Upper temperature limits of tropical marine ectotherms: global warming implications

Animal physiology, ecology and evolution are affected by temperature and it is expected that community structure will be strongly influenced by global warming. This is particularly relevant in the tropics, where organisms are already living close to their upper temperature limits and hence are highly vulnerable to rising temperature. Here we present data on upper temperature limits of 34 tropical marine ectotherm species from seven phyla living in intertidal and subtidal habitats. Short term thermal tolerances and vertical distributions were correlated, i.e., upper shore animals have higher thermal tolerance than lower shore and subtidal animals; however, animals, despite their respective tidal height, were susceptible to the same temperature in the long term. When temperatures were raised by 1°C hour(-1), the upper lethal temperature range of intertidal ectotherms was 41-52°C, but this range was narrower and reduced to 37-41°C in subtidal animals. The rate of temperature change, however, affected intertidal and subtidal animals differently. In chronic heating experiments when temperature was raised weekly or monthly instead of every hour, upper temperature limits of subtidal species decreased from 40°C to 35.4°C, while the decrease was more than 10°C in high shore organisms. Hence in the long term, activity and survival of tropical marine organisms could be compromised just 2-3°C above present seawater temperatures. Differences between animals from environments that experience different levels of temperature variability suggest that the physiological mechanisms underlying thermal sensitivity may vary at different rates of warming.     

Outdoor infrared imaging for spatial and temporal thermography: A case study of necrotic versus healthy leaf areas on woody plants

The temperature of plants can be measured using infrared (IR) thermography. Despite the extensive use of IR imaging indoors, outdoor IR imaging is uncommon. We used IR imaging to compare leaf temperatures between necrotic spots and healthy areas of oriental cherry (Prunus serrulata var. spontanea), Japanese cornel (Cornus officinalis) and sawtooth oak (Quercus acutissima) in the field. There was a significant difference in the mean leaf temperatures between the necrotic spots (26.4°C) and healthy areas (25.6°C) of oriental cherry (p = .01). The mean temperatures in the necrotic spots of Japanese cornel and sawtooth oak leaves were 22.3°C and 29.6°C, respectively, which were not significantly different from the mean temperatures of the healthy areas. A consecutive, 2‐day temporal leaf analysis in October 2018 revealed that the temperatures in the necrotic spots were generally higher than those in the healthy areas of the three species. The temperature difference between the spots and healthy areas (up to 1.4°C) was more pronounced at 13:00 hr in all three species on both days. These results reveal differences in the spatial and temporal thermal state across the necrotic spotted leaves. There is potential for use of outdoor IR imaging to visualize the response of trees to pathogen infection and abiotic stress.     

Adapt,move or die – how will tropical coral reef fishes cope with ocean warming?

Previous studies hailed thermal tolerance and the capacity for organisms to acclimate and adapt as the primary pathways for species survival under climate change. Here we challenge this theory. Over the past decade, more than 365 tropical stenothermal fish species have been documented moving poleward, away from ocean warming hotspots where temperatures 2–3 °C above long‐term annual means can compromise critical physiological processes. We examined the capacity of a model species – a thermally sensitive coral reef fish, Chromis viridis (Pomacentridae) – to use preference behaviour to regulate its body temperature. Movement could potentially circumvent the physiological stress response associated with elevated temperatures and may be a strategy relied upon before genetic adaptation can be effectuated. Individuals were maintained at one of six temperatures (23, 25, 27, 29, 31 and 33 °C) for at least 6 weeks. We compared the relative importance of acclimation temperature to changes in upper critical thermal limits, aerobic metabolic scope and thermal preference. While acclimation temperature positively affected the upper critical thermal limit, neither aerobic metabolic scope nor thermal preference exhibited such plasticity. Importantly, when given the choice to stay in a habitat reflecting their acclimation temperatures or relocate, fish acclimated to end‐of‐century predicted temperatures (i.e. 31 or 33 °C) preferentially sought out cooler temperatures, those equivalent to long‐term summer averages in their natural habitats (~29 °C). This was also the temperature providing the greatest aerobic metabolic scope and body condition across all treatments. Consequently, acclimation can confer plasticity in some performance traits, but may be an unreliable indicator of the ultimate survival and distribution of mobile stenothermal species under global warming. Conversely, thermal preference can arise long before, and remain long after, the harmful effects of elevated ocean temperatures take hold and may be the primary driver of the escalating poleward migration of species.     

Thermal sensitivity of juvenile rabbitfishes Siganus doliatus and S. lineatus (Siganidae): a key role for habitat?

The majority of our understanding of the effects of climate change on coral reef fishes are currently based on studies of small-bodied species such as damselfishes. By contrast, we know little about the potential impacts of ocean warming on larger species of herbivorous and detritivorous reef fish, despite them being a critical functional group and an essential source of food protein for millions of people. In addition, we know little of the role of habitat in determining species’ thermal sensitivity and the legitimacy of extrapolating thermal performance across closely-related species from different habitat types. Here we test the effect of exposure to increased water temperature during juvenile development on key physiological and behavioral traits of two species of rabbitfish typically associated with different habitats: Siganus doliatus (reef-associated) and S. lineatus (estuarine). Wild-caught juveniles were reared for 14 weeks at temperatures representing present-day ambient conditions (28.0 °C), late-summer ambient conditions (30.0 °C), or those projected on reefs under future global warming scenarios (31.5 °C). We then measured the somatic (growth, condition, immune response) and behavioral (feeding rate, latency to feed and activity level) traits of individuals within each treatment to determine the sensitivity of each species to elevated water temperatures. Overall, both species showed comparatively robust levels of thermal tolerance, based on previously-documented responses of small-bodied reef fishes. However, two very different patterns emerged. The reef-associated S. doliatus showed a greater physiological response to temperature, with negative effects on hepatosomatic condition and immune function observed in individuals exposed to the 31.5 °C treatment. By contrast, there were no negative physiological effects of temperature observed in S. lineatus and instead we recorded behavioral changes, with individuals at 30 °C and 31.5 °C displaying altered feeding behavior (increased feeding rate and decreased latency to feed). These distinct responses observed between congeners are likely due to their evolutionary history and flag the potential inaccuracies that could arise from extrapolating effects of ocean warming across even closely-related species adapted to different habitats.

     

Rapid fluctuations in flow and water-column properties in Asan Bay, Guam: implications for selective resilience of coral reefs in warming seas

Hydrodynamics and water-column properties were investigated off west-central Guam from July 2007 through January 2008. Rapid fluctuations, on time scales of 10s of min, in currents, temperature, salinity, and acoustic backscatter were observed to occur on sub-diurnal frequencies along more than 2 km of the fore reef but not at the reef crest. During periods characterized by higher sea-surface temperatures (SSTs), weaker wind forcing, smaller ocean surface waves, and greater thermal stratification, rapid decreases in temperature and concurrent rapid increases in salinity and acoustic backscatter coincided with onshore-directed near-bed currents and offshore-directed near-surface currents. During the study, these cool-water events, on average, lasted 2.3 h and decreased the water temperature 0.57 °C, increased the salinity 0.25 PSU, and were two orders of magnitude more prevalent during the summer season than the winter. During the summer season when the average satellite-derived SST anomaly was +0.63 °C, these cooling events, on average, lowered the temperature 1.14 °C along the fore reef but only 0.11 °C along the reef crest. The rapid shifts appear to be the result of internal tidal bores pumping cooler, more saline, higher-backscatter oceanic water from depths >50 m over cross-shore distances of 100 s of m into the warmer, less saline waters at depths of 20 m and shallower. Such internal bores appear to have the potential to buffer shallow coral reefs from predicted increases in SSTs by bringing cool, offshore water to shallow coral environments. These cooling internal bores may also provide additional benefits to offset stress such as supplying food to thermally stressed corals, reducing stress due to ultraviolet radiation and/or low salinity, and delivering coral larvae from deeper reefs not impacted by surface thermal stress. Thus, the presence of internal bores might be an important factor locally in the resilience of select coral reefs facing increased thermal stress.     

Coral communities and reef growth in the southern Great Barrier Reef

 Fringing reef development is limited around 22° S along the inner Great Barrier Reef, although there is substantial development north and south of this latitude. This study examined the relationships among coral communities and the extent of reef development. Reefs were examined to determine coral composition, colony abundance, colony size and growth form between the latitudes 20°S and 23°S. Major reef framework builders (scler- actinian genus Acropora and families Faviidae and Poritidae) dominated reefs north and south of 22°S, but declined significantly at 22°S where foliose and encrusting corals (Turbinaria and Montipora spp.) were most common. Porites spp. were present at 22° S but had encrusting morphologies. Consistently high turbidity at this latitude, caused by a 10 m tidal range and strong tidal flows, resuspends silts from the shallow shelf, and appears to have precluded reef development throughout the Holocene, by limiting the abundance, stunting the growth, and shortening the life expectancies of reef framework corals. The distinctions between ‘natural’ and ‘human-induced’ degradation may be interpreted on the basis of the relationship between Holocene development and current benthic community longevity. A mismatch between substantial past reef building capacity (a broad and/or thick reef) and non-existent or limited present reef-building capacity could signify anything from a long-period, natural cycle to an unprecedented deterioration in ecosystem function caused by human influence. Accepted: 29 July 1996     

Thermal Heterogeneity in River Floodplains

River floodplains are composed of a shifting mosaic of aquatic and terrestrial habitats. Each habitat type exhibits distinct environmental and ecological properties. Temperature is a key property driving ecological processes and controlling the composition and distribution of biota. However, given the size and complexity of floodplains, ground surveys based on point measurements are spatially limited. In this study, we applied thermal infrared (IR) imagery to quantify surface temperature patterns at 12–15 min intervals over 24 h cycles in two near-natural Alpine river floodplains (Roseg, Tagliamento). Furthermore, vertical temperature distribution was measured at 3–5 min intervals in unsaturated gravel sediment deposits (at 1 cm distances; 0–29 cm depth). Each habitat type exhibited a distinct thermal signature creating a complex thermal mosaic. The diel temperature pulse and maximum daily temperature were the main thermal components that differentiated habitat types. In both floodplains, exposed gravel sediments exhibited the highest diel pulse (up to 23°C), whereas in aquatic habitats the pulse was as low as 11°C (main channel in the Roseg floodplain). In the unsaturated gravel sediment deposits, the maximum diel kinetic temperature pulse ranged from 40.4°C (sediment surface) to 2.7°C (29 cm sediment depth). Vertically, the spatiotemporal variation of temperature was about as high as horizontally across the entire floodplain surface. This study emphasized that remotely sensed thermal IR imagery provides a powerful non-invasive method to quantitatively assess thermal heterogeneity of complex aquatic and terrestrial ecosystems at a resolution required to understand ecosystem processes and the distribution of biota.     

Life on the edge: thermal optima for aerobic scope of equatorial reef fishes are close to current day temperatures

Equatorial populations of marine species are predicted to be most impacted by global warming because they could be adapted to a narrow range of temperatures in their local environment. We investigated the thermal range at which aerobic metabolic performance is optimum in equatorial populations of coral reef fish in northern Papua New Guinea. Four species of damselfishes and two species of cardinal fishes were held for 14 days at 29, 31, 33, and 34 °C, which incorporated their existing thermal range (29–31 °C) as well as projected increases in ocean surface temperatures of up to 3 °C by the end of this century. Resting and maximum oxygen consumption rates were measured for each species at each temperature and used to calculate the thermal reaction norm of aerobic scope. Our results indicate that one of the six species, Chromis atripectoralis, is already living above its thermal optimum of 29 °C. The other five species appeared to be living close to their thermal optima (ca. 31 °C). Aerobic scope was significantly reduced in all species, and approached zero for two species at 3 °C above current‐day temperatures. One species was unable to survive even short‐term exposure to 34 °C. Our results indicate that low‐latitude reef fish populations are living close to their thermal optima and may be more sensitive to ocean warming than higher‐latitude populations. Even relatively small temperature increases (2–3 °C) could result in population declines and potentially redistribution of equatorial species to higher latitudes if adaptation cannot keep pace.     

How do we Measure the Environment? Linking Intertidal Thermal Physiology and Ecology Through Biophysics

Recent advances in quantifying biochemical and cellular-levelresponses to thermal stress have facilitated a new explorationof the role of climate and climate change in driving intertidalcommunity and population ecology. To fruitfully connect thesedisciplines, we first need to understand what the body temperaturesof intertidal organisms are under field conditions, and howthey change in space and time. Newly available data logger technologymakes such an exploration possible, but several potential pitfallsmust be avoided. Body temperature during aerial exposure isdriven by multiple, interacting climatic factors, and extremesduring low tide far exceed those during submersion. Moreover,because of effects of body size and morphology, two organismsexposed to identical climatic conditions can display very differentbody temperatures, which can also be substantially differentfrom the temperature of the surrounding air. These same factorsdrive the temperature recorded by data loggers, and one loggertype is unlikely to serve as an effective proxy for all organismsat a site. Here I describe the difficulties involved in quantifyingpatterns of body temperature in intertidal organisms, and explorethe implications of this complexity for intertidal physiologicalecology. I do so using data from temperature loggers designedto mimic the thermal characteristics of the mussel Mytilus californianus,and deployed at multiple sites along the West Coast of the UnitedStates. Results indicate a highly intricate pattern of thermalstress, where the interaction of climate with the dynamics ofthe tidal cycle determines the timing and magnitude of temperatureextremes, creating a unique "thermal signal" at each site.     

TEMPERATURE-GROWTH RESPONSES OF ALGAL ISOLATES FROM ANTARCTIC OASES1

Thirty-five taxa (128 clonal cultures) of Antarctic algae isolated from various habitats were assayed for growth over a range of 2–34°C. Isolates, all unialgal and two axenic, varied markedly in their temperature-growth responses. Only four taxa belonging to either the Chlamydomonadaceae or Ulotrichaceae were obligately cold-adapted and incapable of growth at ≥20°C. All isolates grew at temperatures ranging from 7.5 to 18°C, and a few were incapable of growth at ≤5°C. Over one-third of the isolates grew at 30°C, but none grew at 34°C. Percentages of cold-adapted clones correlated well with the more stable low temperature habitats. Four chlamydomonad isolates displayed optimum temperatures for growth near their maximum temperatures for growth, both temperatures being well above those of the native habitats. This temperature-growth response suggests a closer relationship to algae from more moderate thermal regions than one might have supposed. However, the ability to grow at low temperatures and the inability to grow at 34°C suggest that these Antarctic algae are cold temperature adapted. Growth capability at low in situ temperatures is considered more useful ecologically than physiologically-defined categories for algae based on their maximum temperature for growth.     

Core temperature variability in diving king penguins (Aptenodytes patagonicus): a preliminary analysis

Core temperature was determined in two king penguins living in the wild at Ile de la Possession, Crozel Archipelago, using implantable four-channel temperature loggers. Core temperatures derived from bird no. 1 (sensor placed under the sternum, in the vicinity of the liver and upper stomach) were closely correlated with diving activity (as determined by an external light recorder), and ranged from 38.3°C, (on land) to a minimum of 37.2°C during a dive. Core temperatures measured in bird no. 2 showed that temperatures near the heart were generally 1°C lower than those under the sternum or in the lower abdomen. Core temperatures declined continuously during dives (by 0.8, 1.2 and 2.7°C in the lower abdomen, under the sternum and near the heart, respectively) and showed precipitous drops to 35°C, probably associated with ingestion of food. Temperatures measured near the heart fluctuated over a period of 288 s, corresponding to the duration (from the literature) of the surface/dive cycle. The relevance of these findings with respect to diving physiology, blood perfusion of tissues, tissue metabolism and aerobic dive limits is discussed.     

Environmental factors associated with the spatial distribution of crustose coralline algae on the Great Barrier Reef

Crustose coralline algae (CCA) fulfill two key functional roles in coral reef ecosystems: they contribute significantly to reef calcification, and they induce larval settlement of many benthic organisms. Percentage cover of CCA, and environmental conditions, were visually estimated on 144 reefs of the Great Barrier Reef between 10 and 24° latitude S. Reefs were located across the shelf and ranged from turbid near-shore reefs close to rivers to clean-water reefs hundreds of kilometers from coastal influences. On each reef, two sites were surveyed between 0.5 and 18 m depth. Strong cross-shelf trends occurred in cover of CCA, amount of sediment deposited, water clarity, and slope angle. Relative distance across the shelf and sedimentation jointly explained 84% of variation in CCA cover. Three regions running parallel to the shore were identified, with a mean CCA cover of <1% on the inner third of the shelf, and >20% cover on the outer half of the shelf, with a narrow transition region between the two. Within each region, the cover of CCA was unrelated to distance across the shelf, but was related to the sedimentary environment, being relatively higher on reefs with low sediment deposits. On the inner third of the shelf, the most sediment-exposed reefs were unsuitable habitats for CCA. The inverse relationship between CCA and sediment has implications for the recruitment of CCA-specialised organisms, and for rates of reef calcification.     

Mapping microhabitat thermal patterns in artificial breakwaters: Alteration of intertidal biodiversity by higher rock temperature

Urbanization is altering community structure and functioning in marine ecosystems, but knowledge about the mechanisms driving loss of species diversity is still limited. Here, we examine rock thermal patterns in artificial breakwaters and test whether they have higher and spatially less variable rock temperature than natural adjacent habitats, which corresponds with lower biodiversity patterns. We estimated rock temperatures at mid‐high intertidal using infrared thermography during mid‐day in summer, in both artificial (Rip‐raps) and natural (boulder fields) habitats. We also conducted diurnal thermal surveys (every 4 hr) in four seasons at one study site. Concurrent sampling of air and seawater temperature, wind velocity, and topographic structure of habitats were considered to explore their influence on rock temperature. Rock temperature was in average 3.7°C higher in the artificial breakwater in two of the three study sites, while air temperature was about 1.5–4°C higher at this habitat at summer. Thermal patterns were more homogeneous across the artificial habitat. Lower species abundance and richness in the artificial breakwaters were associated with higher rock temperature. Mechanism underlying enhanced substrate temperature in the artificial structures seems related to their lower small‐scale spatial heterogeneity. Our study thus highlighted that higher rock temperature in artificial breakwaters can contribute to loss of biodiversity and that integrated artificial structures may alter coastal urban microclimates, a matter that should be considered in the spatial planning of urban coastal ecosystems.     

Thermal Imaging of Aye-Ayes (Daubentonia madagascariensis) Reveals a Dynamic Vascular Supply During Haptic Sensation

Infrared thermography (IRT) is used to visualize and estimate variation in surface temperatures. Applications of IRT to animal research include studies of thermofunctional anatomy, ecology, and social behavior. IRT is especially amenable to investigations of the somatosensory system because touch receptors are highly vascularized, dynamic, and located near the surface of the skin. The hands of aye-ayes (Daubentonia madagascariensis) are thus an inviting subject for IRT because of the prominent middle digit that functions as a specialized haptic sense structure during percussive and probative foraging. It is a vital sensory tool that is expected to feature a high density of dermal mechanoreceptors that radiate heat and impose thermal costs under cool temperatures. Here we explore this premise by acquiring IRT images of 8 aye-ayes engaged in a variety of passive and probative behaviors. We found that the middle digit was typically 2.3°C cooler than other digits when the metacarpophalangeal (MP) joint was extended, and that it warmed an average of 2.0°C when the MP joint was flexed during active touching behavior. These changes in digital surface temperature, which were sometimes as much 6.0°C, stand in sharp contrast with the profoundly invariant temperatures of the other digits. Although the physiological mechanisms behind these temperature changes are unknown, they appear to reveal a uniquely dynamic vascular supply.     

SEASONAL DIFFERENCES IN THE TEMPERATURE RANGES OF GROWTH OF VIRGINIA ALGAE1

One hundred and fifteen clonal, unialgal strains were isolated and tested for their ability to grow over a range of temperatures from 2 to 40° C. Responses of 63 strains isolated from habitats that were 6° C when sampled and 52 strains isolated from habitats that were 20° C when sampled showed trends toward increasing adaptation to cold or warm temperatures commensurate with their seasonal in situ temperatures. Based on temperature-growth responses alone, 24% of the plankton isolates and 17% of the periphyton isolates could be perennial within the natural habitats. At 5° C, 56% of the warm water plankton isolates and 48% of the warm water periphyton isolates were incapable of growth and, therefore, probably could not be important components of the winter algal community. Likewise at 25° C, 25% of the cold water plankton isolates and 13% of the cold water periphyton isolates were incapable of growth. Thus, temperature alone probably is an important variable regulating seasonal changes in algal community structure. Pollution of these habitats by a thermal enrichment averaging + 5° C year-round could effect a pronounced change in algal species composition because many more taxa could be perennial and more taxa would be incapable of growth during naturally warm periods.     

Thermal effects on the biological parameters of the predatory mirid Pilophorus gallicus (Hemiptera: Miridae)

Pilophorus gallicus Remane (Hemiptera: Miridae) is a predatory mirid reported in deciduous trees in the western Mediterranean area. This work aimed to determine the biological and demographic parameters for this species at different temperatures (15, 20, 25 and 30°C). Egg hatching times shortened from 57.8 days at 15°C to 9.2 days at 30°C, and nymphal development times declined from 62.8 days at 15°C to 11.1 days at 30°C. The hatching and nymphal survival rates were low at 15 and 30°C. The lower thermal thresholds for the egg and nymphal development were 12.4 and 12.0°C, respectively. These high thermal thresholds could be a safety mechanism to avoid the emergence of nymphs in the unfavorable winter period. Female weight increased between 15 and 25°C and decreased at 30°C. The fecundity increased from 70.2 eggs per female at 15°C to 212.4 eggs per female at 25°C, and decreased to 88.5 eggs per female at 30°C. Fertility ranged from 9.4% at 15°C to 40.9% at 25°C, being 24.9% at 30°C. The intrinsic rate of natural increase (r_m) rose from 0.001 to 0.081 between 15 and 25°C and decreased to 0.05 at 30°C. In summary, this species performs poorly at low temperatures and has a relative tolerance of high temperatures (30°C); its performance was best at 25°C. Knowledge of the variation in the biological parameters with temperature may be very useful for the understanding of its ecology and population dynamics.     

Extreme climatic events constrain space use and survival of a ground‐nesting bird

Two fundamental issues in ecology are understanding what influences the distribution and abundance of organisms through space and time. While it is well established that broad‐scale patterns of abiotic and biotic conditions affect organisms’ distributions and population fluctuations, discrete events may be important drivers of space use, survival, and persistence. These discrete extreme climatic events can constrain populations and space use at fine scales beyond that which is typically measured in ecological studies. Recently, a growing body of literature has identified thermal stress as a potential mechanism in determining space use and survival. We sought to determine how ambient temperature at fine temporal scales affected survival and space use for a ground‐nesting quail species (Colinus virginianus; northern bobwhite). We modeled space use across an ambient temperature gradient (ranging from ?20 to 38 °C) through a maxent algorithm. We also used Andersen–Gill proportional hazard models to assess the influence of ambient temperature‐related variables on survival through time. Estimated available useable space ranged from 18.6% to 57.1% of the landscape depending on ambient temperature. The lowest and highest ambient temperature categories (35 °C, respectively) were associated with the least amount of estimated useable space (18.6% and 24.6%, respectively). Range overlap analysis indicated dissimilarity in areas where Colinus virginianus were restricted during times of thermal extremes (range overlap = 0.38). This suggests that habitat under a given condition is not necessarily a habitat under alternative conditions. Further, we found survival was most influenced by weekly minimum ambient temperatures. Our results demonstrate that ecological constraints can occur along a thermal gradient and that understanding the effects of these discrete events and how they change over time may be more important to conservation of organisms than are average and broad‐scale conditions as typically measured in ecological studies.     

Latitudinal variation in thermal tolerance thresholds of early life stages of corals

Organisms living in habitats characterized by a marked seasonal temperature variation often have a greater thermal tolerance than those living in more stable habitats. To determine the extent to which this hypothesis applies to reef corals, we compared thermal tolerance of the early life stages of five scleractinian species from three locations spanning 17° of latitude along the east coast of Australia. Embryos were exposed to an 8 °C temperature range around the local ambient temperature at the time of spawning. Upper thermal thresholds, defined as the temperature treatment at which the proportion of abnormal embryos or median life span was significantly different to ambient controls, varied predictably among locations. At Lizard Island, the northern-most site with the least annual variation in temperature, the proportion of abnormal embryos increased and life span decreased 2 °C above ambient in the two species tested. At two southern sites, One Tree Island and Lord Howe Island, where annual temperature variation was greater, upper temperature thresholds were generally 4 °C or greater above ambient for both variables in the four species tested. The absolute upper thermal threshold temperature also varied among locations: 30 °C at Lizard Island; 28 °C at One Tree Island; 26 °C at Lord Howe Island. These results support previous work on adult corals demonstrating predictable differences in upper thermal thresholds with latitude. With projected ocean warming, these temperature thresholds will be exceeded in northern locations in the near future, adding to a growing body of evidence indicating that climate change is likely to be more detrimental to low latitude than high latitude corals.     

Regulate or tolerate: Thermal strategy of a coral reef flat resident,the epaulette shark,Hemiscyllium ocellatum

Highly variable thermal environments, such as coral reef flats, are challenging for marine ectotherms and are thought to invoke the use of behavioural strategies to avoid extreme temperatures and seek out thermal environments close to their preferred temperatures. Common to coral reef flats, the epaulette shark (Hemiscyllium ocellatum) possesses physiological adaptations to hypoxic and hypercapnic conditions, such as those experienced on reef flats, but little is known regarding the thermal strategies used by these sharks. We investigated whether H. ocellatum uses behavioural thermoregulation (i.e., movement to occupy thermally favourable microhabitats) or tolerates the broad range of temperatures experienced on the reef flat. Using an automated shuttlebox system, we determined the preferred temperature of H. ocellatum under controlled laboratory conditions and then compared this preferred temperature to 6 months of in situ environmental and body temperatures of individual H. ocellatum across the Heron Island reef flat. The preferred temperature of H. ocellatum under controlled conditions was 20.7 ± 1.5°C, but the body temperatures of individual H. ocellatum on the Heron Island reef flat mirrored environmental temperatures regardless of season or month. Despite substantial temporal variation in temperature on the Heron Island reef flat (15–34°C during 2017), there was a lack of spatial variation in temperature across the reef flat between sites or microhabitats. This limited spatial variation in temperature creates a low-quality thermal habitat limiting the ability of H. ocellatum to behaviourally thermoregulate. Behavioural thermoregulation is assumed in many shark species, but it appears that H. ocellatum may utilize other physiological strategies to cope with extreme temperature fluctuations on coral reef flats. While H. ocellatum appears to be able to tolerate acute exposure to temperatures well outside of their preferred temperature, it is unclear how this, and other, species will cope as temperatures continue to rise and approach their critical thermal limits. Understanding how species will respond to continued warming and the strategies they may use will be key to predicting future populations and assemblages.     

Large temperature plunges recorded by data loggers at different depths on an Indian Ocean atoll: comparison with satellite data and relevance to coral refuges

Seawater temperature was recorded at two-hourly intervals for two years (March 2006-March 2008) by six data loggers in Diego Garcia atoll, central Indian Ocean. Loggers were substrate mounted, in two transects of 5, 15 and 25 m depth on a seaward reef and in a lagoonal pass. During the season of mixed but predominantly northwest winds, regular plunges of temperature of 5–7°C occurred with a periodicity of 1–4 days. This partly coincided with the period of greatest annual warming. Temperature fluctuations increased with increasing depth. Temperature traces are compared with HadISST1 and AVHRR satellite-derived temperature data which do not capture the deeper water temperature plunges. Reasons for the regular temperature plunges appear to include both tidal cycles and unidentified, internal waves. This is important for issues of coral refuges, complicating use of satellite-derived temperature data for planning optimal coral reef conservation areas.