Reproduction patterns of the bloody cockle Senilia senilis (Linnaeus 1758) in the Sine-Saloum inverse estuary

Understanding the reproductive biology of a species is an important means of determining the renewal capacity of natural stocks, especially in the case of heavily exploited species. It is a fundamental element in supporting the implementation of management measures. Here, we studied the bloody cockle (S. senilis) in the Sine-Saloum, with the aim of describing its seasonal and spatial reproductive cycle. S. senilis reproduction was studied over an annual cycle at two sites chosen for their contrasting situations along the upstream-downstream gradient. The reproductive cycle was studied by histological analysis of a pool of individuals maintained in-situ and sampled throughout the year. Our results showed that gamete maturation is asynchronous within and between individuals. Gametogenesis mostly occurred in October. The maturation stage showed a seasonal pattern with continuous reproduction throughout the year, with two preferred periods between May and July and December and February. The reproductive cycle is highly dependent on temperature and salinity variations, resulting in a seasonal cycle and spatial heterogeneity. The temperature induces gametogenesis and salinity synchronizes the spawning periods

Deciphering the molecular adaptation of the king scallop (Pecten maximus) to heat stress using transcriptomics and proteomics

Background The capacity of marine species to survive chronic heat stress underpins their ability to survive warming oceans as a result of climate change. In this study RNA-Seq and 2-DE proteomics were employed to decipher the molecular response of the sub-tidal bivalve Pecten maximus, to elevated temperatures. Results Individuals were maintained at three different temperatures (15, 21 and 25 °C) for 56 days, representing control conditions, maximum environmental temperature and extreme warming, with individuals sampled at seven time points. The scallops thrived at 21 °C, but suffered a reduction in condition at 25 °C. RNA-Seq analyses produced 26,064 assembled contigs, of which 531 were differentially expressed, with putative annotation assigned to 177 transcripts. The proteomic approach identified 24 differentially expressed proteins, with nine identified by mass spectrometry. Network analysis of these results indicated a pivotal role for GAPDH and AP-1 signalling pathways. Data also suggested a remodelling of the cell structure, as revealed by the differential expression of genes involved in the cytoskeleton and cell membrane and a reduction in DNA repair. They also indicated the diversion of energetic metabolism towards the mobilization of lipid energy reserves to fuel the increased metabolic rate at the higher temperature. Conclusions This work provides preliminary insights into the response of P. maximus to chronic heat stress and provides a basis for future studies examining the tipping points and energetic trade-offs of scallop culture in warming oceans

Modelling paralytic shellfish toxins (PST) accumulation in Crassostrea gigas by using Dynamic Energy Budgets (DEB)

As other filter-feeders, Crassostrea gigas can concentrate paralytic shellfish toxins (PST) by consuming dinoflagellate phytoplankton species like Alexandrium minutum. Intake of PST in oyster tissues mainly results from feeding processes, i.e. clearance rate, pre-ingestive sorting and ingestion that are directly influenced by environmental conditions (trophic sources, temperature). This study aimed to develop a mechanistic model coupling the kinetics of PST accumulation and bioenergetics in C. gigas based on Dynamic Energy Budget (DEB) theory. For the first time, the Synthesizing Units (SU) concept was applied to formalize the feeding preference of oysters between non-toxic and toxic microalgae. Toxin intake and accumulation were both dependent on the physiological status of oysters. The accumulation was modelled through the dynamics of two toxin compartments: (1) a compartment of ingested but non-assimilated toxins, with labile toxins within the digestive gland eliminated via faeces production; (2) a compartment of assimilated toxins with a rapid detoxification rate (within a few days). Firstly, the DEB-PST model was calibrated using data from two laboratory experiments where oysters have been exposed to A. minutum. Secondly, it was validated using data from another laboratory experiment and from three field surveys carried out in the Bay of Brest (France) from 2012 to 2014. To account for the variability in PST content of A. minutum cells, the saxitoxin (STX) amount per energy units in a toxic algae (ρPST) was adjusted for each dataset. Additionally, the effects of PST on the oyster bioenergetics were calibrated during the first laboratory experiment. However, these effects were shown to depend on the strain of A. minutum. Results of this study could be of great importance for monitoring agencies and decision makers to identify risky conditions (e.g. production areas, seawater temperature), to properly assess detoxification step (e.g. duration, modalities) before any commercialization or to improve predictions regarding closing of shellfish areas

Handling enhances the development of brown ring disease signs in Ruditapes philippinarum

International audienceBrown ring disease (BRD) in the Manila clam is characterized by the formation of a brown deposit of conchiolin on the inner surface of the shell that gives the disease its name. The development of the signs of BRD may be favored by the entry of bacteria in the extrapallial compartments via mechanical disruptions of the periostracal lamina and/or chipping of the shell margin. In order to test this hypothesis, we conducted an experiment simulating clam handling under aquaculture conditions and we checked for prevalence of BRD signs. Our results assess that rough handling of R. philippinarum in presence of the bacterium V. tapetis significantly increase the prevalence of BRD signs. As a consequence our results show that minimizing manipulations and transfers of clams during culture is beneficial to avoid the development of BRD signs

Effect of sediment grain-size on development of brown ring disease in the Manila clam Ruditapes philippinarum

International audienceBrown ring disease (BRD) in the Manila clam is induced by the bacterium Vibrio tapetis. During the infection process, the pathogen enters the extrapallial compartment of the Manila clam and induces the formation of a characteristic brown deposit that gives the disease its name. Although post-infection processes have been widely described for this disease, the mechanisms of entry of the bacteria into the extrapallial compartment remains unclear. From relationships between clam size and BRD prevalence, and between grain-size distribution in natural habitats and prevalence, we propose a simple explanation for this step: V. tapetis benefits from mechanical disruptions of the periostracal lamina or valve margins to colonize the extrapallial compartment. Such disruptions may be induced by the presence of large sediment grains in natural habitats, which become lodged in the shell opening. This hypothesis suggests that limiting handling of clams may help to limit development of BRD in cultured clam beds

Respiratory response to combined heat and hypoxia in the marine bivalves Pecten maximus and Mytilus spp.

International audienceCoastal ecosystems are increasingly disturbed by the increase of mean sea surface temperature and expansion of hypoxic areas. The objectives of the present work were to describe and compare the respiratory responses to combined heat and hypoxia in two bivalve species (Pecten maximus and Mytilus spp.) living in two contrasted coastal habitats (subtidal and intertidal, respectively). Results were consistent with the vertical zonation of both species. Mytilus spp. seemed to cope better with a temperature increase than P. maximus, which was found to be outside of its optimal thermal window at 25°C. Concerning respiratory responses to hypoxia at a given temperature, P. maximus displayed greater oxyregulation capacity that was maintained over a larger range of O2 levels, as compared to Mytilus spp. When acclimation temperatures increased, both species showed a decrease in their oxyregulation capacities alongside a reduction in aerobic performance, especially in P. maximus. The comparison between species suggests that subtidal species, such as P. maximus, might be more vulnerable to a combination of heat and hypoxia than intertidal species, such as Mytilus spp. Lastly, this study highlighted the utility of segmented linear models to estimate PcO2 and regulation percentages in marine organisms exposed to hypoxi

Reconstructing physiological history from growth, a method to invert DEB models

Dynamic Energy Budget (DEB) models rely on measurements of food availability to describe the rates at which organisms assimilate and use energy from food for maintenance, growth, maturation and reproduction. Although crucial, the determination of appropriate and accurate energy input variables can be problematic. We developed an inverted DEB model to reconstruct the food intake from temperature and growth trajectories. The method makes use of a reformulation of the DEB model dynamics into a second order linear equation. This formula not only allows the reconstruction of the scaled functional response but also gives access to reserve dynamics, mobilization, and somatic maintenance fluxes. The shell of the great scallop, Pecten maximus, providing high resolution records of incremental growth, was used to explore the potential of this approach to reconstruct the functional response from daily shell growth rates data. In a theoretical case, we investigated the resolution and sensitivity limits of the method. In a validation process, predictions were used to re-simulate growth that was compared to the initial growth trajectory. Moreover, as growth data used in the reconstruction process usually show high-frequency variability, we also developed a smoothing method, based on DEB theory assumptions, to filter growth data time series

Impact of Brown Ring Disease on the energy budget of the Manila clam Ruditapes philippinarum

International audienceBrown Ring Disease (BRD) is a bacterial disease caused by the pathogen, Vibrio tapetis. The disease induces formation of a brown deposit on inner shell of the Manila clam, Ruditapes philippinarum. Development of this disease is correlated with a decrease in the condition index of infected clams. Experiments were conduced in order to assess the effect of the development of BRD on two parameters affecting the energy balance of the clams: the clearance and the respiration rates. Experiments were performed in a physiological measurement system that allowed simultaneous measures of clearance and respiration rates. During both acclimation and measurements clams were fed with cultured T-iso and temperature was close to seasonal field temperature (10°C). Our results showed that severely diseased clams (conchiolin deposit stage, CDS ≥ 4) are subject to weight loss in comparison to uninfected ones, indicating that BRD induces a disequilibrium in the energy balance. We demonstrated a reduction of the clearance rate of severely diseased clams which led to a decrease in energy acquisition. Respiration rate showed a significant decrease with BRD symptoms, but evidence in the literature allowed us to hypothesize that energy mobilised for an immune response and lesion repair increases overall organism maintenance costs. Both factors should thus contribute to the degradation of the energy balance of diseased clams. Because effects of BRD on naturally infected clams only appears significant for CDS ≥ 4, when brown ring assumes a significant place on the inner shell, we consider that the Manila clam is tolerant of low disease levels