetic pigment and MAA analysis via high performance liquid chromatography (HPLC). UVR was a significant factor determining MAA concentration. When UVR was screened from the corals' environment, total MAA concentration decreased by 33% over 6 weeks. However, UVR-exposed corals moved to low water motion also decreased MAA levels, while UVR-exposed corals moved to the control area retained initial levels. Photosynthetic pigments and calcification rate were also significantly reduced in corals moved to low water motion. There was no UVR effect on photosynthetic pigments or calcification rate. This study provides evidence that water motion is important for the maintenance of MAAs. However, there were interesting colony-specific patterns in MAA composition and response to the UVR treatment; some colonies had high total concentrations of MAAs in all treatments, while others displayed a pronounced UVR effect. Also, each genotype seemed to have its own signature MAA composition. These findings indicate a genetic (host, zooxanthellae or both) component to UVR resistance in this population of P. compressa.     

Comparative phylogeography in a genus of coral reef fishes: biogeographic and genetic concordance in the Caribbean

Geographic barriers that limit the movement of individuals between populations may create or maintain phylogenetically discrete lineages. Such barriers are often inferred from geographic surveys of a single mitochondrial marker to identify phylogenetic splits. Mitochondrial DNA, however, has an effective population size one-fourth that of nuclear DNA, which can facilitate the rapid evolution of monophyletic mtDNA lineages in the absence of geographic barriers. The identification of geographic barriers will thus be more robust if barriers are proposed a priori, and tested with multiple independent genetic markers in multiple species. Here, we tested two proposed marine biogeographic breaks located at the Mona Passage in the Caribbean Sea and at the southern end of Exuma Sound in the Bahamas. We sequenced mitochondrial cytochrome b (400 bp) and nuclear rag1 (573 bp) for nine species and colour forms (183 individuals total) within the teleost genus Elacatinus (Gobiidae) that span the proposed breaks. Our results showed that Mona Passage separated mtcyb and rag1 lineages, with no genetic exchange between populations separated by just 23 km. However, the Central Bahamas barrier was only weakly supported by our data. Importantly, neither barrier coincided with deep genetic splits. This suggests that these two barriers did not initially isolate regional populations, but instead disrupt ongoing gene flow between regions. Our inferred relationships further suggested a division of the Caribbean region into northwestern and southeastern regions, a pattern reflected by some freshwater and terrestrial vertebrates. Our results, coupled with genetic and demographic data from other reef fishes and corals, provide robust support for the Mona Passage as a long-term biogeographic barrier for Caribbean animals.     

Depth distributions of coral reef fishes: the influence of microhabitat structure,settlement, and post-settlement processes

Many coral reef fishes have restricted depth ranges that are established at settlement or soon after, but the factors limiting these distributions are largely unknown. This study examines whether the availability of microhabitats (reef substrata) explains depth limits, and evaluates whether juvenile growth and survival are lower beyond these limits. Depth-stratified surveys of reef fishes at Kimbe Bay (Papua New Guinea) showed that the abundance of new settlers and the cover of coral substrata differed significantly among depths. A field experiment investigated whether settling coral reef fishes preferred particular depths, and whether these depth preferences were dependent on microhabitat. Small patch reefs composed of identical coral substrata were set up at five depths (3, 6, 10, 15 and 20 m), and settlement patterns were compared to those on unmanipulated reef habitat at the same five depths. For all species, settlement on patch reefs differed significantly among depths despite uniform substratum composition. For four of the six species tested, depth-related settlement patterns on unmanipulated habitat and on patch reefs did not differ, while for the other two, depth ranges were greater on the patch reefs than on unmanipulated habitat. A second experiment examined whether depth preferences reflected variation in growth and survival when microhabitat was similar. Newly settled individuals of Chrysiptera parasema and Dascyllus melanurus were placed, separately, on patch reefs at five depths (as above) and their survival and growth monitored. D. melanurus, which is restricted to shallow depths, had highest survival and growth at the shallowest depth. Depth did not affect either survival or growth of C. parasema, which has a broader depth range than D. melanurus (between 6 and 15 m). This suggests that the fitness costs potentially incurred by settling outside a preferred depth range may depend on the strength of the depth preference.     

Jaw muscle fiber type distribution in Hawaiian gobioid stream fishes: histochemical correlations with feeding ecology and behavior

Differences in fiber type distribution in the axial muscles of Hawaiian gobioid stream fishes have previously been linked to differences in locomotor performance, behavior, and diet across species. Using ATPase assays, we examined fiber types of the jaw opening sternohyoideus muscle across five species, as well as fiber types of three jaw closing muscles (adductor mandibulae A1, A2, and A3). The jaw muscles of some species of Hawaiian stream gobies contained substantial red fiber components. Some jaw muscles always had greater proportions of white muscle fibers than other jaw muscles, independent of species. In addition, comparing across species, the dietary generalists (Awaous guamensis and Stenogobius hawaiiensis) had a lower proportion of white muscle fibers in all jaw muscles than the dietary specialists (Lentipes concolor, Sicyopterus stimpsoni, and Eleotris sandwicensis). Among Hawaiian stream gobies, generalist diets may favor a wider range of muscle performance, provided by a mix of white and red muscle fibers, than is typical of dietary specialists, which may have a higher proportion of fast-twitch white fibers in jaw muscles to help meet the demands of rapid predatory strikes or feeding in fast-flowing habitats.     

Trematodes of the Great Barrier Reef,Australia: emerging patterns of diversity and richness in coral reef fishes

The Great Barrier Reef holds the richest array of marine life found anywhere in Australia, including a diverse and fascinating parasite fauna. Members of one group, the trematodes, occur as sexually mature adult worms in almost all Great Barrier Reef bony fish species. Although the first reports of these parasites were made 100 years ago, the fauna has been studied systematically for only the last 25 years. When the fauna was last reviewed in 1994 there were 94 species known from the Great Barrier Reef and it was predicted that there might be 2,270 in total. There are now 326 species reported for the region, suggesting that we are in a much improved position to make an accurate prediction of true trematode richness. Here we review the current state of knowledge of the fauna and the ways in which our understanding of this fascinating group is changing. Our best estimate of the true richness is now a range, 1,100–1,800 species. However there remains considerable scope for even these figures to be incorrect given that fewer than one-third of the fish species of the region have been examined for trematodes. Our goal is a comprehensive characterisation of this fauna, and we outline what work needs to be done to achieve this and discuss whether this goal is practically achievable or philosophically justifiable.     

Evolutionary history of the butterflyfishes (f: Chaetodontidae) and the rise of coral feeding fishes

Of the 5000 fish species on coral reefs, corals dominate the diet of just 41 species. Most (61%) belong to a single family, the butterflyfishes (Chaetodontidae). We examine the evolutionary origins of chaetodontid corallivory using a new molecular phylogeny incorporating all 11 genera. A 1759‐bp sequence of nuclear (S7I1 and ETS2) and mitochondrial (cytochrome b) data yielded a fully resolved tree with strong support for all major nodes. A chronogram, constructed using Bayesian inference with multiple parametric priors, and recent ecological data reveal that corallivory has arisen at least five times over a period of 12 Ma, from 15.7 to 3 Ma. A move onto coral reefs in the Miocene foreshadowed rapid cladogenesis within Chaetodon and the origins of corallivory, coinciding with a global reorganization of coral reefs and the expansion of fast‐growing corals. This historical association underpins the sensitivity of specific butterflyfish clades to global coral decline.