•An unusual negative sea level anomaly occurred in the Western Indian Ocean in 2015.•Reef flat benthic communities massively died following the sea level anomaly event.•A 50 % decline in coral cover ...is measured from both in situ and remote sensing data.•Coral cover variation on Reunion Island reef flat are shaped by these disturbances.
Among induced mass-mortality events on coral reef, extreme low tides may ultimately lead to considerable reef community deaths on intertidal reef flats due to unusually long and significant aerial exposure. Here, we report an extensive coral mortality event induced by a negative sea level anomaly (nSLA) that occurred across Reunion Island during the austral winter season between June and October 2015 preceding the 2015–2016 El Niño Southern Oscillation (ENSO) event. The nSLA was strong and long in duration with a rapid drop of 35 cm in the mean sea level over a one-month period. Surveys conducted over seven reef flat sites before and after the nSLA revealed that mean coral cover drastically decreased from 54.5 ± 12.7% in early 2015, to 27.4 ± 6.9% in November 2015, which is an equivalent cover loss of 50% following the 2015 nSLA event. The shallowest sites showed a greater decrease in coral cover while the deepest parts of the reef flat remained unaffected. We found a significant correlation between the bathymetry and the relative coral cover variation. Using this relationship between depth and coral cover changes, high-resolution hyperspectral imagery and Lidar bathymetric airborne data, we mapped the impacts of this event at the scale of the whole reef. Overall the modeled loss reached 13.0 ha, which represents a decrease of 45.5% of all live coral cover in this area during the 2015 nSLA event. The impact of a nSLA on emersion times is much greater than the regular variation in tide amplitude between neap and spring tides, reaching new bathymetric ranges that are usually stable in terms of water submersion. Temporal variation of coral cover on Reunion Island reef flat revealed regular decreases to be compared with mean low-water-level events among other sea and climatic related disturbances and stressors.
The presence of mudworms and other epibionts were studied in three batches of Pacific oyster
Crassostrea gigas from February 2000 to August 2003 at a high productive oyster rearing area in Normandy ...(France). Spatio-temporal variations of mudworm and epibiont presence were analysed and their impacts on oyster mortality, growth (shell and meat) and condition were assessed. The external shells of the oysters were mainly occupied by barnacles. The proportion of epifauna varied seasonally, showing a peak in autumn for the three oyster batches. The heaviest colonization occurred at the highest areas on the foreshore but there was no obvious relationship between distribution and bathymetry. No significant correlation was detected between epifauna biomass and both oyster growth and condition. Inner shell valves of half-grown (second year of life) and marketable (third year of life) oysters were heavily infested by spionid mudworms whose spatial distribution was significantly correlated with intertidal exposure, the oysters at the highest level being almost free of mudworm tubes and/or blisters. Although it remains unclear how spionid mudworms affect summer mortality of oysters, it was demonstrated that they had a significant negative effect on host growth. Abundant mudworm scars on the inner shell were associated with reduced meat and shell weights, thereby revealing a potential decrease in oyster productivity.
For decades, bivalve shells have constituted one of the most common supports for paleoclimatic archives based on stable isotope approaches. In this work, we conducted chemical marking and recapture ...techniques to study the fluctuation of δ18O values of oyster shells of the species Magallana gigas reared in natural environment in Normandy (France) for two years. The results were compared to the continuous monitoring of temperature and salinity and monthly records of seawater δ18O. Isotopic measurements were performed on the hinge area that regroups the whole life of the oysters. Here we demonstrate that oysters mineralized their shells with no significant growth breaks during the two-years experiment, even at temperatures below 6 °C. The results confirm that adult oysters (i.e. > 1 yr) mineralized their shells at equilibrium. However, juvenile specimens exhibit a strong isotopic disequilibrium, with a maximum shift of the δ18O values of 3‰ in winter, likely due to kinetic isotope effects. This corresponds to a reconstructed temperature up to 13 °C warmer than expected. This work indicates that although these oyster shells can be used as an accurate archive of (paleo)environmental conditions, the shell portion mineralized during the juvenile stage (i.e. < 1 yr) should be avoided for paleotemperature reconstructions. Given the wide use of bivalve shells as environmental archives, similar studies on others species are required.
•We analyze the δ18O composition of oyster shells of Normandy (France).•An isotopic disequilibrium is identified during the juvenile portion of the shell.•Isotopic disequilibrium is related to kinetic effect due to fast growth rate.•Adult oysters mineralize their shells in equilibrium.•Juvenile portion of the shell should be avoided for paleoclimatic reconstructions.
While oyster shells are one of the most common mollusks used for the analysis of (paleo)environmental and (paleo)climatic records based on geochemical proxies, high-resolution growth rate changes ...still need to be determined. Promising previous works are restricted to small portions of shell sections due to difficulties in continuous growth increment revelation. Based on a mark and recapture experiment of
Magallana gigas
specimens reared in an intertidal area of Normandy (France) for 22 months, and a sclerochronological approach using cathodoluminescence microscopy, this study provides the longest high-resolution record of growth increments in oyster shells to date. Different growth patterns were identified likely related to the oyster age. After age 1 year, the formation of growth increments follows an expected tide-related model, leading to the mineralization of ~ 2 calcitic increments per day, together with growth rate changes at lunar and semi-lunar periodicities, and a seasonal trend with occasional growth breaks during winter when temperatures fall below ~ 6 °C. However, for oysters younger than 1 year, i.e., before reaching their sexual maturity, the growth increment analysis reveals unconventional patterns. In this case, oysters’ growth is associated with either a large number (~ 5) or less than one increment per day depending on the period. This pattern is also associated with frequent growth cessations, although the growth rate of the shell is high at this period. Our results illustrate that the high-resolution sclerochronological approach is required for accurate paleoenvironmental reconstructions based on oyster shells.
•Field monitoring does not take into account the spatial coral reef heterogeneity.•A spatial approach was developed based on Reunion Island hyperspectral and multispectral images.•Bathymetry, coral ...cover, Hyperspectral Coral to Algae Index (HCAI) and reef tongue displacement were retrieved.•Diachronic analysis was conducted to map and quantify changes over a six-year period.•Our findings open up new opportunities for coral reef management on large areas.
Coral reef degradation due to environmental change, including anthropogenic disturbances, is a major concern worldwide. Detecting and assessing both temporal and spatial changes in benthic cover is a crucial requirement to inform policy makers and guide conservation measures. Here, we introduce a spatial approach based on high resolution multispectral and hyperspectral image analysis, developed in order to detect and quantify changes in benthic cover in a highly heterogeneous shallow coral reef flat in Reunion Island in the South-West Indian Ocean. We propose a new index called HCAI (Hyperspectral Coral to Algae Index), defined as the ratio of living coral cover to the sum of living coral and algal covers. Benthic cover estimates were derived from airborne hyperspectral image processing using water column correction and unmixing models implemented with the four main coral reef benthic components: corals, algae, seagrass and sand. Ground truth and LIDAR data acquired simultaneously were used to validate processing accuracy. A significant positive correlation (adjusted R2 = 0.72, p-value < 0.001) was obtained between coral cover recorded in situ and estimated from image analysis. Moreover, 13 habitat classes based on the four main benthic component covers were mapped at a scale of an entire reef. Diachronic analyses of hyperspectral images between 2009 and 2015 revealed an overall decrease of the HCAI index and a decrease in the area of all the dominant coral classes along the reef (−28.24% for the coral class for example), while the area of habitat classes dominated by algae strongly increased during the same period. Moreover, we detected and documented the spatial and temporal evolutions of coral geomorphological features composed with coral rubble deposits called rubble tongues (RTs) using different available sensors (i.e. hyperspectral, satellite, and orthophotography). Since 2003, four detected (RTs) have spread shoreward at a mean rate of 8.4 m.y−1 including significant loss of reef structural complexity and heterogeneity, a spreading pattern which was confirmed by 2009 and 2015 hyperspectral data. Remote sensing and more specifically airborne hyperspectral approaches open new perspectives for coral reef monitoring, at high temporal and spatial resolutions.
Feeding competition between the Pacific cupped oyster
Crassostrea gigas and the polychaete
Lanice conchilega was studied by assessing the polychaete suspension feeding activity. Retention efficiency ...was estimated by comparing particle size distributions at the output of experimental chambers containing the species and controls. Although particles ranging from 4 to 12 μm were collected by
L. conchilega, no upper threshold or maximum retention rate was reached within this range. In contrast,
C. gigas showed retention starting at 2 μm, and reaching an upper threshold at 6 to 8 μm. Based on our results, feeding competition is likely to occur between
C. gigas and
L. conchilega. Standardised filtration rates reached 0.225 l h
−1 g dmw
−1 (±0.08) for
L. conchilega and 2.43 l h
−1 g dmw
−1
C. gigas for animals of 1 g dry meat weight (dmw). Assimilation rates, 0.44 for
L. conchilega and 0.49 for
C. gigas, were similar for the two species. Respiration rates were estimated at 0.113 and 0.68 ml O
2 h
−1 for
L. conchilega (Allometric coefficient=0.534) and
C. gigas respectively. Therefore polychaete scope for growth (SFG) (4.01 J h
−1 g dmw
−1) was significantly lower when compared with
C. gigas SFG (61.96 J h
−1 g dmw
−1). The impact of the
L. conchilega population on that of cultivated oysters was evaluated from these results and field population assessment of both species. Based on field population estimates,
L. conchilega was responsible for a 19% decrease in the carrying capacity and 30% of the oxygen depletion from the total activity of both species. However,
L. conchilega SFG was only 16% of that of the
C. gigas population. Several hypotheses regarding population interactions are discussed.
Oyster culture structures support a host of epibionts belonging to the same suspension-feeding guild, which are considered to be potential competitors for food with cultivated oysters. In an ...intertidal shellfish ecosystem on the northern French coast, an approach based on stable isotopes (13C and15N) was used to investigate intra- and interspecific food resource partitioning among cultivated oysters and the main associated wild sessile epibionts such as polychaetes, barnacles, mussels and ascidians. The main objective of the present study was to determine inter- and intraspecific food partitioning, along with small-scale spatial variability, within the guild of suspension feeders. We demonstrated that interspecific competition was limited among co-occurring suspension-feeders (ascidians, serpulid and terebellid polychaetes, bivalves and barnacles). None of the studied species had similar δ13C and δ15N signatures, indicating that relative contributions of organic matter sources may differ for each suspension-feeding species. Spatial variability was investigated both from the view of intra- and interspecific variability. Intraspecific variability was examined with regard to species’ feeding biology and the trophic plasticity of co-occurring suspension-feeders. Mantel tests indicated that spatial heterogeneity resulted not only from environmental conditions, such as elevation above sea level (a.s.l.) and sediment features, but also from the inherent spatial structure of isotopic signatures. Our results show that isotopic approaches that are limited to sampling in one area and at one time are at risk of mistaking trophic interactions.
We developed a growth model for
Crassostrea gigas oyster shells based on the use of
in situ temporal manganese markings to calibrate natural cathodoluminescence (CL) changes in the shell hinge ...sections. A 30
min to 4-h exposure period with Mn
2+ (90–120
mg
l
−
1
) was sufficient to create a detectable mark in the shells. This makes the Mn
2+ markings the fastest mollusc shells marking technique to date. The natural CL from juvenile and adult shells cultured in four standard shellfish-farming locations along the English Channel and French Atlantic coasts, exhibited a seasonal pattern (maximum CL intensity occurring during summer periods, minimum CL intensity occurring during winter). Hydrobiological data recorded at Baie des Veys site allows us to attribute the seawater temperature as the main parameter controlling CL of shells. Chlorophyll a and seawater manganese concentration were not decisive in the luminescence intensity of the shells. A relationship between oyster hinge growth and the length of shells makes the umbo investigations a promising tool for oyster-farming and/or wild stock assessments. Shell growth varied at spatial and temporal scales (higher growth rates were observed during summer–autumn and lower during the winter period), depending on seawater temperature changes. Sub-monthly Mn
2+ markings support the fact that shell deposition can occur under temperatures below 6
°C, which has to be taken into account for both shellfish production and environmental monitoring derived from chemical compositions of the shells. Finally, our results point out the efficiency of age and shell growth rate determination by CL analysis in further shellfish ecosystem researches.
Oyster mortality was monitored at multiple sites along the French coastline (∼
5 500 km) between 1993 and 2005. Mortality data for one- and two-year-old oysters were collected over 10–12 years in 39 ...oyster culture sites bordering 3 different “seas”: the English Channel, Bay of Biscaye and Mediterranean. Combining these data with records from environmental monitoring databases, 11 of these sites had consistent chronological data sets including both environmental data and mortality records. Mortality in one-year-old oysters was clearly summer mortality (49% of their annual mortality) whereas mortality of two-year-olds occurred mostly in spring (51%). Analysis of variance revealed that “coastal area” was the main influence on mortality of one-year-olds (77.5%% of the variance) and that “year studied” was the main influence on mortality of two-year-olds (60.6% of the variance). The highest mortalities occurred in Marennes and in several sites in Brittany for both age groups, and in Veys Bay (Normandy) for two-year-old oysters only. Environmental parameters were then analysed to investigate which of these might influence summer mortality. Principal Component Analyses revealed that environmental factors such as chlorophyll
a (food resource indicator) and salinity (watershed effect) influence oyster mortality. Chlorophyll
a concentration (10% of the variance), water temperature (7% of the variance) and turbidity (5% of the variance) are the main significant factors for the mortality of one-year-olds, while salinity and chlorophyll
a have more effect on the mortality of two-year-old oysters (respectively 5% and 4% of the variance).
Summer mortality of Pacific oysters is known in several countries. However no specific pathogen has been systematically associated with this phenomenon. A complex combination of environmental and ...biological parameters has been suggested as the cause and is now starting to be identified. A high genetic basis was found for survival in oysters when a first generation (G1) was tested in three sites during summer. This paper presents a synthesis on physiological characteristics of two selected groups (‘R’ and ‘S’, from families selected for resistance and susceptibility to summer mortality respectively), of the second and third generations. R and S showed improvement or reduction of survival compared with the control in both field and laboratory trials confirming the high heritability of survival of juveniles <
1 year old. Interestingly, no correlation was observed between growth and survival.
Comparison between the two selected groups showed that S oysters invested more energy in reproduction and stayed a longer time without spawning than R oysters which had high synchronous spawning. This was mainly shown with high rather than low dietary rations (respectively 12% and 4% DW algae/DW oyster) in a controlled experiment. Moreover, early partial spawning was detected in S oysters and not R ones in the high dietary ration. S showed a higher respiration rate and an earlier decrease in absorption efficiency than R during gametogenesis, but they were not significantly different in glycogen or ATP utilisation. Two months before a mortality episode, hemocytes from S oysters had a higher adhesive capacity than R hemocytes and significantly higher reactive oxygen species production capacity. One month before mortality, S oysters had the highest hyalinocyte concentration and their expression of genes coding for glucose metabolism enzymes (Hexokinase, GS, PGM, PEPCK) was significantly lower in the labial palps. After a thermal increase from 13 °C to 19 °C, during 8 days in normoxia, S oysters showed a large HSP70 increase under hypoxia contrary to R oysters, suggesting their high susceptibility to stress. Their catalase activity was lower than in R oysters and showed no further change to subsequent hypoxia and pesticide stresses, in contrast to R oysters.
These observations suggest possible links between higher reproductive effort in S oysters, their specific stress response to temperature and hypoxia, ROS production, partial spawning, hyalinocyte increase and the infection process. To compare R and S oysters in a more integrated way, a suppression subtractive hybridisation (SSH) library and a micro-array strategy are being undertaken.
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