Effects of territorial damselfish on an algal-dominated coastal coral reef

Territorial damselfish are important herbivores on coral reefs because they can occupy a large proportion of the substratum and modify the benthic community to promote the cover of food algae. However, on coastal coral reefs damselfish occupy habitats that are often dominated by unpalatable macroalgae. The aim of this study was to examine whether damselfish can maintain distinctive algal assemblages on a coastal reef that is seasonally dominated by Sargassum (Magnetic Island, Great Barrier Reef). Here, three abundant species (Pomacentrus tripunctatus, P. wardi and Stegastes apicalis) occupied up to 60% of the reef substrata. All three species promoted the abundance of food algae in their territories. The magnitudes of the effects varied among reef zones, but patterns were relatively stable over time. Damselfish appear to readily co-exist with large unpalatable macroalgae as they can use it as a substratum for promoting the growth of palatable epiphytes. Damselfish territories represent patches of increased epiphyte load on macroalgae, decreased sediment cover, and enhanced cover of palatable algal turf.     

Interactions between herbivorous fish guilds and their influence on algal succession on a coastal coral reef

Herbivory is an important mechanism affecting algal succession, particularly on coral reefs where the relationship between algae and corals is largely controlled by herbivores. However, different functional groups of herbivores may have contrasting effects on succession, which may explain different trajectories of coral reef recovery after disturbance. Here, the effects of different herbivore groups (roving herbivores = foragers and territorial damselfish = farmers) were isolated by a multi-factorial experiment carried out on a coastal coral reef with high macroalgal cover, high farmer densities and relatively low forager abundance. The effects of foragers and farmers were distinguished by monitoring algal succession on settlement tiles placed inside and outside exclusion cages, with orthogonal treatments established inside and outside damselfish territories (with appropriate cage controls). Within 12 months, algal assemblages on ungrazed tiles inside exclusion cages proceeded rapidly from fine filamentous turfs, to corticated algae, to tough erect (e.g. Amphiroa spp.) and foliose (e.g. Peyssonnellidae) calcified algae. Farmers had a dramatic impact on succession, essentially arresting the development of the algal community at a point where it was dominated by palatable filamentous algae of the genus Polysiphonia. Fleshy macroalgae such as Sargassum spp. were excluded from farmer territories. In contrast, foragers did not suppress fleshy macroalgae, but rather, appeared to decelerate succession and promote a relatively diverse assemblage. In contrast to forager-dominated reefs, farmer territories did not appear to function solely as forager exclusion areas or promote algal diversity as a result of intermediate grazing pressure. The relatively strong effects of farmers observed here may represent a future scenario for coral reefs that are increasingly subject to overfishing of large grazing fishes.     

Caribbean reefs of the Anthropocene: Variance in ecosystem metrics indicates bright spots on coral depauperate reefs

Dramatic coral loss has significantly altered many Caribbean reefs, with potentially important consequences for the ecological functions and ecosystem services provided by reef systems. Many studies examine coral loss and its causes—and often presume a universal decline of ecosystem services with coral loss—rather than evaluating the range of possible outcomes for a diversity of ecosystem functions and services at reefs varying in coral cover. We evaluate 10 key ecosystem metrics, relating to a variety of different reef ecosystem functions and services, on 328 Caribbean reefs varying in coral cover. We focus on the range and variability of these metrics rather than on mean responses. In contrast to a prevailing paradigm, we document high variability for a variety of metrics, and for many the range of outcomes is not related to coral cover. We find numerous “bright spots,” where herbivorous fish biomass, density of large fishes, fishery value, and/or fish species richness are high, despite low coral cover. Although it remains critical to protect and restore corals, understanding variability in ecosystem metrics among low‐coral reefs can facilitate the maintenance of reefs with sustained functions and services as we work to restore degraded systems. This framework can be applied to other ecosystems in the Anthropocene to better understand variance in ecosystem service outcomes and identify where and why bright spots exist.     

Effects of monsoon-driven wave action on coral reefs of Guam and implications for coral recruitment

Benthic cyanobacteria can respond rapidly to favorable environmental conditions, overgrow a variety of reef organisms, and dominate benthic marine communities; however, little is known about the dynamics and consequences of such cyanobacterial blooms in coral reef ecosystems. In this study, the benthic community was quantified at the time of coral spawnings in Guam to assess the substrate that coral larvae would encounter when attempting settlement. Transects at 9, 18, and 25-m depths were surveyed at two reef sites before and after heavy wave action driven by westerly monsoon winds. Communities differed significantly between sites and depths, but major changes in benthic community structure were associated with wave action driven by monsoon winds. A shift from cyanobacteria to crustose coralline algae (CCA) accounted for 44% of this change. Coral recruitment on Guam may be limited by substrate availability if cyanobacteria cover large areas of the reef at the time of settlement, and consequently recruitment may in part depend upon wave action from annual monsoon winds and tropical storms which remove cyanobacteria, thereby exposing underlying CCA and other substrate suitable for coral settlement.     

Resilience of predators to fishing pressure on coral patch reefs

Numbers and biomass of piscivorous fish and their predation on other fish may often be high in undisturbed coral reef communities. The effects of such predation have sometimes been studied by removal of piscivores (either experimentally or by fishermen). Such perturbations have usually involved removal of large, highly vulnerable, mobile piscivores that are often actively sought in fisheries. The effects of fishing on smaller, demersal, semi-resident piscivores have been little studied. We studied such effects on the fish communities of patch reefs at Midway atoll by experimentally removing major resident, demersal, piscivorous fishes. First, four control reefs and four experimental reefs were selected, their dimensions and habitats mapped, and their visible fish communities censused repeatedly over 1 year. Census of all control and experimental reefs was continued for the following 39 months, during which known piscivores were collected repeatedly by hand spearing. Records were kept of catch and effort to calculate CPUE as an index of predator density. Spearfishing on the experimental reefs removed 2504 piscivorous fish from 12 families and 43 taxa (mostly species). The species richness of the catch did not show an overall change over the duration of the experiment. Spearman rank correlation analysis showed some unexpected positive correlations for density in numbers and biomass of major fished piscivorous groups (especially lizardfish) over the experiment. Only two relatively minor fished piscivorous taxa declined in abundance over the experiment, while the overall abundance of piscivores increased. Visual censuses of fish on the experimental reefs also failed to show reduction of total piscivores over the full experimental period. No significant trend in the abundance of lizardfish censused over the full period was apparent on any of the control reefs. The high resilience of piscivores on these experimental reefs to relatively intense fishing pressure could result from their protracted recruitment seasons, high immigration rates, cryptic habits, or naturally high abundances. A major factor was the high immigration rates of lizardfish, replacing lizardfish and other less mobile piscivores removed from the reefs by spearing. On the fished reefs, the removed lizardfish population replaced itself >20 times during the experiment; other piscivorous taxa replaced themselves only 5 times.     

Ecological structure of assemblages of coral reef fishes on isolated patch reefs

A 9-year study of the structure of assemblages of fish on 20 coral patch reefs, based on 20 non-manipulative censuses, revealed a total of 141 species from 34 families, although 40 species accounted for over 95% of sightings of fish. The average patch reef was 8.5 m² in surface area, and supported 125 fish of 20 species at a census. All reefs showed at least a two-fold variation among censuses in total numbers of fish present, and 12 showed ten-fold variations. There was also substantial variation in the composition and relative abundances of species present on each patch reef, such that censuses of a single patch reef were on average about 50% different from each other in percent similarity of species composition (Czekanowski's index). Species differed substantially in the degree to which their numbers varied from census to census, and in the degree to which their dispersion among patch reefs was modified from census to census. We characterize the 40 most common species with respect to these attributes. The variations in assemblage structure cannot be attributed to responses of fish to a changing physical structure of patch reefs, nor to the comings and goings of numerous rare species. Our results support and extend earlier reports on this study, which have stressed the lack of persistant structure for assemblages on these patch reefs. While reef fishes clearly have microhabitat preferences which are expressed at settlement, the variations in microhabitat offered by the patch reefs are insufficient to segregate many species of fish by patch reef. Instead, at the scale of single patch reefs, and, to a degree, at the larger scale of the 20 patch reefs, most of the 141 species of fish are distributed without regard to differences in habitat structure among reefs, and patterns of distribution change over time. Implications for general understanding of assemblage dynamics for fish over more extensive patches of reef habitat are considered.     

Impact of herbivores on nitrogen cycling: contrasting effects of small and large species

Herbivores are reported to slow down as well as enhance nutrient cycling in grasslands. These conflicting results may be explained by differences in herbivore type. In this study we focus on herbivore body size as a factor that causes differences in herbivore effects on N cycling. We used an exclosure set-up in a floodplain grassland grazed by cattle, rabbits and common voles, where we subsequently excluded cattle and rabbits. Exclusion of cattle lead to an increase in vole numbers and a 1.5-fold increase in net annual N mineralization at similar herbivore densities (corrected to metabolic weight). Timing and height of the mineralization peak in spring was the same in all treatments, but mineralization in the vole-grazed treatment showed a peak in autumn, when mineralization had already declined under cattle grazing. This mineralization peak in autumn coincides with a peak in vole density and high levels of N input through vole faeces at a fine-scale distribution, whereas under cattle grazing only a few patches receive all N and most experience net nutrient removal. The other parameters that we measured, which include potential N mineralization rates measured under standardized laboratory conditions and soil parameters, plant biomass and plant nutrient content measured in the field, were the same for all three grazing treatments and could therefore not cause the observed difference. When cows were excluded, more litter accumulated in the vegetation. The formation of this litter layer may have added to the higher mineralization rates under vole grazing, through enhanced nutrient return through litter or through modification of microclimate. We conclude that different-sized herbivores have different effects on N cycling within the same habitat. Exclusion of large herbivores resulted in increased N annual mineralization under small herbivore grazing.     

Limited functional redundancy in high diversity systems: resilience and ecosystem function on coral reefs

Abstract Biodiversity is frequently associated with functional redundancy. Indo‐Pacific coral reefs incorporate some of the most diverse ecosystems on the globe with over 3000 species of fishes recorded from the region. Despite this diversity, we document changes in ecosystem function on coral reefs at regional biogeographical scales as a result of overfishing of just one species, the giant humphead parrotfish (Bolbometopon muricatum). Each parrotfish ingests over 5 tonnes of structural reef carbonates per year, almost half being living corals. On relatively unexploited oceanic reefs, total ingestion rates per m² balance estimated rates of reef growth. However, human activity and ecosystem disruption are strongly correlated, regardless of local fish biodiversity. The results emphasize the need to consider the functional role of species when formulating management strategies and the potential weakness of the link between biodiversity and ecosystem resilience.     

Visual censuses of tropical fish aggregations on artificial reefs: slate versus video recording techniques

Artificial coastal reefs were initially installed off Reunion Island (SW Indian Ocean) in order to attract and catch small pelagic fish of economic importance for the local fishery. These artificial reefs also attract a high density of juvenile coral reef fish, especially after recruitment. Because of high species richness and density, a suitable method of evaluating diversity and abundance in such assemblages is needed. An experiment was conducted in situ for 1 year in order to test which of the two visual techniques, slate or video recording, would be the most suitable for sampling artificial reefs. For the quantitative study, a rope framework (four belt transects each measuring 24×3 m) was used to ease the movement of the divers. To avoid bias due to time variations, the study was conducted under limited time in situ. The advantages and disadvantages of the two techniques regarding the accuracy of qualitative and quantitative results are discussed. The qualitative study results show that the slate technique is the most accurate one for determining species richness. In the quantitative study, factors such as species characteristics (ecological type), attractiveness (relationship between fish and diver) and abundance of fish communities around the artificial reefs appear to influence our results on the comparison of the two techniques. Of 13 ‘permanent’ (observed in more than 75% of dives) species, 6 returned a similar frequency of detection by slate and video (Lutjanus kasmira, Lutjanus bengalensis, Priacanthus hamrur, Chaetodon kleinii, Heniochus diphreutes, Mulloides vanicolensis); these were then selected for comparison of the techniques using quantitative data. The correlation between video and slate census results is high, but, in comparison with the slate census, the video census underestimates abundance, especially for low values. A corrected index is proposed to remedy this underestimation; it could be used for video surveys conducted on species responding well to video detection (as below).     

An analysis of fish‐habitat associations on disturbed coral reefs: chaetodontid fishes in New Caledonia

A concordance analysis was used to study the simultaneous influence of several environmental data sets on chaetodontid (butterflyfish) distributions. This multivariate and multitable method enabled the correlation of three types of benthic characteristics (mineral substratum, coverage of structural species and large echinoderms) with butterflyfish abundances in two bays of the urban centre of Nouméa (New Caledonia). The first concordance axis was related to a gradient in the coverage of branching corals. This disturbance gradient compared damaged reef areas dominated by long‐spined sea urchins to areas with an extensive coverage of branching corals. The abundance of corallivorous chaetodontids was related to this gradient, supporting the view of corals as a food and shelter source for these fishes. The second concordance axis was interpreted as a gradient of heterogeneity in the coverage of benthic life‐forms. The abundance of omnivorous chaetodontids was related to this gradient. Thus, the concordance axes defined two key components of habitat structure that were related to the entire fish community structure.     

Climate change and coral reefs: different effects in two high-latitude areas (Arabian Gulf,South Africa)

The findings in this paper show that Arabian Gulf (Abu Dhabi, Dubai, Sharjah) corals have already been measurably affected by climate change and further negative impacts are expected. Corals in South Africa have been only weakly impacted and are expected to persist in this likely refuge. The Arabian Gulf has recently experienced high-frequency recurrences of temperature-related bleaching (1996, 1998, 2002). First evidence may suggest that bleaching patterns in corals changed due to phenotypic adaptation after two strong bleaching events in rapid succession, because Acropora, which during the 1996 and 1998 events always bleached first and suffered heaviest mortality, bleached less than all other corals in 2002 at Sir Abu Nuair and recovered at Jebel Ali and Ras Hasyan. In South Africa, reef corals largely escaped the mass mortalities observed across the tropics in the late 1990s, although bleaching has also increased since 1999. These reefs are protected by local small-scale upwelling events in summer that, if they occur at the right time, keep temperatures below bleaching levels. Both areas, the Arabian Gulf and South Africa, have rich coral faunas but little to no recent reef-framework production. It is possible that many reefs worldwide may have similar dynamics in the future, if the changed climate (recurrence of temperature anomalies, changes in aragonite saturation state, etc.) suppresses sustained reef building at least temporarily. Global climate models predict the possibility of significant environmental changes, including increases in atmospheric temperature, sea-surface temperature (SST), and sea level. Monsoon and El Niño Southeastern Oscillation (ENSO) patterns might change, but climate models are not conclusive. Sea-level rise by up to 0.88 m is expected to be a problem in some low-lying areas, like the southern Arabian Gulf. Ocean aragonite saturation state is predicted to fall throughout the ocean but may not change reef dynamics in the two study areas.     

Ocurrence of Ostreopsis in two temperate coastal bays (SW iberia): Insights from the plankton

The benthic genus Ostreopsis contains toxic-bloom forming species and is an important cause of concern in warm-temperate and tropical waters. On the coast of Portugal, NE Atlantic, the occurrence of Ostreopsis cf. siamensis and Ostreopsis cf. ovata has been reported since 2008 and 2011, respectively. This work aims to understand the favorable conditions for high concentrations of Ostreopsis cells in the plankton at two sites, Lagos and Lisbon Bays, located in the South and West coast of Portugal, respectively. This study is based on weekly Ostreopsis abundance data in the plankton, from 2011 to 2017, daily satellite and in situ sea surface temperature (SST), and meteorological and sea state parameters, namely wind stress and significant wave height. The molecular identification of local Ostreopsis spp. is also presented. The maximum cell densities occur between late-summer and autumn. The distribution range of Ostreopsis cf. ovata is restricted to the South coast, while Ostreopsis cf. siamensis has a wider distribution range, being also present on the West coast. In the study period, there was only one occurrence of Ostreopsis spp., in Lagos Bay, with concentrations within the alert phase of monitoring. In Lagos Bay, high Ostreopsis spp. concentrations were related with positive SST anomalies. These high concentrations were often recorded after a period of almost 2-weeks to more than 4-weeks of low sea state (<0.6 m), followed by short time events of onshore wind and moderate waves (0.6–1 m). The former conditions are interpreted as favoring bloom development on the substrate and the latter as causing the re-suspension of Ostreopsis cells in the water column. In Lisbon Bay, O. cf. siamensis occurred in the plankton in few occasions and no clear relation could be established with the studied environmental variables. It is here hypothesized that the recent records of O. cf. siamensis in Lisbon Bay may correspond to an early colonization stage of an invasion process. Knowledge gained on Ostreopsis dynamics along the Portuguese coast can be used for both the improvement of benthic harmful algal blooms (BHABs) monitoring in the region and as a basis to design forecasting models.     

When predator odour makes groups stronger: effects on behavioural and chemical adaptations in two termite species

1. Being able to detect a predator before any physical contact is crucial for individual survival. Predator presence can be detected thanks to several types of signal, such as chemical cues. Chemical signals are produced by predators for their protection against desiccation, for their communication, or possibly as a side‐effect of their activity. In insects, chemical communication plays a key role in diverse biological processes, including prey‐predator or plant‐insect interactions, courtship behaviour, and kin or species recognition. 2. Cuticular hydrocarbons (CHCs) are specifically involved in recognition processes and social organisation (division of labour, caste ratios) in social insects. Here, the questions raised are whether termites can detect a predator with their cuticular compounds and whether the predator‐produced compounds can influence their prey. 3. The responses of termites Reticulitermes grassei (Clément, 1978) and Reticulitermes flavipes (Kollar, 1837) to the presence of the cuticular compounds produced by a predator, the ant species Lasius niger (Linnaeus, 1758), were investigated. More specifically, the study quantified termite traits such as caste ratios, mortality rates, CHC profile homogeneity and aggressiveness of workers after 2 months' exposure to predator‐produced compounds. 4. The results show that the predator odour did affect the aggressiveness of the native species R. grassei but not of the invasive R. flavipes. The caste ratios and the mortality rates were not affected for both species. 5. Differences between species are discussed around the native or invasive status of each species, along with the role played by chemical cues on behavioural and chemical adaptations.     

Orientation behavior of butterflyfishes (family Chaetodontidae) on coral reefs: spatial learning of route specific landmarks and cognitive maps

Synopsis Foraging butterflyfishes follow predictable paths as they swim from one food patch to another within their territories and home ranges. The pattern is repeated throughout the day. The behavior is described in species belonging to the coral feeding guild. Habit formation and spatial learning are implicated. Foraging paths are based on learned locations of route specific landmarks. When a coral head is removed the fish look for it in its former location. If pairs of foraging fish are deflected from the path, they resume their routine pattern at the first landmark they encounter. Periodically, fish make excursions of 30 m or more to distant parts of the reef. Usually they follow different paths on the outbound and homeward legs of these excursions. The critical question is: Are the paths novel? If they are, it is evidence for the use of cognitive maps. Certainly fishes living in the highly structured coral reef environment are prime candidates to use cognitive maps in their orientation behavior.     

Effects of coral transplantation and giant clam restocking on the structure of fish communities on degraded patch reefs

Active restoration is being practiced to supplement conservation activities for the purpose of reversing the trend of reef degradation. In the last decade, the feasibility of different restoration approaches such as coral transplantation and restocking of other marine biota has been the focus of research and relatively few have examined experimentally its effects on the resultant communities. In this study, coral transplantation and giant clam restocking were applied on 25 degraded patch reefs (~ 25 m²) inside a marine sanctuary in Pangasinan, northwestern Philippines to examine their effects on the community structure of reef fishes. Five interventions or treatments were employed: 1) “coral” consisted of transplantation of a combination of Acropora spp. and Pocillopora spp. on concrete blocks; 2) “clam” consisted of restocking of Tridacna gigas; 3) “clam+coral” consisted of restocking of T. gigas with Acropora spp. transplanted on their shells; 4) “shell” consisted of deployment of T. gigas shells; and 5) “control” consisted of no intervention. Fish communities on the patch reefs were monitored monthly for 3 months before the intervention and were monitored further for 11 months after the intervention, including 1 recruitment season. After the intervention, the coral cover and the “other biota” category increased in the coral and clam+coral treatments, due to the transplanted corals and deployed giant clams. Consequently, the complexity of the substrate was enhanced. A month after the intervention, a rapid increase in the abundance and species richness of reef fishes on the coral, clam+coral and clam treatments was observed compared to the shell and control treatments. A change in species composition of reef fish assemblage was also apparent in the coral and clam+coral treatments relative to the clam, shell and control, especially 4 months after the intervention. The present experiment demonstrates the feasibility of improving the condition of degraded patch reefs, which can subsequently enhance the fish community. Results also show the importance of the underlying substratum and the abundance of live corals and clams to reef fishes.     

棘冠海星暴发及其对珊瑚礁的生态影响研究进展

棘冠海星的反复暴发是导致印度—太平洋区域珊瑚礁生态系统退化的最主要原因之一。然而,我国对棘冠海星的研究非常有限。本文综述了国内外关于棘冠海星及其暴发的生态影响和应对策略的研究进展,得出以下主要结论：1)雌性棘冠海星个体每年产卵数量高达50万—2亿个,环境因素变化只要导致幼虫和幼体存活率的轻微提高,成体就将得到大量补充;2)棘冠海星暴发的阈值为1000—1500个/km²,暴发周期为10—27 a,每次暴发持续1—10 a,最终可能以“种群集体感染疾病而崩溃”结束;3)棘冠海星暴发对印度洋及太平洋东部和北部珊瑚礁的破坏性非常小,却直接导致太平洋的西部和南部珊瑚礁90%以上的珊瑚死亡,并通过改变珊瑚群落组成、减少珊瑚和鱼类多样性而对珊瑚礁产生间接影响;4)关于棘冠海星暴发原因的假说中“陆地营养物质输入假说”和“捕食者过度捕捞假说”得到了最普遍的认可,但都不能解释所有的暴发事件;5)应对棘冠海星暴发的主要策略有改善水质、设立保护区、投放天敌和人工清理等,其中人工清理是最直接有效的策略,但迄今并没有发现可长期抑制棘冠海星暴发的方法。因此,急需加强对棘冠海星的深入研究,探查...     

Cellular and nuclear morphological variability within a single species of the toxigenic dinoflagellate genus Gambierdiscus: Relationship to life-cycle processes

Dinoflagellates belonging to the genus Gambierdiscus are the causative agent of ciguatera fish poisoning (CFP). This syndrome, which is widespread in tropical and subtropical regions, has recently been reported also in temperate latitudes. Taxonomic studies of Gambierdiscus have yet to completely couple the morphological features of member species with their genetics. In this study, the cellular and nuclear morphology of a single strain of one species of Gambierdiscus was determined in cells grown under different culture conditions. The results showed a wide-ranging variability of cell sizes, together with a clear relationship between cell size and nuclear morphology. Thus, small cells were associated with round to oval or slightly U-shaped nuclei and large cells with obviously U-shaped nuclei. Most cells exhibited the typical anterio-posteriorly compressed lenticular, shape of Gambierdiscus, with the exception of a few small globular-shaped specimens. In all cells, regardless of their size, the arrangement of the thecal plates was typical of lenticular Gambierdiscus. Dividing cells were consistently the largest. In these cells, nuclear morphology, karyokinesis, and cytokinesis were characterized. Cells underwent division only during the dark period, thus demonstrating their spontaneous synchronized division. Cellular forms related to the sexual cycle were also present in the cultures and included gamete pairs and putative meiotic planozygotes. The effect of the culture medium was studied by means of principal component analyses, which showed a positive correlation between the medium used and nuclear size and shape but not cell size.