Analyse des données collectées lors des embarquements à bord des senneurs basés aux Seychelles (1986-1989)

La pêcherie thonière à la senne basée aux Seychelles est analysée au travers de données collectées lors d'embarquements d'observateurs à bord des navires français, espagnols, japonais et soviétiques. De 1986 à 1989, ont été récoltées des données ayant trait à certains paramètres météorologiques et océanographiques, à l'aspect et à l'importance des bancs observés, aux activités des senneurs, aux caractéristiques des pêches (prises, rendements et durée des coups de filet). La fiabilité des données d'observateurs se rapportant aux captures est évaluée par comparaison avec les données des livres de bord des patrons de senneurs français. (Résumé d'auteur

Is apparent low productivity of the invasive marine mollusc Crepidula fornicata Linnaeus, 1758 related to biased age determination?

The slipper limpet Crepidula fornicata Linnaeus, 1758 is a major exotic invader of East North Atlantic coasts. Individuals live on top of each other and form stacks with the youngest on the top. Earlier studies reported that one individual typically settles per year. If true, it is a simplified means to provide a “shell length-age” relationship for population dynamics studies, especially Production/Biomass ratio (P/B) assessment. However, estimated P/B ranges between 0.15 and 0.45 yr-1 seem low compared to those of closely-related marine invertebrates and considering the invasiveness of C. fornicata in coastal habitats. In this study, we placed artificial substrates (“tiles”) for one year in the middle of a C. fornicata colony and measured settlement. We sometimes observed more than one individual settling per stack in a year: 4% of stacks were composed by 3 individuals, 27% by 2 individuals and 69% were single specimen. On this basis, we formulated a model to better link the position of each C. fornicata within a stack to its age. In addition, the C. fornicata population was annually sampled for 5 years. Then, population dynamics parameters, density at recruitment, mortality rate, growth performance, production and P/B were estimated. We compared two cases: (i) individual age was defined by its position in stacks; (ii) individual age was corrected by the model. Recalculation moderately increased growth performance expectation (+2 to 8% per year) but greatly enhanced production and P/B (ca. 1.2 to 2.6-fold per year). Recalculated P/B values ranged from 0.55 to 0.72 yr-1. While still low, they were more consistent with published values for similar large marine invertebrates, in particular for invasive species

Intraspecific trade-offs between facilitation and competition in the non-native mollusc Crepidula fornicata

We tested the hypothesis that high-density populations of the non-indigenous gastropod Crepidula fornicata increase settlement of conspecific recruits (facilitation process), and that this facilitation is balanced by competition processes. To verify our hypothesis, we sampled C. fornicata at 2 drastically different densities for 10 yr. We found that at high densities, the number of 1 yr old individuals per square meter colonizing the habitat and individual growth performances were higher in comparison with the low-density condition (Allee effect). In contrast, the production/biomass ratio (P/B), a good indicator of population fitness, was lower at higher densities. We relate this lower P/B ratio to the deficit of young individuals compared to adult biomass. We conclude that the net effect of high density on the conspecific colonization processes of the population is positive, thanks to the higher available hard substrate for larvae (facilitation). However, intraspecific competition also occurs and mitigates this positive effect. Therefore, we suggest that it is particularly important to take into account the ‘net’ balance between costs and benefits (i.e. what we observe) when analyzing population growth, in order to better understand its dynamics

Large scale patterns of trematode parasites communities infecting Cerastoderma educe along the Atlantic coast from Portugal to Morocco

In this study, spatial sampling was performed from north Portugal to south Morocco to analyse trematode communities of the widespread bivalve Cerastoderma edule, host of several trematode species. From the twelve trematode species found in this study, nine were present in multiple aquatic systems demonstrating high trematode dispersal ability, driven by the presence of all hosts. Multivariate analysis related to trematode communities in cockles clustered: 1) Portuguese aquatic systems influenced by cold waters, leading to low trematode abundance; 2) coastal systems characterized by dominance of trematode Parvatrema minutum and muddy sediments; 3) lagoons (or bays) with high oceanic influence and high trematode diversity. These findings suggested that, besides host species presence, temperature is an important trigger for parasite infection, with coastal upwelling operating as a shield against trematode infection in Portugal and masking latitudinal gradients. Results highlighted the possible consequences of thermal modification mediated by oceanographic global circulation change on cockle populations.publishe

Production of marine trematode cercariae: a potentially overlooked path of energy flow in benthic systems

Parasites, in particular trematodes, are unseen but ubiquitous components of marine intertidal ecosystems. Although parasites are known to affect population dynamics and food web structure, their potential function as an unrecognized path of energy flow in these ecosystems is yet to be quantified. We use published data on rates at which trematodes produce free-swimming infective larvae (cercariae) that are released from their gastropod intermediate hosts to investigate patterns in cercarial output as a function of different variables, and to calculate the annual production of cercariae in different marine benthic systems. Across 18 trematode species, cercarial output (no. cercariae shed snail–1 d–1) ranged over 4 orders of magnitude and was positively correlated with snail host species size. While cercarial output did not correlate with latitude, it did correlate negatively with the size of cercariae, and was influenced by the type of downstream host sought by cercariae, being highest when this host was a vertebrate. Our estimates of annual cercarial production (kJ m–2 yr–1), which take into account the density of infected snails in the habitat, were within the range of production values reported for free-living invertebrates inhabiting benthic ecosystems. These estimates would be much higher if they included all trematode species in an ecosystem, and not just single-species values. Overall, results suggest that trematode cercariae represent potentially important paths of energy flow in benthic systems as well as a potentially important food supply to benthic organisms

Metallothionein gene identification and expression in the cockle ( Cerastoderma edule) under parasitism (trematodes) and cadmium contaminations

Parmi les organismes benthiques, les bivalves sont souvent utilisés comme bioindicateurs des pollutions environnementales, du fait de leur importante capacité de bioaccumulation des métaux lourds conduisant à l'induction des métallothionéines (MT). Les MT sont de petites protéines cytosoliques capables de fixer les métaux et qui sont impliquées dans l'homéostasie et la détoxication de ces métaux chez les organismes. Ces protéines peuvent être induites par un grand nombre de facteurs tels que les hormones, les stress physiques, le parasitisme. La quantification des MT en relation avec le parasitisme est rarement reportée dans la littérature, bien que le parasitisme soit omniprésent et très délétère chez les bivalves. De plus, seulement un petit nombre de gènes de MT ont été identifiés chez les mollusques. Cette étude décrit la caractérisation de la séquence partielle d'un gène de MT (Cemt1) chez la coque Cerastoderma edule, qui présente plus de 80 % d'homologie avec de nombreuses séquences de MT de bivalves. A partir de cette séquence, des amorces spécifiques, utilisables en PCR quantitative en temps réel, ont été déterminées. Le niveau d'expression du gène Cemt1, ainsi que la quantité de protéine MT ont été analysés chez des individus soumis à différents paramètres : sans parasites ou infestés par le trématode digène Himasthla elongata, et lors d'expositions au cadmium à 15 µg Cd L−1. Des résultats concordants ont été obtenus lors de la quantification des protéines MT et de l'analyse de l'expression du gène Cemt1. Ceux-ci mettent en évidence que les concentrations en MT augmentent significativement par les deux types de traitement (infestation parasitaire et exposition au cadmium).Among benthic organisms, bivalves are often used as bioindicators of environmental pollution because of their high bioaccumulation capacities for heavy metals leading to metallothioneins (MT) induction. MT are small cytosolic metal-binding proteins involved in metal homeostasis and detoxification in living organisms. These proteins can also be induced by a wide range of factors, such as hormones, physical stress, parasitism. MT quantification in relation to parasitism is rarely reported in literature, while parasites are omnipresent and have deleterious impacts on bivalves. Moreover, only a few number of MT genes have been characterized in molluscs. This study describes the partial sequence of the MT gene (Cemt1) in the edible cockle Cerastoderma edule. The cockle's MT cDNA was sequenced and showed over 80% homology to several other bivalve MT sequences. This sequence was then used to determine MT specific primers which can be used in quantitative real time PCR. MT protein and gene expression levels were quantified for individuals selected under different conditions: free from or infected by the digenean trematode Himasthla elongata, and under cadmium exposure at 15 mu g Cd L-1. Results evidenced that MT concentrations were significantly increased by both treatments; parasite infection and Cd exposure. Moreover, congruent results between MT protein and gene expression levels were obtained

Mobilisation of data to stakeholder communities. Bridging the research-practice gap using a commercial shellfish species model

Knowledge mobilisation is required to “bridge the gap” between research, policy and practice. This activity is dependent on the amount, richness and quality of the data published. To understand the impact of a changing climate on commercial species, stakeholder communities require better knowledge of their past and current situations. The common cockle (Cerastoderma edule) is an excellent model species for this type of analysis, as it is well-studied due to its cultural, commercial and ecological significance in west Europe. Recently, C. edule harvests have decreased, coinciding with frequent mass mortalities, due to factors such as a changing climate and diseases. In this study, macro and micro level marine historical ecology techniques were used to create datasets on topics including: cockle abundance, spawning duration and harvest levels, as well as the ecological factors impacting those cockle populations. These data were correlated with changing climate and the Atlantic Multidecadal Oscillation (AMO) index to assess if they are drivers of cockle abundance and harvesting. The analyses identified the key stakeholder communities involved in cockle research and data acquisition. It highlighted that data collection was sporadic and lacking in cross-national/stakeholder community coordination. A major finding was that local variability in cockle populations is influenced by biotic (parasites) and abiotic (temperature, legislation and harvesting) factors, and at a global scale by climate (AMO Index). This comprehensive study provided an insight into the European cockle fishery but also highlights the need to identify the type of data required, the importance of standardised monitoring, and dissemination efforts, taking into account the knowledge, source, and audience. These factors are key elements that will be highly beneficial not only to the cockle stakeholder communities but to other commercial species

Brown muscle disease and Manila clam Ruditapes Philippinarum dynamics in Arcachon Bay, France

Brown Muscle Disease (BMD) affects Manila clam Ruditapes philippinarum. It was described for the first time in 2005 in Arcachon Bay, France. The pathology consists in a progressive necrosis of the posterior adductor muscle, valve gaping, clam migration to the sediment surface, and death. This study aims to quantify the prevalence of BMD in the bay and to evaluate the effect of BMD on Manila clam dynamics. The prevalence was assessed on 50 stations spread within Arcachon Bay. About 62% of Manila clam habitat surface was infected by BMD. A survey of buried and surface clams was conducted from November 2006 to March 2008 in Lanton, a site infected by BMD. Modal progression analysis separated confidently cohorts from 2003 to 2005 recruitments. This pathology only affected adult clams (>25 mm, >2 years). For both buried and surface individuals, shell length was significantly correlated with BMD infection. Surface clams had prevalence (67%) higher than buried clams (23%) and showed greater mortality rate after 15 d in running water: 82% against 12% for buried individuals. The final disease index (FDI) and the condition index (CI) were monthly evaluated on 50 clams located at each position in the sediment. CI displayed a significant decrease after BMD's infection from light to severe disease stages. Length data analysis through Bhattacharya's method (FISAT II software) allowed identifying four cohorts. The 2003s cohort enabled to calculate mortality rate that was 39% for 5 months and to estimate that BMD was responsible of 95% of that mortality. The temperature was certainly an important factor in BMD transmission, because cohort dynamics results argued that BMD developed during spring and summer

Long-term evolution (1988-2008) of Zostera spp. meadows in Arcachon Bay (Bay of Biscay)

The spatial variability of seagrass meadows in Arcachon Bay, was studied between 1988 and 2008 using a combination of mapping techniques based on aerial photographs for intertidal dwarf-grass (Zostera noltti) beds and acoustic sonar for permanently submerged eelgrass (Zostera marina) populations. The results show a severe decline over the period for both species, as well as an acceleration of the decline since 2005 for Z. noltii. The total surface regression over the studied period is estimated to be 22.8 km(2) for Z. noltii and 2.7 km(2) for Z. marina, which represent declines of 33 and 74% respectively. Environmental data time series spanning the same period were investigated in order to seek the causes for such a decline. The calculated inter-annual trends for temperature, salinity, nitrate plus nitrite, ammonia, suspended sediment and chlorophyll a did not identify any clear environmental change capable of explaining the observed seagrass regression. For instance, no evident sign of eutrophication was observed over the study period. On the other hand, we suggest that the observed variations of ammonia in the inner part of the lagoon are a symptom of the seagrasses' disappearance and thus, a first sign indicating a change of the Arcachon Bay ecosystem towards more instability and vulnerability. Several hypotheses to explain the observed seagrass decay are proposed. (C) 2010 Elsevier Ltd. All rights reserved

Molecular confirmation of pearl formation in arctic mussels (Mytilus edulis) caused by Gymnophallus bursicola (Odhner 1900) metacercariae

Abstract In recent field studies, suspected gymnophallid metacercariae were histologically located in the mantle of mussels from the Norwegian Sea. Mussels from the sites in which that infection was detected also presented abnormally high pearl numbers. It has been previously described that gymnophallid metacercariae could cause pearl formation processes in mussels, as a host reaction to encapsulate these metacercariae. Given the pathological host reaction these parasites elicit, a study was performed to identify gymnophallid metacercariae found in mussels collected from Tromsø at morphological and molecular level and to assess, by the use of molecular tools, the relationship between the parasite and the biological material inside the pearls. As a result, Gymnophallus bursicola metacercariae infecting Norwegian Mytilus edulis were identified according to morphological characters, along with the first 18S rDNA and COI sequences for this trematode species. In addition, parasite DNA from the core of the pearls was extracted and amplified for the first time, confirming the parasitological origin of these pearls. This procedure could allow identifying different parasitic organisms responsible for the generation of pearls in bivalves