Multiple marine invertebrate species have been utilized as model organisms for investigations on the effects of climate change and bleaching on animal-algal symbioses. However, one potential model host that has not been addressed is the acoel Convolutriloba retrogemma, which associates with chlorophytes. Therefore, this study assessed whether thermal stress reduces survival, size, symbiont density and chlorophyll-a concentration for C. retrogemma. Acoels were subjected to an experiment to examine the combined effects of temperature (26, 28 and 30 degrees C) and exposure (1, 2 and 4 weeks) on each of the four variables. The population size and total length of acoels were monitored, followed by sampling and maceration for quantifying symbionts and extracting chlorophyll-a, which was measured using a fluorometer. Results showed significant acoel mortality at both 28 and 30 degrees C, with populations decreasing by 50 % and 100 %, respectively, after four weeks. Length did not differ among conditions. Acoels underwent bleaching as symbiont density decreased at 30 degrees C, and chlorophyll-a content decreased at both 28 and 30 degrees C. In the current context of climate change, the association between C. retrogemma and Tetraselmis sp. appears to be considerably fragile compared to other model organisms. With the predicted increase in marine heatwaves, this holobiont may be severely threatened.

Giant clams are photosymbiotic molluscs, hosting Symbiodiniaceae dinoflagellates. Serving as an alternative model organism for ecophysiological studies within reef environments, giant clams differ from corals due to their anatomical complexity, with extracellular symbionts present in multiple organs. We aimed to determine if clams, under thermal stress, exhibit symbiont shuffling both at the organism level and across individual organs. Therefore, the fluted giant clam, Tridacna squamosa, was exposed to control and heat-stress temperatures of 26 and 30 ºC, respectively, for 45 days. Subsequently, the degree of bleaching was assessed through quantification of symbiont cells and chlorophyll-a loss via fluorometric detection and photometric analysis. The relative composition of Symbiodiniaceae ITS2 rDNA profiles across ten different organs was determined using metabarcoding by next-generation sequencing. Findings show that the outer mantle of heat-stressed clams lost approximately 30

Mobulidae is a monophyletic family within the Myliobatiformes that comprises pelagic species represented by manta and devil rays. Among the genus Mobula, the Atlantic Pygmy Devil Ray - Mobula hypostoma - is reported in coastal regions exclusively in tropical and subtropical Atlantic Ocean from 1 to 100 m deep. In Brazil, M. hypostoma is one of the least studied Mobula species. It is regularly misidentified, especially as Mobula thurstoni, and is commonly listed as bycatch, in fishery inventories, or related to opportunistic sightings in the national territory. Here, we describe the complete nucleotide sequence of the mitochondrial genome (mitogenome) from Mobula hypostoma, which is 18,141 bp in length and comprises 13 protein-coding, two ribosomal RNA, and 22 transfer RNA genes. The M. hypostoma mitochondrial genes organisation and mitochondrial genome length are similar to other Mobula species, and the phylogenetic reconstruction indicates M. hypostoma as closely related to Mobula munkiana. The Brazilian mitogenome of M. hypostoma is expected to be a valuable resource for molecular-based species identification, and evolutionary and phylogeography studies.

The influence of abiotic variables and anthropogenic pressure on symbiodiniaceans associated with the scleractinian corals Mussismilia hispida and Siderastrea stellata were assessed quarterly at Arma & ccedil;a o dos B & uacute;zios, Brazil, for over 18 months. Thirty-eight Symbiodiniaceae ITS2 rDNA phylotypes were found by metabarcoding, with eight comprising new phylotypes. Both hosts maintained their generalist pattern, with 1 -3 dominant lineages. An environmental pressure index and changes in seawater temperature explained the variations in the structure and diversity of Symbiodiniaceae assemblages over time and space. A mild bleaching event affected the photosymbiotic assemblage structure, even in non -bleached colonies. The highly dynamic and diverse photosymbiont assemblages were constantly driven by the influence of environmental variables and human -induced impacts. Furthermore, new strains of Symbiodiniaceae might be associated with lower temperatures caused by upwelling, which is characteristic of this subtropical coral community, highlighting the region 's idiosyncrasy and the need for further studies of this coral system.

Mass bleaching events are growing in duration and intensity. Besides causing extensive mortality, the progressively shorter time between events disrupts the ability of reefs to recover. The unique reefs of the Southwestern Atlantic are often considered climate refugia as they have suffered less bleaching-related mortality when compared to Indo–Pacific and Caribbean reefs. However, their recovery capacity still requires investigation. In 2019, an unprecedented heatwave triggered the most severe bleaching episode recorded for Southwestern Atlantic reefs. Therefore, this study aimed to (i) document the bleaching incidence and mortality during the heatwave, and (ii) assess coral recovery over 3 years. We measured bleaching incidence and monitored coral cover through surveys in three Southern Bahia (central Brazilian coast) reefs before, during and after thermal stress. Our findings show that coral assemblages were exposed to a 5-month-long thermal anomaly, experiencing thermal stress peaking at 14.1 ºC-weeks. Roughly 70

Reef corals have been threatened by climate change, with more frequent and intense bleaching events leading to extensive coral mortality and loss of coral cover worldwide. In the face of this, the corals’ photosymbiont assemblages have received special attention as a key to better understand the bleaching process and its recovery. To assess the effects of thermal anomalies, the coral Mussismilia harttii and the hydrocoral Millepora alcicornis were monitored through the El Niño 2015/2016 at a Southwestern Atlantic (SWA) coral reef. A severe bleaching event (57% of colonies bleached) was documented, triggered by a < 3 °C-week heatwave, but no mortality was detected. The hydrocoral was more susceptible than the scleractinian, displaying bleaching symptoms earlier and experiencing a longer and more intense bleaching event. The composition of photosymbionts in the M. alcicornis population was affected only at the rare biosphere level (< 5% relative abundance), with the emergence of new symbionts after bleaching. Conversely, a temporary dysbiosis was observed in the M. harttii population, with one of the dominant symbiodiniaceans decreasing in relative abundance at the peak of the bleaching, which negatively affected the total β-diversity. After colonies’ complete recovery, symbiodiniaceans’ dominances returned to normal levels in both hosts. These results highlight critical differences in how the two coral species cope with bleaching and contribute to the understanding of the role of photosymbionts throughout the bleaching-recovery process.

Intertidal environments and shallow reefs are subject to natural disturbances and impacts by humans that influence their dynamics and therefore their conservation. These environments are among the most subject to anthropogenic impacts worldwide, ranging from global warming to local pollution and disordered tourism. Fernando de Noronha Marine National Park is an important Marine Protected Area that covers marine habitats up to 50 m deep of an oceanic archipelago off North East Brazil. It is a no-take area, so the main potential impacts in intertidal and shallow environments there are related to tourism, pollution and climate change. One of the chief tourist attractions in this Park is a tide pool where snorkelers can dive under a series of regulations. We monitored corals and benthic organisms in this tide pool for two years and evaluated their response to different disturbances: burial by sand, temperature anomalies, and tourism. We observed high temporal variability in the benthic cover in the Atalaia pool, with an important role of the sand being naturally brought by waves coming from the southeast to south-southeast direction on a seasonal basis. Despite the highly variable environment of this pool leading to periodic visual coral cover decrease when sand accumulated, coral health was not significantly affected neither by burial nor by the number of visitors accessing the pool. However, sea temperature anomaly played a role, being the main environmental driver affecting coral health, leading to increased paleness, bleaching and unhealthy purple/pink color and causing changes on the associated symbionts. Regardless of the local scope of this monitoring, this is an interesting study case to be used by other marine parks, explicitly showing how biological monitoring can aid the evaluation and potential adjustments to management strategies.(c) 2023 Elsevier B.V. All rights reserved.

The symbiotic relationship between corals and symbiodiniaceans can favor reef formation, but is easily rupted when these organisms are exposed to thermal anomalies. Here, we assessed the ITS2 rDNA phylotype diversity of dominant Symbiodiniaceae lineages associated with the hydrocoral Millepora alcicornis and investigated host–symbiont distribution patterns in the Atlantic Ocean. This is the first effort to assess the symbiont community of this hydrocoral over nearly its entire distributional range. Millepora alcicornis is highly generalist in the composition of its photosymbiont community. We found 16 ITS2 phylotypes, mainly of the genus Breviolum but also of the genera Symbiodinium and Cladocopium ; nine of them are new lineages. The distribution patterns of the M. alcicornis –Symbiodiniaceae associations were explained by differences in primary productivity, photosynthetically active radiation, water turbidity, and temperature. Six geographic sections were identified. Colonies from the Brazilian Northeastern Region showed the most stable associations, with two Breviolum phylotypes, while those from the Brazilian Eastern Region showed the most diverse symbiont community, composed of three genera of Symbiodiniaceae. A new and dominant phylotype of Breviolum was identified in the Brazilian Southern Region. Our results suggest a radiation of Breviolum lineages associated with M. alcicornis through the Atlantic Ocean. The impressive diversity of symbiotic associations observed characterizes an adaptable host–symbiont relationship, which can be key for colonization of new habitats and the resilience of milleporids to environmental changes.

Bleaching events are becoming more frequent and intense worldwide. Southwestern Atlantic corals are considered stress-tolerant species that suffer less during bleaching episodes. Therefore, we investigated if the reef-building corals Mussismilia hispida and M. harttii are capable of spawning while fully bleached. We compared spermatozoa concentration and motility, egg diameter and embryo viability between seemingly healthy and bleached colonies for both species. Findings show that both species spawned viable gametes. Concentration and motility of newly-released spermatozoa were similar between healthy and bleached colonies for both species. Unlike M. hispida, a 10% reduction in size was found in egg diameter for M. harttii. Embryo viability for bleached M. hispida colonies did not decrease; for bleached M. harttii there was a reduction from 14.7 to 6.3%, but fertilization and larval development still took place. These findings describe the unprecedented episode of corals spawning viable gametes when entirely bleached, supporting the suggestion of greater resilience for Southwestern Atlantic corals.

Threatened by global warming and extreme climatic events, such as El Niño Southern Oscillation (ENSO) and Marine Heatwaves (MHW), coral reefs worldwide faced the worst bleaching and mortality event between 2014 and 2017, induced by the 2015/2016 ENSO. We evaluated the impacts of ENSO and MHW episodes on bleaching and mortality frequencies of Siderastrea stellata at Rocas Atoll, Southwestern Atlantic, using visual censuses conducted in 2016, 2017 and 2019. Bleaching rate varied significantly along the sampling period (11.71% in 2016, 1.52% in 2017, and 88% in 2019), but mortality was always less than 4%. Bleaching events in Atlantic reefs have been constantly associated with ENSO, until these recent events of the last two years. We suggest that MHW were probably the primary driver of the observed bleaching, especially in 2019, when much higher bleaching rates were observed than in ENSO periods. Although Southwestern Atlantic massive corals are considered more resistant to thermal stress than reefs corals worldwide, the strong events registered since 2019 highlight the need for continuous monitoring to better understand coral bleaching dynamics and improve predictions on the effects of global change in the region.

Mass coral bleaching has increased in intensity and frequency and has severely impacted shallow tropical reefs worldwide. Although extensive investigation has been conducted on the resistance and resilience of coral reefs in the Indo-Pacific and Caribbean, the unique reefs of the South Atlantic remain largely unassessed. Here we compiled primary and literature data for reefs from three biogeographical regions: Indo-Pacific, Caribbean and South Atlantic and performed comparative analyses to investigate whether the latter may be more resistant to bleaching. Our findings show that South Atlantic corals display critical features that make them less susceptible to mass coral bleaching: (i) deeper bathymetric distribution, as species have a mean maximum depth of occurrence of 70 m; (ii) higher tolerance to turbidity, as nearly 60% of species are found in turbid conditions; (iii) higher tolerance to nutrient enrichment, as nitrate concentration in the South Atlantic is naturally elevated; (iv) higher morphological resistance, as massive growth forms are dominant and comprise two thirds of species; and (v) more flexible symbiotic associations, as 75% of corals and 60% of symbiont phylotypes are generalists. Such features were associated with occurrence of fewer bleaching episodes with coral mortality in the South Atlantic, approximately 60% less than the Indo-Pacific and 50% less than the Caribbean. In addition, no mass coral mortality episodes associated with the three global mass bleaching events have been reported for the South Atlantic, which suffered considerably less bleaching. These results show that South Atlantic reefs display several remarkable features for withstanding thermal stress. Together with a historic experience of lower heat stress, our findings may explain why climate change impacts in this region have been less intense. Given the large extension and latitudinal distribution of South Atlantic coral reefs and communities, the region may be recognized as a major refugium and likely to resist climate change impacts more effectively than Indo-Pacific and Caribbean reefs.

Phycosphere is the region where algal exudates influence microbial communities. Since the term was coined, research has been focused on understanding free-living-phytoplankton and bacteria interactions. Coral-Symbiodiniaceae symbiosis and its associated microbiomes have also been widely studied due to the increasing number of global and local threats endangering coral reefs worldwide. Nevertheless, little is known about the specific interactions between Symbiodiniaceae and bacterial communities within a coral host. In this review, we focus on Symbiodiniaceae-bacteria relationships, occurring in a zone we call here 'Symbiodiniaceae phycosphere', within the coral holobiont, in view of 1) identifying and outlining Symbiodiniaceae-bacteria interactions; 2) highlighting interkingdom signaling among phycosphere partners and 3) characterizing microbiome diversity and its relevance to 'Symbiodiniaceae phycosphere' as an adaptability generator. Since free-living Symbiodiniaceae are associated with a diverse, structured and dynamic microbial community that are attracted by chemotaxis to the microalgae exudates' gradient, the maintenance of this gradient and its effect on the microbiota inside the coral host is a must. Here, we raise evidence that sustains the 'Symbiodiniaceae phycosphere' as a relevant portion of the coral holobiont, where essential roles in the holobiont's fitness, acclimatization, adaptability and survival are presented, both in homeostatic and adverse conditions.

Zooxanthellate corals live in symbiosis with phototrophic dinoflagellates of the family Symbiodiniaceae, enabling the host coral to dwell in shallow, nutrient-poor marine waters. The South Atlantic Ocean is characterized by low coral diversity with high levels of endemism. However, little is known about coral-dinoflagellate associations in the region. This study examined the diversity of Symbiodiniaceae associated with the scleractinian coral Favia gravida across its distributional range using the ITS-2 marker. This brooding coral endemic to the South Atlantic can be found across a wide range of latitudes and longitudes, including the Mid-Atlantic islands. Even though it occurs primarily in shallower environments, F. gravida is among the few coral species that live in habitats with extreme environmental conditions (high irradiance, temperature, and turbidity) such as very shallow tide pools. In the present study, we show that F. gravida exhibits some degree of flexibility in its symbiotic association with zooxanthellae across its range. F. gravida associates predominantly with Cladocopium C3 (ITS2 type Symbiodinium C3) but also with Symbiodinium A3, Symbiodinium linucheae (ITS2 type A4), Cladocopium C1, Cladocopium C130, and Fugacium F3. Symbiont diversity varied across biogeographic regions (Symbiodinium A3 and S. linucheae were found in the Tropical Eastern Atlantic, Cladocopium C1 in the Mid-Atlantic, and other subtypes in the Southwestern Atlantic) and was affected by local environmental conditions. In addition, Symbiodiniaceae diversity was highest in a southwestern Atlantic oceanic island (Rocas Atoll). Understanding the relationship between corals and their algal symbionts is critical in determining the factors that control the ecological niches of zooxanthellate corals and their symbionts, and identifying host-symbiont pairs that may be more resistant to environmental changes.

Scleractinian corals are key organisms in structuring reef habitats and coral cover is being lost due to local and global stressors caused and/or exacerbated by anthropogenic activities. Despite being hardly touched upon, studies of size-frequency distributions serve as snapshots of coral populations’ status and provide information on population decline or growth over time. In our study we have intermittently monitored two Brazilian scleractinians species, the endemic Mussismilia hispida and Siderastrea stellata, since 2000 in an important coral marginal reef site at Armação dos Búzios, Rio de Janeiro, Brazil. We measured length, width and arc of all colonies from both species found across transects at eleven sites. In total, over 5,000 colonies have been measured over the past 17 years. Although the frequency of small and medium colonies remained relatively constant, we observed a clear decline in the frequency of larger colonies (> 30 cm) for both species, particularly the most common S. stellata. Additionally, we have been monitoring colonies for bleaching in five of these sites throughout 2017 to assess health status. The relationship between observed bleaching prevalence/intensity and environmental variables (temperature, light availability and sedimentation) may elucidate how changes in local conditions influence coral health.

The hologenome theory of evolution (HTE), which is under fierce debate, presupposes that parts of the microbiome are transmitted from one generation to the next [vertical transmission (VT)], which may also influence the evolution of the holobiont. Even though bacteria have previously been described in early life stages of corals, these early life stages (larvae) could have been inoculated in the water and not inside the parental colony (through gametes) carrying the parental microbiome. How Symbiodinium is transmitted to offspring is also not clear, as only one study has described this mechanism in spawners. All other studies refer to incubators. To explore the VT hypothesis and the key components being transferred, colonies of the broadcast spawner species Mussismilia hispida were kept in nurseries until spawning. Gamete bundles, larvae and adult corals were analyzed to identify their associated microbiota with respect to composition and location. Symbiodinium and bacteria were detected by sequencing in gametes and coral planula larvae. However, no cells were detected using microscopy at the gamete stage, which could be related to the absence of those cells inside the oocytes/dispersed in the mucus or to a low resolution of our approach. A preliminary survey of Symbiodinium diversity indicated that parental colonies harbored Symbiodinium clades B, C and G, whereas only clade B was found in oocytes and planula larvae [5 days after fertilization (a.f.)]. The core bacterial populations found in the bundles, planula larvae and parental colonies were identified as members of the genera Burkholderia, Pseudomonas, Acinetobacter, Ralstonia, Inquilinus and Bacillus, suggesting that these populations could be vertically transferred through the mucus. The collective data suggest that spawner corals, such as M. hispida, can transmit Symbiodinium cells and the bacterial core to their offspring by a coral gamete (and that this gamete, with its bacterial load, is released into the water), supporting the HTE. However, more data are required to indicate the stability of the transmitted populations to indicate whether the holobiont can be considered a unit of natural selection or a symbiotic assemblage of independently evolving organisms.