Oysters modify the planktonic microbial community structure by their filtration and NH 4 excretion activities. While many studies have been conducted on this subject with adult oysters, none had been carried out in situ with juveniles. Pacific oyster juveniles (Magallana gigas, previously Crassostrea gigas) died massively all over the world since 2008 in relation with OsHV-1 infection. During mortality episodes, sick and dead oysters are not separated from healthy live ones, and left to decay in the surrounding environment, with unknown consequences for the nutrient cycle and planktonic microbial components (PMC). The present study aimed to elucidate for the first time the interactions between oyster juveniles and PMC during a mortality episode. Innovative 425-L pelagic chambers were deployed weekly in situ around oyster lanterns along a stocking-density gradient in the Thau Mediterranean lagoon (France) before, during and after an oyster mortality episode, from April to May 2015. This study reveals (i) significant changes of planktonic microbial community structure during mortality episodes, with a proliferation of picoplankton (<3 μm) and ciliates (Balanion sp., Uronema sp.) within 2 weeks when mortality rates and numbers of moribund juvenile oysters were highest. These changes were probably induced by oyster tissue leaching, decomposition and mineralization, which probably began during the moribund period, as suggested by an increase of PO 4 concentration and N:P ratio decrease, (ii) oyster juveniles mainly retained 3-20 μm plankton. In contrast to adults, picophytoplankton and small heterotrophic flagellates (<3 μm) were significantly depleted in the presence of oyster juveniles. Depletion of picoplankton occurred only at the starting of the mortality episode and during the moribund phase. (iii) Oyster juvenile filtration and mortality shifted the planktonic microbial structure toward a heterotrophic microbial system, where ciliates and heterotrophic flagellates acted as a trophic link between picoplankton and oysters. The next stage of our investigation is to examine the effect of a mortality episode on pathogen fluxes in the water column, exploring their relationships with planktonic Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.components and dead oyster flesh. Highlights► The planktonic microbial components (PMC) change during OsHV-1 oyster juvenile mortality. ► Picophytoplankton and ciliates increase during infection and mortality periods. ► Filtration and mortality of juvenile oysters shift PMC toward a heterotrophic system.
Within populations, phenotypic plasticity may allow adaptive phenotypic variation in response to selection generated by environmental heterogeneity. For instance, in multivoltine species, seasonal changes between and within generations may trigger morphological and physiological variation enhancing fitness under different environmental conditions. These seasonal changes may irreversibly affect adult phenotypes when experienced during development. Yet, the irreversible effects of developmental plasticity on adult morphology have rarely been linked to life-history traits even though they may affect different fitness components such as reproduction, mobility and self-maintenance. To address this issue, we raised larvae of Pieris napi butterflies under warm or cool conditions to subsequently compare adult performance in terms of reproduction performance (as assessed through fecundity), displacement capacity (as assessed through flight propensity and endurance) and self-maintenance (as assessed through the measurement of oxidative markers). As expected in ectotherms, individuals developed faster under warm conditions and were smaller than individuals developing under cool conditions. They also had more slender wings and showed a higher wing surface ratio. These morphological differences were associated with changes in the reproductive and flight performances of adults, as individuals developing under warm conditions laid fewer eggs and flew larger distances. Accordingly, the examination of their oxidative status suggested that individuals developing under warm conditions invested more strongly into self-maintenance than individuals developing under cool conditions (possibly at the expense of reproduction). Overall, our results indicate that developmental conditions have long-term consequences on several adult traits in butterflies. This plasticity likely acts on life history strategies for each generation to keep pace with seasonal variations and may facilitate acclimation processes in the context of climate change.
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