Bleached Anemones Research Articles

Interspecific competition among anemonefishes: pink anemonefish engage in hostile takeovers of sea anemones occupied by clown anemonefish

AbstractUnderstanding drivers of species co-existence on coral reefs is important to predict community responses to environmental change. Here, we present, for the first time, observations of takeovers of anemones occupied by one species, Amphiprion percula, by another species, A. perideraion, and ask if habitat characteristics, fish characteristics, or environmental conditions predict these occurrences. Comparing survey data from multiple years, we find that takeovers occur at a frequency of 0.48–6.84 per 100 groups/year and that this does not depend on disturbances (anemone bleaching) in the population. Takeovers are not predicted by depth, anemone size, A. percula rank 1 size or the number of A. percula in the original group. Instead, we hypothesize that proximity of the A. percula group to the A. perideraion group is a key predictor. Our study demonstrates how long-term observations of tagged individuals and groups could reveal the drivers and consequences of interspecific conflicts on coral reefs.

Retraction of: 'Reef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selection' (2016) by Scott and Dixson.

Retraction of: 'Reef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selection' (2016) by Scott and Dixson.

Anemone bleaching impacts the larval recruitment success of an anemone-associated fish

In marine environments, mutualisms such as those between corals or sea anemones and their algal symbionts (Symbiodiniaceae) play a key role for supporting surrounding biodiversity. However, as the breakdown of the mutualism between corals and/or anemones and Symbiodiniaceae (i.e. bleaching) become increasingly frequent and severe, the risk of losing the additional species that rely on them may also increase. While the effects of anemone bleaching on the biology and ecology of anemone-associated fishes have been the subject of recent research, relatively little is known about the impacts that anemone bleaching might have on the recruitment of larval fish. Here, we report that climate change-induced anemone bleaching impairs a secondary mutualism between anemones and an anemone-associated fish species, the threespot dascyllus (Dascyllus trimaculatus). Field-based monitoring over a 1-year period showed anemones that bleached experienced decreased recruitment of larval D. trimaculatus compared to those that did not bleach, with abundances of newly settled D. trimaculatus three times lower in bleached versus unbleached anemones. A visual choice experiment showed that this pattern is associated with fish being less attracted to bleached anemones, and a predation experiment demonstrated that fish associated with bleached anemones experienced higher mortality compared to those associated with unbleached anemones. These results suggests that the decreased recruitment of D. trimaculatus observed in bleached anemones may be driven by hampered pre-settlement (habitat selection) and post-settlement (survival to predation) processes for larval D. trimaculatus in bleached hosts. This study highlights the risk of cascading mutualism breakdowns in coral reefs as conditions deteriorate and stresses the importance of protecting these mutualisms for the maintenance of coral reef biodiversity.

Correction to: 'Reef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selection'.

Open AccessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Scott Anna and Dixson Danielle L. 2022Correction to: ‘Reef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selection’Proc. R. Soc. B.2892022121720221217http://doi.org/10.1098/rspb.2022.1217SectionOpen AccessCorrectionCorrection to: ‘Reef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selection’ Anna Scott Anna Scott Google Scholar Find this author on PubMed Search for more papers by this author and Danielle L. Dixson Danielle L. Dixson http://orcid.org/0000-0003-1493-1482 Google Scholar Find this author on PubMed Search for more papers by this author Anna Scott Anna Scott Google Scholar Find this author on PubMed Search for more papers by this author and Danielle L. Dixson Danielle L. Dixson http://orcid.org/0000-0003-1493-1482 Google Scholar Find this author on PubMed Search for more papers by this author Published:08 July 2022https://doi.org/10.1098/rspb.2022.1217This article corrects the followingResearch ArticleReef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selectionhttps://doi.org/10.1098/rspb.2015.2694 Anna Scott and Danielle L. Dixson volume 283issue 1831journalTitle Proceedings of the Royal Society B: Biological Sciences25 May 2016Proc. R. Soc. B283, 20152694 (Published online 25 May 2016) (https://doi.org/10.1098/rspb.2015.2694)The published article [1] requires a correction. First, the experimental dates listed in the materials and methods section were incorrectly reported. The correct experimental timeframe during which we recorded host selection using two flumes was 5 October–7 November 2014. Of note, this error does not impact or change any of the results or conclusions of our work.Second, a summary table rather than the raw dataset was uploaded to a public repository. This has been corrected, and the raw data are available at https://doi.org/10.5281/zenodo.6565204.Additionally, we would like to clarify that the strong treatment effects found in this experiment are likely the result of a combination of factors including, but not limited to, the ecology of the focal species, the flume apparatus, the chemical comparisons being tested as well as the concentration of the chemical cues tested. The chemical cue concentration is likely higher than the organisms would experience in nature, and therefore likely a supernormal stimulus. Simply put, supernormal stimuli are bigger and more intense than normal, and elicit a larger than normal response from the animal [2]. Here, the naturally occurring olfactory cues indicate habitat; the heightened preference when the stimulus is offered at an intense concentration follows this behavioural pattern. The research presented in this study purposefully used strong chemical cues to determine if chemical cues are used in habitat selection, rather than determining a detection threshold for this species of a concentration gradient.Footnotes© 2022 The Authors.Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Just keep swimming: Long-distance mobility of tomato clownfish following anemone bleaching.

Just keep swimming: Long-distance mobility of tomato clownfish following anemone bleaching.

Deep Heat: A Comparison of Water Temperature, Anemone Bleaching, Anemonefish Density and Reproduction between Shallow and Mesophotic Reefs

French Polynesia is experiencing increasing coral bleaching events in shallow waters triggered by thermal anomalies and marine heatwaves linked to climate change, a trend that is replicated worldwide. As sea surface thermal anomalies are assumed to lessen with depth, mesophotic deep reefs have been hypothesized to act as refuges from anthropogenic and natural disturbances, the ‘deep reef refugia hypothesis’ (DRRH). However, evidence supporting the DRRH is either inconclusive or conflicting. We address this by investigating four assumptions of the DRRH focusing on the symbiotic association between anemones and anemonefish. First, we compare long-term temperature conditions between shallow (8 m) and mesophotic sites (50 m) on the island of Moorea from 2011–2020. Second, we compare the densities of the orange-fin anemonefish, Amphiprion chrysopterus between shallow and mesophotic (down to 60 m) reefs across three archipelagos in French Polynesia. Finally, we compare the percentage of anemone bleaching, as well as anemonefish reproduction, between shallow and mesophotic reefs. We found that the water column was well mixed in the cooler austral winter months with only a 0.19 °C difference in temperature between depths, but in the warmer summer months mixing was reduced resulting in a 0.71–1.03 °C temperature difference. However, during thermal anomalies, despite a time lag in warm surface waters reaching mesophotic reefs, there was ultimately a 1.0 °C increase in water temperature at both 8 and 50 m, pushing temperatures over bleaching thresholds at both depths. As such, anemone bleaching was observed in mesophotic reefs during these thermal anomalies, but was buffered compared to the percentage of bleaching in shallower waters, which was nearly five times greater. Our large-scale sampling across French Polynesia found orange-fin anemonefish, A. chrysopterus, in mesophotic zones in two high islands and one atoll across two archipelagos, extending its bathymetric limit to 60 m; however, orange-fin anemonefish densities were either similar to, or 25–92 times lower than in shallower zones. Three spawning events were observed at 50 m, which occurred at a similar frequency to spawning on shallower reefs at the same date. Our findings of thermal anomalies and bleaching in mesophotic reefs, coupled with mainly lower densities of anemonefish in mesophotic populations, suggest that mesophotic reefs show only a limited ability to provide refugia from anthropogenic and natural disturbances.

Prevalence and Photobiology of Photosynthetic Dinoflagellate Endosymbionts in the Nudibranch Berghia stephanieae.

Simple SummarySome sea slugs have evolved highly specialized feeding habits and solely prey upon a reduced number of species. This is the case of Berghia stephanieae, a sea slug that feeds exclusively on glass anemones, such as Exaiptasia diaphana. Glass anemones host photosynthetic microalgae that B. stephanieae ingest when preying upon E. diaphana. The association between these photosynthetic microalgae and sea slugs appears to be limited in time, particularly if B. stephanieae is deprived of prey hosting these microalgae. In the present study, we validate the use of a non-invasive and non-destructive approach that allows monitoring the persistence of this association in live sea slugs by measuring chlorophyll fluorescence. A complete loss of photosynthetic microalgae was observed within 8 days when animals were deprived of food or fed glass anemones with no microalgae (bleached anemones). As such, the association between B. stephanieae and photosynthetic microalgae acquired when preying glass anemones is not a true symbiosis. Future studies may use the technique here described to monitor the prevalence of the association between sea slugs and photosynthetic microalgae, particularly under bleaching events that will impair sea slugs to acquire microalgae by preying upon their invertebrate hosts.Berghia stephanieae is a stenophagous sea slug that preys upon glass anemones, such as Exaiptasia diaphana. Glass anemones host photosynthetic dinoflagellate endosymbionts that sea slugs ingest when consuming E. diaphana. However, the prevalence of these photosynthetic dinoflagellate endosymbionts in sea slugs appears to be short-lived, particularly if B. stephanieae is deprived of prey that host these microalgae (e.g., during bleaching events impacting glass anemones). In the present study, we investigated this scenario, along with food deprivation, and validated the use of a non-invasive and non-destructive approach employing chlorophyll fluorescence as a proxy to monitor the persistence of the association between sea slugs and endosymbiotic photosynthetic dinoflagellates acquired through the consumption of glass anemones. Berghia stephanieae deprived of a trophic source hosting photosynthetic dinoflagellate endosymbionts (e.g., through food deprivation or by feeding on bleached E. diaphana) showed a rapid decrease in minimum fluorescence (Fo) and photosynthetic efficiency (Fv/Fm) when compared to sea slugs fed with symbiotic anemones. A complete loss of endosymbionts was observed within 8 days, confirming that no true symbiotic association was established. The present work opens a new window of opportunity to rapidly monitor in vivo and over time the prevalence of associations between sea slugs and photosynthetic dinoflagellate endosymbionts, particularly during bleaching events that prevent sea slugs from incorporating new microalgae through trophic interactions.

Ocean temperature, but not acidification, causes sea anemone bleaching under a near-future climate scenario

Climate change is causing ocean temperature and partial pressure of carbon dioxide (pCO2) to increase. For sea anemones that have Symbiodiniaceae, high temperatures induce bleaching, whereas rises in pCO2 can enhance photosynthesis and increase host growth and abundance. It is, however, not clear how the interaction of these two stressors impacts sea anemones that provide habitat for anemonefishes. Here, we investigated the bleaching response of the sea anemone Entacmaea quadricolor, under four conditions: (i) current temperature and current pCO2 (control); (ii) future pCO2; (iii) future temperature; and (iv) future temperature and future pCO2. After 16 days of exposure, future temperature, but not pCO2 nor their interaction, significantly reduced the Symbiodiniaceae density and total chlorophyll Symbiodiniaceae cell−1. Colour score was lower in the sea anemones exposed to future temperature than current temperature from day 4 onwards. In contrast, total chlorophyll symbiont cell−1 increased in the future temperature treatments, and light-adapted effective quantum yield remained similar in all treatments. Although pCO2 had no impact within the time frame of our experiment, the predicted future temperature induced bleaching in E. quadricolor. As bleaching events increase in frequency and severity, this will likely impact the abundance of host sea anemones and their symbiotic anemonefishes.

Physiological and behavioural effects of anemone bleaching on symbiont anemonefish in the wild

Abstract Climate change causes extreme heat waves that have induced worldwide mass coral bleaching. The impacts of temperature‐induced bleaching events on the loss of algal endosymbionts in both corals and anemones are well documented. However, the cascading impacts of bleaching on animals that live in association with corals and anemones are understudied. We performed a field‐based experiment to investigate how host anemone bleaching affected the metabolic rate, growth, behaviour and survival of wild juvenile orange‐fin anemonefish Amphiprion chrysopterus over 1, 2 and (for survival) 9 months. We found that the standard metabolic rate of anemonefish residing in bleached anemones decreased over time but was unaffected in fish from healthy anemones. Despite the reduced metabolic cost, the growth rate of fish from bleached anemones was significantly lower compared to fish from healthy anemones, suggesting that animals residing in bleached hosts are at an energetic disadvantage. This was corroborated by our finding that fish from bleached anemones spent more time out of their anemones, suggestive of a greater need to forage in the water column. However, fish from bleached anemones were overall less active and used less space around the anemone, resulting in a negative correlation between space use and survival after 4 weeks. Our results provide insight into the physiological and behavioural effects of host bleaching on juvenile fish in the wild, and highlight how relatively short‐term thermal anomalies can have long‐lasting impacts beyond the bleached anemones or corals themselves. A free Plain Language Summary can be found within the Supporting Information of this article.

Physiological and behavioural effects of anemone bleaching on symbiont anemonefish in the wild

Physiological and behavioural effects of anemone bleaching on symbiont anemonefish in the wild

Five-year study on the bleaching of anemonefish-hosting anemones (Cnidaria: Anthozoa: Actiniaria) in subtropical Okinawajima Island

Since the 1980s, coral bleaching mainly due to rising seawater temperatures has become a serious problem for subtropical and tropical coral reefs around the world. Coral bleaching and the associated loss of corals have a negative impact on taxonomic richness of creatures. Bleaching is seen not only in hard corals (order Scleractinia), but also in other zooxanthellate animals, including anemonefish-hosting anemones (order Actiniaria), where it is known to significantly impact symbiotic anemonefish as well as other organisms. However, to date, there have been few continuous studies on bleaching and its effects on anemones and anemonefish. Therefore, in this study, we recorded the water temperature and the status of bleaching for individual host anemones and symbiotic anemonefish on the western coast of Okinawajima Island over five years from November 2014 to October 2019. In the summers of 2016 and 2017, when abnormally higher water temperatures were observed during ENSO events, five out of 30 host anemones bleached. All individuals of host anemone that bleached in 2016 again bleached in 2017, and these anemones took longer to recover in 2017 than in 2016. Anemonefish pairs inhabiting the five bleached host anemones survived these bleaching periods; however, one of the bleached anemones along with its associated anemonefish disappeared subsequent to the 2017 bleaching. The present study indicates that if abnormally high seawater temperatures continue in consecutive summers, recovery of host anemones from bleaching may become impaired, and associated mortality events may increase.

Chemical Communication in the Symbiotic Interaction between the anemone Exaiptasia diaphana (ex Aiptasia pallida) Rapp and the Dinoflagellate Symbiodinium spp.

The dinoflagellate Symbiodinium sp. establishes symbiotic relationships with the anemone Exaiptasia diaphana Rapp. The process that leads to the reciprocal recognition of the two symbiotic partners is still not very well understood. We hypothesize that chemical signals are exchanged between the Exaiptasia diaphana-Symbiodinium holosymbiont and aposymbiotic anemones or between free living Symbiodinium and holo- and aposymbiotic Exaiptasia, leading to changes in organic and elemental compositions in the aposymbiotic anemones. In order to test these hypotheses, bleached anemones were exposed to the presence of either free living Symbiodinium, previously extracted from the same Exaiptasia clone, or to holobionts. The ex-hospite algae and the holobionts were included in dialyses membranes with a cut-off of 14 000 Da. In the control treatments, the experimental samples were exposed to the presence of empty dialysis tubes. The organic composition and the elemental composition of the anemones were determined by Fourier Transform Infrared Spectroscopy and Total Reflection X-Ray Fluorescence Spectroscopy/Gas Chromatography, respectively. The fact that both the organic and elemental composition of the experimental aposymbiotic anemones differed significantly from the controls, in the absence of any obvious nutritional effect, is suggestive of an exchange of chemical signals between the aposymbiotic and holosymbiotic anemones.

Live imaging of Aiptasia larvae, a model system for coral and anemone bleaching, using a simple microfluidic device

Coral reefs, and their associated diverse ecosystems, are of enormous ecological importance. In recent years, coral health has been severely impacted by environmental stressors brought on by human activity and climate change, threatening the extinction of several major reef ecosystems. Reef damage is mediated by a process called ‘coral bleaching’ where corals, sea anemones, and other cnidarians lose their photosynthetic algal symbionts (family Symbiodiniaceae) upon stress induction, resulting in drastically decreased host energy harvest and, ultimately, coral death. The mechanism by which this critical cnidarian-algal symbiosis is lost remains poorly understood. The larvae of the sea anemone, Exaiptasia pallida (commonly referred to as ‘Aiptasia’) are an attractive model organism to study this process, but they are large (∼100 mm in length, ∼75 mm in diameter), deformable, and highly motile, complicating long-term imaging and limiting study of this critical endosymbiotic relationship in live organisms. Here, we report ‘Traptasia’, a simple microfluidic device with multiple traps designed to isolate and image individual, live larvae of Aiptasia and their algal symbionts over extended time courses. Using a trap design parameterized via fluid flow simulations and polymer bead loading tests, we trapped Aiptasia larvae containing algal symbionts and demonstrated stable imaging for >10 hours. We visualized algae within Aiptasia larvae and observed algal expulsion under an environmental stressor. To our knowledge, this device is the first to enable time-lapsed, high-throughput live imaging of cnidarian larvae and their algal symbionts and, in further implementation, could provide important insights into the cellular mechanisms of cnidarian bleaching under different environmental stressors. The ‘Traptasia’ device is simple to use, requires minimal external equipment and no specialized training to operate, and can easily be adapted using the trap optimization data presented here to study a variety of large, motile organisms.

Response of bleached and symbiotic sea anemones to plastic microfiber exposure

Microplastics are emerging contaminants in the marine environment. They enter the ocean in a variety of sizes and shapes, with plastic microfiber being the prevalent form in seawater and in the guts of biota. Most of the laboratory experiments on microplastics has been performed with spheres, so knowledge on the interactions of microfibers and marine organisms is limited. In this study we examined the ingestion of microfibers by the sea anemone Aiptasia pallida using three different types of polymers: nylon, polyester and polypropylene. The polymers were offered to both symbiotic (with algal symbionts) and bleached (without algal symbionts) anemones. The polymers were introduced either alone or mixed with brine shrimp homogenate. We observed a higher percentage of nylon ingestion compared to the other polymers when plastic was offered in the absence of shrimp. In contrast, we observed over 80% of the anemones taking up all types of polymers when the plastics were offered in the presence of shrimp. Retention time differed significantly between symbiotic and bleached anemones with faster egestion in symbiotic anemones. Our results suggest that ingestion of microfibers by sea anemones is dependent both on the type of polymers and on the presence of chemical cues of prey in seawater. The decreased ability of bleached anemones to reject plastic microfiber indicates that the susceptibility of anthozoans to plastic pollution is exacerbated by previous exposure to other stressors. This is particularly concerning given that coral reef ecosystems are facing increases in the frequency and intensity of bleaching events due to ocean warming.

Contrasting Antibacterial Capabilities of the Surface Mucus Layer From Three Symbiotic Cnidarians

Coral reefs are affected by the deterioration of the oceans due to global warming and other anthropogenic perturbations, increasing the frequency and severity of bleaching and disease. To overcome some of these conditions, reef corals and other cnidarians rely on a mucus layer housing a diverse community of beneficial microorganisms and mechanisms of innate immune response. The antimicrobial defense has been associated with the bacterial community in these organisms, but the potential antimicrobial activity of the mucus layer itself has not been explored fully. We hypothesized that the bacteria-free mucus layer of different cnidarians would show differential and specific antimicrobial and immunological responses when challenged with two potentially pathogenic bacteria. We evaluated this capability through antimicrobial properties, immune response and biochemical composition of the mucus. Results clearly showed that the mucus of healthy cnidarians has the capability of inhibiting the growth of Serratia marcescens and Aurantimonas sp. in a species-specific way that includes differences in the potency of the response. The anemone Exaiptasia pallida was particularly potent against Aurantimonas sp. while the coral Pseudodiploria strigosa and the medusa Cassiopea xamachana had similar capabilities against both bacterial strains. In coral affected by black band disease, this antibacterial capability diminished in the mucus layer, but the associated bacteria remained potent. Results showed that hydroxyproline and phenoloxidase increased in the transition zone of diseased corals, although melanin was not detected in any of the animals tested. Bleaching of anemones and medusas also diminished the antibacterial capability of the surface mucus layer, but in anemones, the associated bacteria did not show a significant reduction in their ability to inhibit the growth of the bacterial strains. The mucus of bleached medusas showed an increased inhibitory activity against Aurantimonas sp. that may be associated with a specific bacterial strain we isolated. Mucus collected from bleached anemones and medusas did not show a significant immune response. In this work, we show that the surface mucus layer itself has antibacterial properties not associated with the bacteria this layer houses; such properties diminished due to disease or bleaching, while immunological responses increased in the mucus of diseased animals.

Anemone bleaching increases the metabolic demands of symbiont anemonefish

Increased ocean temperatures are causing mass bleaching of anemones and corals in the tropics worldwide. While such heat-induced loss of algal symbionts (zooxanthellae) directly affects anemones and corals physiologically, this damage may also cascade on to other animal symbionts. Metabolic rate is an integrative physiological trait shown to relate to various aspects of organismal performance, behaviour and locomotor capacity, and also shows plasticity during exposure to acute and chronic stressors. As climate warming is expected to affect the physiology, behaviour and life history of animals, including ectotherms such as fish, we measured if residing in bleached versus unbleached sea anemones (Heteractis magnifica) affected the standard (i.e. baseline) metabolic rate and behaviour (activity) of juvenile orange-fin anemonefish (Amphiprion chrysopterus). Metabolic rate was estimated from rates of oxygen uptake [Formula: see text], and the standard metabolic rate [Formula: see text] of anemonefish from bleached anemones was significantly higher by 8.2% compared with that of fish residing in unbleached anemones, possibly due to increased stress levels. Activity levels did not differ between fish from bleached and unbleached anemones. As [Formula: see text] reflects the minimum cost of living, the increased metabolic demands may contribute to the negative impacts of bleaching on important anemonefish life history and fitness traits observed previously (e.g. reduced spawning frequency and lower fecundity).

Cascading effects of thermally-induced anemone bleaching on associated anemonefish hormonal stress response and reproduction

Organisms can behaviorally, physiologically, and morphologically adjust to environmental variation via integrative hormonal mechanisms, ultimately allowing animals to cope with environmental change. The stress response to environmental and social changes commonly promotes survival at the expense of reproduction. However, despite climate change impacts on population declines and diversity loss, few studies have attributed hormonal stress responses, or their regulatory effects, to climate change in the wild. Here, we report hormonal and fitness responses of individual wild fish to a recent large-scale sea warming event that caused widespread bleaching on coral reefs. This 14-month monitoring study shows a strong correlation between anemone bleaching (zooxanthellae loss), anemonefish stress response, and reproductive hormones that decreased fecundity by 73%. These findings suggest that hormone stress responses play a crucial role in changes to population demography following climate change and plasticity in hormonal responsiveness may be a key mechanism enabling individual acclimation to climate change.

Reef fishes can recognize bleached habitat during settlement: sea anemone bleaching alters anemonefish host selection.

Understanding how bleaching impacts the settlement of symbiotic habitat specialists and whether there is flexibility in settlement choices with regard to habitat quality is essential given our changing climate. We used five anemonefishes (Amphiprion clarkii, Amphiprion latezonatus, Amphiprion ocellaris, Amphiprion percula and Premnas biaculeatus) and three host sea anemones (Entacmaea quadricolor, Heteractis crispa and Heteractis magnifica) in paired-choice flume experiments to determine whether habitat naive juveniles have the olfactory capabilities to distinguish between unbleached and bleached hosts, and how this may affect settlement decisions. All anemonefishes were able to distinguish between bleached and unbleached hosts, and responded only to chemical cues from species-specific host anemones irrespective of health status, indicating a lack of flexibility in host use. While bleached hosts were selected as habitat, this occurred only when unbleached options were unavailable, with the exception of A. latezonatus, which showed strong preferences for H. crispa regardless of health. This study highlights the potential deleterious indirect impacts of declining habitat quality during larval settlement in habitat specialists, which could be important in the field, given that bleaching events are becoming increasingly common.

Thermal and menthol stress induce different cellular events during sea anemone bleaching

Cnidarian-dinoflagellate symbiosis disruption and subsequent bleaching are major concerns, especially regarding their ecological consequences on coral reefs and temperate coralligenous communities. Cnidarian bleaching is caused by a variety of environmental stressors, such as elevated seawater temperature associated with global climate change, and by pollutants, such as herbicides and metals. Several cellular events have been described to explain symbiosis dysfunction and bleaching. Excess or damaged Symbiodinium symbionts are removed through a variety of mechanisms, including exocytosis, apoptosis, necrosis and autophagy. However, few studies have compared in the same species the relative involvement of these mechanisms, according to the stress inducing the bleaching. In this study, we used two different treatments —temperature and menthol— to induce bleaching in the sea anemone Anemonia viridis. By monitoring the ultrastructural tissue modifications, in control specimens we observed a basal rate of in situ symbiont digestion —or symbiophagy— induced by starvation. Symbiophagy was strongly induced in menthol-treated specimens and was the main cellular process of bleaching, whereas apoptosis and necrosis predominated in hyperthermal-induced bleaching. These results suggested a host effect through autophagy in menthol-treated specimens. These observations also suggested that symbiont removal may result from reengagement of the phagosomal maturation process in the host. These overall data demonstrate that several Symbiodinium cell removal mechanisms coexist and that stressors can activate one or more of these pathways, depending on the stress type, intensity or duration.

Habitat bleaching disrupts threat responses and persistence in anemonefish

Climate-induced habitat bleaching is linked to dramatic declines in diversity and abundance of coral reef fish; however, mechanisms underlying these declines are poorly understood. Here, we used in situ studies to demonstrate that bleaching can influence persistence of reef fish by affecting behaviours, including responses to a potential predation threat. When encountering the predatory rock cod Cephalophalis cyanostigma, anemonefish Amphiprion akindynos occupying healthy unbleached host anemones Heteractis crispa respond by feeding less and spending more time within the anemone tentacles. When the host anemone was experimentally bleached, these visual risk responses were compromised: A. akindynos continued to feed and did not seek shelter. The impaired behavioural response may prove detrimental to anemonefish populations as abundance levels of fish on bleached anemones was reduced by 60% within 3 d, which may have been the result of increased predation. Our data illustrate how climate-induced habitat degradation can drive declines of reef fish by potentially altering outcomes of predator–prey interactions.