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Lethal and sublethal effects of thermal stress on octocorals early life history stages

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Abstract

The frequency and severity of marine heatwaves causing mass mortality events in tropical and temperate coral species increases every year, with serious consequences on the stability and resilience of coral populations. Although recovery and persistence of coral populations after stress events is closely related to adult fitness, as well as larval survival and settlement, much remains unknown about the effects of thermal stress on early life history stages of temperate coral species. In the present study, the reproductive phenology and the effect of increased water temperature (+4 °C and +6 °C above ambient, 20 °C) on larval survival and settlement was evaluated for two of the most representative Mediterranean octocoral species (Eunicella singularis and Corallium rubrum). Our study shows that reproductive behavior is more variable than previously reported and breeding period occurs over a longer period in both species. Thermal stress did not affect the survival of symbiotic E. singularis larvae, but drastically reduced the survival of the non‐symbiotic C. rubrum larvae. Results on larval biomass and caloric consumption suggest that higher mortality rates of C. rubrum exposed to increased temperature were not related to depletion of endogenous energy in larvae. The results also show that settlement rates of E. singularis did not change in response to elevated temperature after 20 days of exposure, but larvae may settle fast and close to their native population at 26 °C (+6 °C). Although previous experimental studies found that adult colonies of both octocoral species are mostly resistant to thermal stress, our results on early life history stages suggest that the persistence and inter‐connectivity of local populations may be severely compromised under continued trends in ocean warming.

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Sessile marine species such as Anthozoans act as ecosystem engineers due to their three-dimensional structure. Gorgonians, in particular, can form dense underwater forests that give shelter to other species increasing local biodiversity. In the last decades, several Mediterranean gorgonian populations have been affected by natural and anthropogenic impacts which drastically reduced their size. However, some species showed unexpected resilience, mainly due to the supply of new individuals. To understand the mechanisms underlying recovery processes, studies on the first life history stages (i.e. larval dispersal, settlement and recruitment) are needed. In tropical coral reefs, crustose coralline algae (CCA) are known to influence coral larvae habitat selection and settlement. This capacity however is not ubiquitous among CCA species and larvae of different coral species may have different preferences. The present work focuses on three Mediterranean gorgonians (Eunicella singularis, Paramuricea clavata and Corallium rubrum) with the objective of quantifying settlement and recruitment in presence of two common CCA species (Litophyllum stictaeforme and Litophyllum incrustans). Results showed that the presence of CCA activates earlier settlement in E. singularis and increases the density of recruits, with different trends for the three species. Our results suggest that CCA should be taken into account in the implementation of conservation strategies. Moreover, a deeper comprehension of settlement mechanisms could help improving restoration techniques based on sexual reproduction.
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Recurrent thermal stress events and intensified precipitation alter the ocean environment resulting in the decline of coral populations. However, the influence of these changes on larval survival and settlement is not well understood. We examined the effect of salinity (15, 20, 25, 30, 35, and 40 ppt) and temperature (27 °C, 30 °C, and 33 °C) on settlement and survival of larvae of the octocoral, Heliopora coerulea. Larvae settled successfully at salinities from 25 to 30 ppt. On the other hand, larval survival and settlement decreased with increasing temperature. A combination of 25-35 ppt and 27-30 °C resulted in highest survival and settlement. These results indicate that early life stages of H. coerulea are negatively impacted by thermal stress but may be able to survive at reduced salinity. The wider tolerance range of H. coerulea larvae compared to most scleractinian larvae may thus contribute to the success of this coral on disturbed reef ecosystems.
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1.Global warming and overexploitation both threaten the integrity and resilience of marine ecosystems. Many calls have been made to at least partially offset climate change impacts through local conservation management. However, a mechanistic understanding of the interactions of multiple stressors is generally lacking for habitat‐forming species; preventing the development of sound conservation strategies. 2.We examined the effectiveness of no‐take marine protected areas (MPAs) at enhancing structural complexity and resilience to climate change on populations of an overexploited and long‐lived octocoral. We used long‐term data over eight populations, subjected to varying levels of disturbances, and Integral Projection Models to understand how the interaction between overfishing and mass‐mortality events shapes the stochastic dynamics of the Mediterranean red coral Corallium rubrum. 3.MPAs largely reduced colony partial mortality (i.e. shrinkage), enhancing the structural complexity of coral populations. However, there were no significant differences in individual mortality or population growth rates between protected and exploited populations. In contrast, warming had detrimental consequences for the long‐term viability of red coral populations, driving steady declines and potential local extinctions due to sharp effects in survival rates. Stochastic demographic models revealed only a weak compensatory effect of MPAs on the impacts of warming. 4.Policy implications. Our results suggest that marine protected areas (MPAs) are an effective local conservation tool for enhancing the structural complexity of red coral populations. However, MPAs may not be enough to ensure red coral's persistence under future increases in thermal stress. Accordingly, conservation strategies aiming to ensure the persistence and functional role of red coral populations should include management actions at both local (well‐enforced MPAs) and global scales (reductions in greenhouse gas emissions). Finally, this study unravels the divergent demographic consequences that can arise from multiple stressors and highlights the key role of demography in better understanding and predicting the consequences of combined impacts for vulnerable ecosystems. This article is protected by copyright. All rights reserved.
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Predicted increases in sea-surface temperatures due to climate change are likely to alter the physiology of marine organisms and ultimately influence the distribution and abundance of their populations. The consequences of increased temperatures for marine species, including decreased survival and altered rates of development, growth and settlement, are well known and often attributed to imbalances between energy supply and demand. To test this hypothesis, we calibrated the effect of temperature on rates of survival and lipid depletion for larvae of the common stony coral Acropora tenuis over a 7 °C temperature range. Temperature had a pronounced, linearly increasing effect on larval mortality, with a sixfold decrease in median survival time. Contrary to expectation, however, temperature had a quasi-parabolic effect on lipid use; rates declined as temperatures either increased above or decreased below the ambient temperature at the time of spawning. This contrasts with previous work suggesting that increased energy depletion is the cause of larval mortality at higher temperatures. Our results highlight the sensitivity of coral larvae to temperature and have implications for dispersal potential because fewer larvae will survive to disperse. Such projected declines in connectivity among coral populations are likely to undermine reef resilience.
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This study investigates the energetic investment during spawning of two Mediterranean gorgonians characterized by different reproductive strategies: Corallium rubrum (internal brooder) and Paramuricea clavata (surface brooder). Sexual products (number of oocytes and spermatic sacs) were quantified, and biochemical characteristics (lipid content and free fatty acid content and composition) were determined to investigate the parental energetic investment and demand in reproduction. Results suggested that the majority of the energetic cost was due to reproductive activity (i.e., gametogenesis and spawning). The two species exhibited different life history strategies, with P. clavata investing more energy in reproduction than C. rubrum. However, P. clavata is reproductively more sensitive to inter-annual changes in environmental conditions.
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The behaviour of Mediterranean octocoral planulae was studied in light-dark situations and in a light gradient. Larvae of Eunicella singularis (Esper, 1794) reacted photopositively but it is uncertain which mechanism (klinotaxis or klinokinesis) determines this property. The blind larvae probably possess a dermal light sense, but it cannot be excluded that the yolk contains photosensitive carotenoids while the symbiotic zooxanthellae may also play a role. The photopositive behaviour of planulae of this species explains some aspects of the distributional ecology of adult colonies. It was also found that for the induction of settlement and metamorphosis the chemical properties of a given substratum seem to be far more important than its roughness. Larvae of Corallium rubrum (Linnaeus, 1758) are geonegative and indifferent to light. This latter fact is surprising, since in nature the colonies are exclusively found in dark places. It is supposed, therefore, that tolerance of the colonies rather than larval choice determines light-dependent zonation of this species in nature.
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Interactions between organisms are a major determinant of the distribution and abundance of species. Ecology textbooks (e.g., Ricklefs 1984, Krebs 1985, Begon et al. 1990) summarise these important interactions as intra- and interspecific competition for abiotic and biotic resources, predation, parasitism and mutualism. Conspicuously lacking from the list of key processes in most text books is the role that many organisms play in the creation, modification and maintenance of habitats. These activities do not involve direct trophic interactions between species, but they are nevertheless important and common. The ecological literature is rich in examples of habitat modification by organisms, some of which have been extensively studied (e.g. Thayer 1979, Naiman et al. 1988).
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Coralligenous concretions, the unique calcareous formations of biogenic origin in Mediterranean benthic environments, are produced by the accumulation of encrusting algae growing in dim light conditions. This review provides an overview of the results obtained by the main studies dealing with these formations, including the environmental factors which influence the development of coralligenous communities, their distribution, types, assemblages, builders and eroders, the biotic relationships and processes that create and destroy coralligenous assemblages, their dynamics and seasonality, and the functioning of several outstanding and key species. Special attention is devoted to the biodiversity of coralligenous communities and a first estimation of the number of species reported for this habitat is provided. Major disturbances affecting coralligenous communities are discussed, ranging from large-scale events that are probably related to global environmental changes to degradation by waste water or invasive species. Degradation by fishing activities and by divers is also considered. Finally, the main gaps in current scientific knowledge of coralligenous communities are listed and some recommendations are made regarding their protection. © R. N. Gibson, R. J. A. Atkinson, and J. D. M. Gordon, Editors Taylor & Francis.
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The combined effects of disparate impacts on population health are a continuing problem in risk assessment and management for threatened species. Populations of red gorgonian Paramuricea clavata in the NW Mediterranean Sea are threatened both by chronic and localized increases in mortality due to high diving activity and by widespread but episodic mass mortality events related to climatic anomalies. Using demographic data obtained from 3 populations (2 under contrasting levels of diving and one affected by a 1999 mass mortality event), we developed size-structured matrix models to forecast the long-term consequences of both disturbances and their combined effects. When we considered only the effects of diver damage, our results showed that population stability could be achieved with an increase in the annual survival of adult colonies of between 3 and 7%, demonstrating the need for diving reduction in the study locations, where there are estimated to be between 30000 and 70000 dives yr(-1). Modeling the effects of mass mortality events alone showed a low annual population growth rate (0.886) and near certain extinction risk over even short time scales. Considering these 2 types of impacts together, we found that the effects of mass mortality events aggravate the decline in gorgonian populations subjected to high diving impact, reaching the extinction threshold after 36 to 55 yr under the actual frequency of mass mortality events. Simulated reduction of diving effects dramatically increased the time to quasi-extinction for populations subjected to realistic frequencies of mass mortality events. Our simulations reveal the need of management actions to ensure the gorgonian viability in the face of climate change, and suggest that management of more controllable impacts, such as diving, can help buffer populations against the less controllable effects of climate change.
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Satellite observations from 1985-2006 indicate that in the last 2 decades the temperature in the upper layer of the Mediterranean Sea has been increasing at an average (+/- SD) rate of 0.03 +/- 0.008 degrees C yr(-1) for the western basin and 0.05 +/- 0.009 degrees C yr(-1) for the eastern basin. The increases in temperature are not constant throughout the year but occur primarily during May, June and July. Maximum increases of 0.16 degrees C yr(-1) are found in June in the Tyrrhenian, Ligurian and Adriatic Seas and close to the African coast. The Aegean Sea shows maximum change in sea surface temperature during August. Only the statistically significant results are presented.