The future of evolutionary diversity in reef corals

One-third of the world''s reef-building corals are facing heightened extinction risk from climate change and other anthropogenic impacts. Previous studies have shown that such threats are not distributed randomly across the coral tree of life, and future extinctions have the potential to disproportionately reduce the phylogenetic diversity of this group on a global scale. However, the impact of such losses on a regional scale remains poorly known. In this study, we use phylogenetic metrics in conjunction with geographical distributions of living reef coral species to model how extinctions are likely to affect evolutionary diversity across different ecoregions. Based on two measures—phylogenetic diversity and phylogenetic species variability—we highlight regions with the largest losses of evolutionary diversity and hence of potential conservation interest. Notably, the projected loss of evolutionary diversity is relatively low in the most species-rich areas such as the Coral Triangle, while many regions with fewer species stand to lose much larger shares of their diversity. We also suggest that for complex ecosystems like coral reefs it is important to consider changes in phylogenetic species variability; areas with disproportionate declines in this measure should be of concern even if phylogenetic diversity is not as impacted. These findings underscore the importance of integrating evolutionary history into conservation planning for safeguarding the future diversity of coral reefs.     

How much time can herbivore protection buy for coral reefs under realistic regimes of hurricanes and coral bleaching?

Coral reefs have been more severely impacted by recent climate instability than any other ecosystem on Earth. Corals tolerate a narrow range of physical environmental stress, and increases in sea temperature of just 1 °C over several weeks can result in mass coral mortality, often exceeding 95% of individuals over hundreds of square kilometres. Even conservative climate models predict that mass coral bleaching events could occur annually by 2050. Unfortunately, managers of coral‐reef resources have few options available to meet this challenge. Here, we investigate the role that fisheries conservation tools, including the designation of marine reserves, can play in altering future trajectories of Caribbean coral reefs. We use an individual‐based model of the ecological dynamics to test the influence of spatially realistic regimes of disturbance on coral populations. Two major sources of disturbance, hurricanes and coral bleaching, are simulated in contrasting regions of the Caribbean: Belize, Bonaire, and the Bahamas. Simulations are extended to 2099 using the HadGEM1 climate model. We find that coral populations can maintain themselves under all levels of hurricane disturbance providing that grazing levels are high. Regional differences in hurricane frequency are found to cause strikingly different spatial patterns of reef health with greater patchiness occurring in Belize, which has less frequent disturbance, than the Bahamas. The addition of coral bleaching led to a much more homogenous reef state over the seascape. Moreover, in the presence of bleaching, all reefs exhibited a decline in health over time, though with substantial variation among regions. Although the protection of herbivores does not prevent reef degradation it does delay rates of coral loss even under the most severe thermal and hurricane regimes. Thus, we can estimate the degree to which local conservation can help buy time for reefs with values ranging between 18 years in the Bahamas and over 50 years in Bonaire, compared with heavily fished systems. Ultimately, we demonstrate that local conservation measures can benefit reef ecosystem services but that their impact will vary spatially and temporally. Recognizing where such management interventions will either help or fail is an important step towards both achieving sustainable use of coral‐reef resources and maximizing resource management investments.     

Disaster taxa in microbially mediated metazoans: how endosymbionts and environmental catastrophes influence the adaptive capacity of reef corals

Reef corals are examples of metazoans that engage in mutualisms with a variety of microorganisms, including dinoflagellates, Bacteria, Archaea, and viruses. The high adaptive capacity of these microbial symbionts can be co‐opted by their coral hosts, and various emergent traits of these associations, such as thermotolerance, are undergoing strong selection due to climate change. This selection may spur the rise of microbial ‘disaster taxa’: opportunistic, cosmopolitan generalists that can proliferate and increase host survivorship following disturbances. Coral bleaching (a stress‐induced loss of dinoflagellates) constitutes one type of catastrophic disturbance for resident symbiont communities, and opens novel patches of host for colonization by microbial disaster taxa. Moreover, the compartmentalization of microbial symbionts within coral polyps reduces their effective population size and thus facilitates the spread of disaster taxa during times of environmental change. These phenomena suggest that, despite widespread loss of coral cover as a result of climate disturbances, the potential spread of resilient microbial disaster taxa in surviving colonies can have important implications for coral reef persistence over the coming decades.     

Escaping the heat: range shifts of reef coral taxa in coastal Western Australia

One of the most critical challenges facing ecologists today is to understand the changing geographic distribution of species in response to current and predicted global warming. Coastal Western Australia is a natural laboratory in which to assess the effect of climate change on reef coral communities over a temporal scale unavailable to studies conducted solely on modern communities. Reef corals composing Late Pleistocene reef assemblages exposed at five distinct localities along the west Australian coast were censused and the results compared with coral occurrence data published for the modern reefs offshore of each locality. The resulting comparative data set comprises modern and Late Pleistocene reef coral communities occurring over approximately 12° of latitude. For the modern reefs this gradient includes the zone of overlap between the Dampierian and Flindersian Provinces. Modern reef coral communities show a pronounced gradient in coral composition over the latitudinal range encompassed by the study, while the gradient in community composition is not as strong for Pleistocene communities. Tropical‐adapted taxa contracted their ranges north since Late Pleistocene time, emplacing two biogeographic provinces in a region in which a single province had existed previously. Beta diversity values for adjacent communities also reflect this change. Modern reefs show a distinct peak in beta diversity in the middle of the region; the peak is not matched by Pleistocene reefs. Beta diversity is correlated with distance only for comparisons between modern reefs in the north and the fossil assemblages, further supporting change in distribution of the biogeographic provinces in the study area. Coral taxa present in modern communities clearly expanded and contracted their geographic ranges in response to climate change. Those taxa that distinguish Pleistocene from modern reefs are predicted to migrate south in response to future climate change, and potentially persist in ‘temperature refugia’ as tropical reef communities farther north decline.     

Effects of low light and high temperature on pediveligers of the fluted giant clam Tridacna squamosa

ABSTRACT

This paper examines the responses of the fluted giant clam Tridacna squamosa pediveligers to elevated temperature and reduced light levels. In a light reduction experiment, a total of 104,000 T. squamosa pediveligers were exposed to four different levels of shading for approximately one month. The most heavily shaded treatment, at 0.4% of ambient light, had significantly lower survival than the other groups, which all received 1% or more of ambient light. In a second experiment, for approximately two weeks 13,000 T. squamosa pediveligers were divided among three treatments: one at ambient temperature averaging 29.5 °C, and two with elevated temperatures averaging 32.2 °C and 34.8 °C. The elevated temperature treatments resulted in near total mortality. The highest temperature survived by any pediveliger was 32.8 °C. Our results indicate a potential synergetic effect, with turbidity causing giant clam pediveligers to settle in shallower water―where they will likely be exposed to higher temperatures.     

Ten polymorphic STR loci in the cosmopolitan reef coral, Pocillopora damicornis

We report the development of 10 polymorphic molecular markers containing short tandem repeats in the cosmopolitan reef-building coral, Pocillopora damicornis, an important model species for coral health, physiology, ecology, and genetics. The availability of polymorphic DNA markers in P. damicornis can act as impetus for investigations into inheritance and population genetics, as well as novel investigations into host-symbiont ecology and evolution. Coral bleaching and gene flow studies performed with these markers can have direct conservation implications.     

Reef fish assemblages in north-western Sri Lanka: distribution patterns and influences of fishing practises

The distribution and abundance of reef fishes in relation to habitat structure were studied within Bar Reef Marine Sanctuary (BRMS) and on an adjacent reef, disturbed by destructive fishing techniques, in north-western Sri Lanka, by visually censusing 135 species groups using fifty metre belt-transects. Two types of continental shelf patch-reefs are found in the study area: coral reefs and sandstone reefs, which are divided into distinct habitats, four for the coral reef (shallow reef flat, shallow patch reef, deep reef flat and Porites domes) and two for the sandstone reef (structured sandstone-reef and flat sandstone-reef). Fish assemblages varied in structure between reef types and among habitats within reef types. Functional aspects of habitat structure and composition, such as available food and shelter, seemed to be important factors influencing distribution patterns. The strongest separation in the organisation of fish assemblages in BRMS was between reef types: 19% of all species were confined to the coral-reef patches while 22% were restricted to the sandstone reef patches and 59% were represented on both reef types. In terms of distribution among habitats, 21% of all species were restricted to one habitat while only 1.5% were present in all. The highest density of fish was in the coral reef habitats while highest species diversity was found in the most structurally complex habitat: the structured sandstone-reef. This habitat also had the highest proportion of species with restricted distribution. Planktivores were the most abundant trophic group in BRMS, and the species composition of the group varied among habitats. The comparison of the disturbed reef with BRMS suggested that habitat alteration caused by destructive fishing methods has strongly influenced the fish community. Within the fished area the structure of the fish assemblages was more heterogeneous, fish abundance was lower by an order of magnitude and species numbers were lower than in BRMS.