Increasing Seawater Temperature Research Articles

Ontogenetic shifts in swimming capacity of echinoderm propagules: a comparison of species with planktotrophic and lecithotrophic larvae

While developmental strategies can modulate the dispersal and recruitment of marine benthic species, the significance and drivers of propagule motility throughout ontogeny remain incompletely understood. Species with lecithotrophic (non-feeding) development are rarely studied, despite their predominance in some taxa, including echinoderms. Quantification of the swimming capacity (i.e. speed and trajectory) of early life history stages and its variability with environmental factors is required to improve the ability to predict population connectivity and assess trade-offs associated with complex life histories. In general, lecithotrophic larvae of echinoderms are ascribed weak swimming abilities relative to planktotrophic larvae, although explicit measures are scarce. Here, we explored selected metrics of swimming capacity in four co-occurring species of North Atlantic echinoderms displaying different types of pelagic development: planktotrophs represented by the sea star Asterias rubens and the sea urchin Strongylocentrotus droebachiensis, and lecithotrophs represented by the sea star Crossaster papposus and the sea cucumber Cucumaria frondosa. Swimming was characterized in still water based on the horizontal speed and path straightness of early life-history stages, from late blastula (hatched embryo) to late-stage larva. We tested the hypotheses that swimming capacity of propagules increases with progression through developmental stages and with increasing seawater temperature. Swimming speed increased with ontogeny in two of the four species (A. rubens and C. papposus) and with temperature in all species, although the effects of temperature were not uniform across life stages. The fastest swimming speeds across all species and temperatures were recorded in lecithotrophic propagules (i.e. max speed 1.2 mm s− 1 in the brachiolaria of C. papposus), whereas propagules of species with planktotrophic development displayed faster relative speeds (body lengths s− 1). Relative speeds increased with temperature in all tested species except C. papposus. Swimming paths typically increased in straightness from early to later developmental stages, and also became straighter with increased temperature in most species, except in C. papposus where they became more circular and complex. In general, planktotrophic and lecithotrophic propagules had similar swimming capacities when tested at the same level of increased temperature, though several stage-specific differences were detected; propagules of species with planktotrophic development had greater relative speeds at the gastrula stage and greater path-corrected speeds at the larval stage. Swimming paths and swimming speeds were similar between propagules of species with planktotrophic development and lecithotrophic development, suggesting that phylogenetically conserved, ontogenetic patterns of swimming capacity (seen here between two sea stars) may supersede the contribution of larval nutritional mode.

Response of the Coral Associated Nitrogen Fixing Bacteria Toward Elevated Water Temperature

Coral reefs are among the most biologically diverse and economically important ecosystem on the planet. Despite the importance, reef habitat is being under threat from human exploitation, and its most serious stressor is increasing seawater temperature, an aftermath of global warming phenomenon. The increasing seawater temperature causes bleaching, diseases and insufficiency of nutrients of corals. Despite being surrounded by ocean waters were nutrient are in short supply, the reef ecosystem is a significant source of new nitrogen. Biological nitrogen fixation is a significant internal source of marine organism. The growth of all organism lies on the availability of mineral nutrients particularly of nitrogen (N 2 ). Approximately 80% of atmosphere is made of nitrogen, however, N 2 can only be available for use by organism unless it undergoes a process of nitrogen fixation. In this aspect, related literature on biological nitrogen fixation seems sparse especially on the effects of increasing seawater temperature, a well-known contributing factor of coral bleaching. In this study, an investigation was conducted on nitrogen fixing bacterial communities associated in the coral Acropora digitifera , exploring its responses towards elevated water temperature. The study shows that exposure to high temperature causes a drastic change in the community of nitrogen fixing bacteria which are abundant in coral mucus. These changes, is correlated with the shift in the metabolic function in coral holobiont, thus, affecting both health and resiliency of corals. Overall, the finding highlights the impact of elevated seawater temperature on the nitrogen fixing bacterial composition and its diversity as well as its effects of this on host metabolism.

Temperature and Salinity Effects on Bioaccumulation, Gill Structure, and Radiation Dose Estimation in the Milkfish Chanos chanos Exposed to <sup>137</sup>Cs

The present trend of global warming has led to an increase in seawater temperature and salinity. The effects of increasing salinity and temperature on the accumulation of 137 Cs by milkfish Chanos chanos was studied under laboratory conditions to obtain information on Chanos chanos adaptability under environmental changes. The uptake of radioactive cesium by Chanos chanos increased with temperature of seawater. The concentration factors (CF) of 137 Cs for temperatures of 25°C, 27°C, 29°C, and 31°C at steady state period were 5.25, 5.91, 6.78, and 9.98 mL g -1 for the whole-body of Chanos chanos . The concentration factors at steady state (CF ss ) of 137 Cs for salinities of 26‰, 29‰, 32‰, and 35‰ were 6.23, 9.93, 9.24, and 6.86 mL g -1 , respectively. After temperature exposure to 31°C, the fish gills showed hyperplasia of epithelial cells in branchial secondary lamellae, congestion of blood vessels, and hypertrophy of pillar cells. The fish from the treatment group exhibited hemorrhage between the branchial secondary lamellae and an abundance of mucous substance in comparison with control group. This study links radionuclide bioaccumulation data and monitoring data obtained in the field and laboratory experiment with radiation dose determined by ERICA Tools, an approach that will enable better linkages to be made between exposure and dose in Chanos chanos and its marine food web. Received: 2 November 2015; Revised: 1 July 2016; Accepted: 17 July 2016

Elevated Temperature and Allelopathy Impact Coral Recruitment.

As climate change continues to alter seawater temperature and chemistry on a global scale, coral reefs show multiple signs of degradation. One natural process that could facilitate the recovery of reef ecosystems is coral recruitment, which can be influenced by the benthic organisms in a local habitat. We experimentally tested both a global stressor (increased seawater temperature) and a local stressor (exposure to microcolin A, a natural product from a common marine benthic cyanobacterium) to determine how these stressors impacted coral larval sublethal stress, survival and settlement. Larvae of Porites astreoides had the same survival and settlement as the controls after exposure to increased temperature alone, but elevated temperature did cause oxidative stress. When exposed to natural concentrations of microcolin A, larval survival and settlement were significantly reduced. When larvae were exposed to these two stressors sequentially there was no interactive effect; but when exposed to both stressors simultaneously, there was a synergistic reduction in larval survival and an increase in oxidative stress more than in either stressor treatment alone. Increased seawater temperatures made larvae more susceptible to a concurrent local stressor disrupting a key process of coral reef recovery and resilience. These results highlight the importance of understanding how interactive stressors of varying spatial scales can impact coral demographics.

Effects of seawater temperature increase on economically relevant native and introduced clam species

As a consequence of climate change, global warming is expected to increase during the 21st century. Taking this into account, the impact of rising temperatures on the native Ruditapes decussatus and introduced R. philippinarum bivalve species was assessed, through biochemical and mRNA transcription analyses. Our findings showed that at 21 °C the electron transport system and antioxidant enzyme activity, as well as the expression of Hsp70 gene were induced in R. decussatus when compared with 17 °C. On the other hand, at 25 °C results suggested that R. decussatus closed their valves during short periods, as a behavioral strategy, down-regulating the expression of genes associated with mitochondrial metabolism (Cox-1 and 16S) and chaperone function (Hsp70) compared with organisms at 17 °C. In addition, the introduced species (R. philippinarum) increased the electron transport system and antioxidant activities, as well as gene expression of antioxidant enzymes and molecular chaperone (Hsp70) at 21 °C. However, antioxidant mechanisms were not enough to prevent lipid membrane damages at 21 °C. At 25 °C R. philippinarum presented increased electron transport system and antioxidant activity, as well as the expression of genes associated with apoptosis regulation and molecular chaperone. Overall, the present findings indicate that in a global warming scenario both species are able to induce different mechanisms to mitigate the impacts of temperature increase.

Air–Water CO2 Fluxes and Net Ecosystem Production Changes in a Baja California Coastal Lagoon During the Anomalous North Pacific Warm Condition

The present study examines the temporal variability of air–water CO2 fluxes (FCO2) and seawater carbonate chemistry in a Baja California coastal lagoon during an exceptionally warm anomaly that was developed in Northeast Pacific coasts during 2014. This oceanographic condition led to a summer-like season (weak upwelling condition) during the study period, which reached a maximum surface temperature anomaly of 2 °C in September 2014. San Quintin Bay acts as a source of CO2 to the atmosphere in 2014 (3.3 ± 4.8 mmol C m−2 day−1) with the higher positive fluxes mainly observed in summer months (9.0 ± 5.3 mmol C m−2 day−1). Net ecosystem production (NEP) switched seasonally between net heterotrophy and net autotrophy during the study period, with an annual average of 2.2 ± 7.1 mmol C m−2 day−1, which indicates that San Quintin Bay was a net autotrophic system during the atypical warm oceanographic condition in 2014. This pattern of seasonal variations in the carbon balance at San Quintin Bay appears to be linked to the life cycle of benthic communities, which play an important role in the whole-ecosystem metabolism. Under the limited input from external sources coupled with an increase in seawater temperatures, the recycled benthic carbon and nutrient fluxes play a major role to sustain water-column processes within the bay. Since the upwelling condition may influence the magnitude of the air–water CO2 fluxes, our results clearly indicated that San Quintin Bay is a net source of carbon to the atmosphere regardless of the adjacent oceanic conditions. Our study sheds light on the carbon dynamics and its metabolic implications in a shallow coastal ecosystem under a regional warm anomaly and contributes potentially relevant information in view of the likely future scenario of global climate change.

Numerical Modeling of Water Thermal Plumes Emitted by Thermal Power Plants

This work focuses on the study of thermal dispersion of plumes emitted by power plants into the sea. Wastewater discharge from power stations causes impacts that require investigation or monitoring. A study to characterize the physical effects of thermal plumes into the sea is carried out here by numerical modeling and field measurements. The case study is the thermal discharges of the Presidente Adolfo López Mateos Power Plant, located in Veracruz, on the coast of the Gulf of Mexico. This plant is managed by the Federal Electricity Commission of Mexico. The physical effects of such plumes are related to the increase of seawater temperature caused by the hot water discharge of the plant. We focus on the implementation, calibration, and validation of the Delft3D-FLOW model, which solves the shallow-water equations. The numerical simulations consider a critical scenario where meteorological and oceanographic parameters are taken into account to reproduce the proper physical conditions of the environment. The results show a local physical effect of the thermal plumes within the study zone, given the predominant strong winds conditions of the scenario under study.

Predictive modeling of European flat oyster (Ostrea edulis L.) fatty acid composition

This study investigates polyunsaturated fatty acids (PUFAs) of European flat oyster (Ostrea edulis L.) and their composition shift in relation to the interaction of selected factors in the marine environment of the Adriatic Sea, over an annual farming cycle. We advance the hypothesis that fatty acid composition may be predicted regarding the changes of abiotic environment parameters. Novel approach was developed as a tool for interpretation of PUFA ratio using mathematical models and algorithms, providing association and better prediction of observed variables compared to standard statistical techniques. Data analysis using R language package resulted in models that primarily provide the possibility to predict the PUFA value based on certain fluctuations in the environment. In addition to predictions, results present a useful tool for insight into the oysters’ quality and understanding of their basic biology in different environments. This is the first report of using mathematical modeling to investigate interactions between PUFA compositions of oyster meat relative to interactions of particular environmental variables. We furthermore discuss the potential of environmental variables in affecting the oyster PUFA composition and apply an approach providing a better insight into the health status and quality of oysters throughout the season. Model projection showed a nonlinear, parabolic dependence between the tested variables and dissolved oxygen. Both, the n-3 PUFA and DHA alone, were associated with an open-down parabolic relationship; the maximum levels were reached by 95 % oxygen saturation. The n-3 PUFAs and n-3/n-6 ratio were likely negatively affected by increase in sea water temperature, where a 15 % reduction in n-3 PUFAs content and 50 % reduction in n-3/n-6 proportion were predicted. Also, we detected FA ratios (AA/DHA, AA/EPA) and AA positively affected by increase in sulphate concentration and dissolved oxygen, respectively, and strongly negative prediction of ammonia level and sea water pH with AA, DHA and ARA/EPA ratio, respectively. These models might be applied to other aquatic species, for promotion of sea food quality, improvement of farming conditions and production, as well as for optimizing conditions in controlled trails.

Gametogenic cycle of the rough cockle <em>Acanthocardia tuberculata</em> (Mollusca: Bivalvia) in the M’diq Bay (SW Mediterranean Sea)

The reproductive cycle of Acanthocardia tuberculata (Linnaeus, 1758) was studied in M’diq Bay. The gonadal development was determined by means of standard histological techniques, mean oocyte diameters and a condition index. Rough cockle is a gonochoric species with synchronous gonadal development and spawning in males and females. The sex ratio obtained was not significantly different to 1:1. Gametogenesis began in late winter (November) coinciding with the temperature drop. In June, with increasing sea water temperature, most of the population was spent and resting oocytes appeared dispersed in the gonad. Resting stage occurred from August to October, during which time sex could not be determined in 100% of the population. A. tuberculata showed a clear seasonality in its gametogenic cycle, with one spawning peak per year in June. Quantitative measurements of 6318 oocyte diameters indicated the patterns of development observed in the qualitative staging. The results obtained revealed the direct influence of temperature on the reproductive cycle. First sexual maturity occurred at 42.77 mm shell length. The information gathered in this study allowed preliminary management measures to be suggested for the fishery of this species, including a closed season during the main spawning season (May-June) and the establishment of a minimum landing size (at least 50 mm shell length) for A. tuberculata from the Moroccan Mediterranean coast.

Genotype-by-environment interactions during early development of the sea urchin Evechinus chloroticus

The increase in seawater temperature due to anthropogenic climate change is likely to affect population persistence and changes in distributional ranges of marine species. Adaptation to warmer environmental conditions will be determined by the presence of tolerant genotypes within a population. The present study determined the genotype-by-environment (G × E) interactions during early development of the New Zealand sea urchin Evechinus chloroticus cultured at 18 °C (mean annual temperature), 21 °C (ambient summer temperature) and 24 °C (+3 °C above ambient summer temperature). The experiment was performed in 3 experimental blocks using gametes from 3 males and 3 females crossed in all combinations (North Carolina II cross-breeding design), resulting in 9 families per experimental block (i.e., total of 27 families). Differences between female and male identities were quantified during cleavage and gastrulation: Reaction norms (i.e., interaction plots) showed a clear G × E interaction, with some genotypes performing better than others at high temperatures. Heritability during gastrulation was 0.51, indicating that 51 % of the variability corresponds to genetic variation. Overall, the present study shows that seawater temperature has a negative effect on early development of E. chloroticus; however, there are resilient genotypes in the studied population that could provide the genetic potential to adapt to future ocean conditions.

Probabilistic remaining life estimation for deteriorating steel marine infrastructure under global warming and nutrient pollution

The longer-term serviceability and structural safety of steel infrastructure exposed to seawater conditions may be affected by global warming and by seawater nutrient pollution. These may affect abiotic and biotic (microbial) corrosion. A model for long-term corrosion is developed from data obtained from steel piling exposed for 33 years in a seawater harbour. The effects on corrosion losses on the structural reliability of steel sheet piling as used in harbours world-wide were investigated as a function of seawater temperature rise from global warming and of seawater nutrient pollution. The results show that structural reliability is more sensitive to likely nutrient pollution than to predicted increases in seawater temperature, noting also that global warming also could increase nutrient pollution from anthropological sources.

Thermal discharge-created increasing temperatures alter the bacterioplankton composition and functional redundancy.

Elevated seawater temperature has altered the coupling between coastal primary production and heterotrophic bacterioplankton respiration. This shift, in turn, could influence the feedback of ocean ecosystem to climate warming. However, little is known about how natural bacterioplankton community responds to increasing seawater temperature. To investigate warming effects on the bacterioplankton community, we collected water samples from temperature gradients (ranged from 15.0 to 18.6 °C) created by a thermal flume of a coal power plant. The results showed that increasing temperatures significantly stimulated bacterial abundance, grazing rate, and altered bacterioplankton community compositions (BCCs). The spatial distribution of bacterioplankton community followed a distance similarity decay relationship, with a turnover of 0.005. A variance partitioning analysis showed that temperature directly constrained 2.01 % variation in BCCs, while temperature-induced changes in water geochemical and grazing rate indirectly accounted for 4.03 and 12.8 % of the community variance, respectively. Furthermore, the relative abundances of 24 bacterial families were linearly increased or decreased (P < 0.05 in all cases) with increasing temperatures. Notably, the change pattern for a given bacterial family was in concert with its known functions. In addition, community functional redundancy consistently decreased along the temperature gradient. This study demonstrates that elevated temperature, combined with substrate supply and trophic interactions, dramatically alters BCCs, concomitant with decreases in functional redundancy. The responses of sensitive assemblages are temperature dependent, which could indicate temperature departures.Electronic supplementary materialThe online version of this article (doi:10.1186/s13568-016-0238-4) contains supplementary material, which is available to authorized users.

Effect of Acute Temperature Change on Antioxidant Enzyme Activities and Lipid Peroxidation in the Ark ShellScapharca subcrenata(Lischke, 1869)

The present study investigated whether an acute temperature change affected hemolymph antioxidant enzyme activities and lipid peroxidation in ark shell Scapharca subcrenata transferred from 22°C to 17°C, 27°C, and 32°C for 72 h and then transferred back quickly to 22°C for a 6-h recovery period. Superoxide dismutase (SOD) and glutathione peroxidase (GPX) activities and malondialdehyde (MDA) contents [in cell-free hemolymph (CFH) and hemocyte lysate (HL)] were chosen as biomarkers of acute temperature stress. The results demonstrated that CFH and HL SOD activities decreased significantly (P < 0.05) after ark shells were exposed to 32°C for 72 h and returned to normal temperature. Glutathione peroxidase activity in CFH of ark shells exposed to 32°C increased significantly (P < 0.01) at 12 h, but then decreased significantly (P < 0.01) at the end of the recovery period. Malondialdehyde contents in the CFHandHL of stressed ark shells, regardless of temperature (17°C, 27°C, or 32°C), were lowest at 1 h of the recovery period, and recovered completely by the end of the experiment. In contrast, no difference was observed in CFH or HLMDA contents between the 17°C and 27°C groups. In conclusion, S. subcrenata had high tolerance to an acute decrease and increase in seawater temperature, but oxidative stress in ark shell hemolymph increased significantly with temperature and time, such as 32°C and greater than 72 h.

Response of Subtropical Coastal Sediment Systems of Okinawa, Japan, to Experimental Warming and High pCO2

Increasing seawater temperatures and CO2 levels associated with climate change affect the shallow marine ecosystem function. In this study, the effects of elevated seawater temperature and partial pressure of CO2 (pCO2) on subtropical sediment systems of mangrove, seagrass, and coral reef lagoon habitats of Okinawa, Japan, were examined. Sediment and seawater from each habitat were exposed to four pCO2-temperature treatments, including ambient pCO2- ambient temperature, ambient pCO2-high temperature (ambient temperature + 4°C), high pCO2 (936 ppm)-ambient temperature, and high pCO2-high temperature. Parameters including primary production, nutrient flux, pigment content, photosynthetic community composition, and bacterial abundance were examined. Neither high temperature nor high pCO2 alone impacted mangrove and seagrass sediment primary production significantly (Tukey’s test, P > 0.05). However, the combined stress significantly (Tukey’s test, P 0.05) under the combined stress, suggesting that heterotrophic processes were less affected by the combined stress than autotrophic processes. In summary, mangrove and seagrass sediments minimize the negative impacts of elevated temperature and pCO2 via increased primary production and carbon storage. Lagoonal sediments also act as a carbon sink under temperature and ocean acidification stress.

Transcriptional responses in Ecklonia cava to short-term exposure to hyperthermal stress

Rapid climate change accelerates the damage to many valuable marine ecosystems, including macroalgal forests and coral reefs. To understand the biological responses of kelp species to increased seawater temperatures, we isolated the genes that responded to hyperthermal stress conditions in Ecklonia cava. Thalli of E. cava were exposed to seawater at 20 or 24°C, and the genes whose transcript levels changed in response to hyperthermal stress were identified with microarray hybridization. Eighty-nine candidate genes were identified, 49 of which were upregulated and 40 were downregulated by hyperthermal stress. Gene Ontology mapping showed that cellular oxidant detoxification, the regulation of cellular metabolic processes, and photosynthesis processes were affected by increased ocean temperatures. The functions of some isolated genes were also analyzed to predict the possible changes in the metabolism of E. cava under hyperthermal stress. The differentially expressed genes isolated here may serve as molecular biomarkers, allowing us to better understand the biological responses of marine organisms to environmental changes.

Reproductive biology of the black coral Antipathella wollastoni (Cnidaria: Antipatharia) in the Azores (NE Atlantic)

Antipathella wollastoni is an antipatharian species common to Macaronesia, occurring in depths between 20 and 1425m. Despite its importance as a habitat-forming species, there is no information about its basic biology. The aim of the current study is to describe its reproductive strategy, gametogenic cycle and reproductive timing. Sampling was performed monthly on six tagged colonies during one year and specimens were histologically processed. A. wollastoni was found to be a gonochoric broadcast spawner. Gametogenesis took place within the primary transversal mesenteries, and followed an annual cycle. The reproductive season coincided with an increase in seawater temperature but spawning, inferred from the disappearance of gametes, likely happened after the sea surface temperature peak of the year (September 2009). Polyp fecundity ranged from 1 to 309 oocytes/polyp. A decrease in polyp fecundity was detected in samples at the higher pre-spawning maturity stage, indicating possible repetitive spawning or oocyte absorption. Intra-colonial comparisons revealed a longer duration of the reproductive cycle in the medial colony section, and a gradient of increased oocyte size towards the apical section, possibly due to intra-colonial differences in energy allocation between reproduction and other biological processes, or as a strategy against predation on gametes/larvae. Colony height was positively correlated with polyp fecundity indicating that the reproductive output increases with colony size.

Recent Advances in Understanding the Effects of Climate Change on Coral Reefs

Climate change is one of the greatest threats to the persistence of coral reefs. Sustained and ongoing increases in ocean temperatures and acidification are altering the structure and function of reefs globally. Here, we summarise recent advances in our understanding of the effects of climate change on scleractinian corals and reef fish. Although there is considerable among-species variability in responses to increasing temperature and seawater chemistry, changing temperature regimes are likely to have the greatest influence on the structure of coral and fish assemblages, at least over short–medium timeframes. Recent evidence of increases in coral bleaching thresholds, local genetic adaptation and inheritance of heat tolerance suggest that coral populations may have some capacity to respond to warming, although the extent to which these changes can keep pace with changing environmental conditions is unknown. For coral reef fishes, current evidence indicates increasing seawater temperature will be a major determinant of future assemblages, through both habitat degradation and direct effects on physiology and behaviour. The effects of climate change are, however, being compounded by a range of anthropogenic disturbances, which may undermine the capacity of coral reef organisms to acclimate and/or adapt to specific changes in environmental conditions.

Effects of Ocean Acidification and Temperature Increases on the Photosynthesis of Tropical Reef Calcified Macroalgae.

Climate change is a global phenomenon that is considered an important threat to marine ecosystems. Ocean acidification and increased seawater temperatures are among the consequences of this phenomenon. The comprehension of the effects of these alterations on marine organisms, in particular on calcified macroalgae, is still modest despite its great importance. There are evidences that macroalgae inhabiting highly variable environments are relatively resilient to such changes. Thus, the aim of this study was to evaluate experimentally the effects of CO2-driven ocean acidification and temperature rises on the photosynthesis of calcified macroalgae inhabiting the intertidal region, a highly variable environment. The experiments were performed in a reef mesocosm in a tropical region on the Brazilian coast, using three species of frondose calcifying macroalgae (Halimeda cuneata, Padina gymnospora, and Tricleocarpa cylindrica) and crustose coralline algae. The acidification experiment consisted of three treatments with pH levels below those occurring in the region (-0.3, -0.6, -0.9). For the temperature experiment, three temperature levels above those occurring naturally in the region (+1, +2, +4°C) were determined. The results of the acidification experiment indicate an increase on the optimum quantum yield by T. cylindrica and a decline of this parameter by coralline algae, although both only occurred at the extreme acidification treatment (-0.9). The energy dissipation mechanisms of these algae were also altered at this extreme condition. Significant effects of the temperature experiment were limited to an enhancement of the photosynthetic performance by H. cuneata although only at a modest temperature increase (+1°C). In general, the results indicate a possible photosynthetic adaptation and/or acclimation of the studied macroalgae to the expected future ocean acidification and temperature rises, as separate factors. Such relative resilience may be a result of the highly variable environment they inhabit.

Growth and reproductive biology of the sea star Astropecten aranciacus (Echinodermata, Asteroidea) on the continental shelf of the Catalan Sea (northwestern Mediterranean)

The growth and reproductive biology of the sea star Astropecten aranciacus was investigated on the continental shelf of the northwestern Mediterranean Sea. Sea stars were captured monthly in two bathymetric ranges (5–30 and 50–150 m) between November 2009 and October 2012. Bathymetric segregation by size in A. aranciacus was detected: small individuals inhabit shallow areas (5–30 m), while large individuals inhabit deeper areas of the continental shelf (50–150 m). Recruitment was recorded twice nearshore but no recruitment was detected offshore during the whole study period. Three cohorts were identified in each bathymetric range and growth rates were estimated. A. aranciacus population exhibited a seasonal growth pattern, being higher from June to October in the nearshore cohorts and from February to October in the offshore ones. Histology and organ indices revealed that spawning likely started in March, coinciding with the spring phytoplankton bloom and the increase in sea water temperature, and extended until June–July. Ratio between males and females was approximately 1:1 throughout the year and in both bathymetrical ranges. The size at first maturity (R50 %) was estimated to be R = 112 mm. A. aranciacus did not show an inverse relationship between gonad index and pyloric caeca index.

Exploring the Symbiodinium rare biosphere provides evidence for symbiont switching in reef-building corals.

Reef-building corals possess a range of acclimatisation and adaptation mechanisms to respond to seawater temperature increases. In some corals, thermal tolerance increases through community composition changes of their dinoflagellate endosymbionts (Symbiodinium spp.), but this mechanism is believed to be limited to the Symbiodinium types already present in the coral tissue acquired during early life stages. Compelling evidence for symbiont switching, that is, the acquisition of novel Symbiodinium types from the environment, by adult coral colonies, is currently lacking. Using deep sequencing analysis of Symbiodinium rDNA internal transcribed spacer 2 (ITS2) PCR amplicons from two pocilloporid coral species, we show evidence consistent with de novo acquisition of Symbiodinium types from the environment by adult corals following two consecutive bleaching events. Most of these newly detected symbionts remained in the rare biosphere (background types occurring below 1% relative abundance), but one novel type reached a relative abundance of ~33%. Two de novo acquired Symbiodinium types belong to the thermally resistant clade D, suggesting that this switching may have been driven by consecutive thermal bleaching events. Our results are particularly important given the maternal mode of Symbiodinium transmission in the study species, which generally results in high symbiont specificity. These findings will cause a paradigm shift in our understanding of coral-Symbiodinium symbiosis flexibility and mechanisms of environmental acclimatisation in corals.