Soutenance mémoire
Sampling of adult populations and natural seston
The laboratory experiment was conducted from March 6th to April 26th 2012 at the Centre for Marine Biology (CEBIMar) of the University of Sâo Paulo (USP), in Sâo Sebastiâo, Brazil. Rock chips were collected at Calhetas (23°45’S; 45°24’W) on March 6th (T,), 10 km apart from the CEBIMar, where the barnacle cover is denser. In CEBIMar, 6 X 2 L of seawater pumped from 0.5 to 1 m from the surface and pre-filtered through a square mesh sieve (80 J.lm) have been filtered on 25 mm GFIF filter (Whatman, Maidstone, UK). Three filters have been used for dry mass determination (70°C, 24h) and three for fatty acids characterization of natural seston. This characterization of natural seston will be used to estimate the level of differentiation with the experimental diet.
Impact on fatty acids profiles
The total FA concentration of the neutral fraction has been used as an indicator of the lipid energetic reserves accumulation in both adults and nauplii. In adults, we observed a significant effect of time (F2,23 = 4.68; P = 0.02), with values 40% lower in T3 compared to Tl (Fig. 10). Total mass of neutral fatty acids in nauplii showed a triple interaction (time X food supply X thermal stress; F2,23 = 3.65; P = 0.042), but without conclusive trends. No interpretation had been visible. To obtain an indicator of the nutritional quality of experimental diets, we used the ratio of the polar fraction of fatty acids (structural phospholipids of cell membranes) in adults to the fatty acids of the food (microalgae mix). This ratio indicates selective incorporation or elimination in the cell membrane (polar lipid) of adults of a given dietary fatty acid. We assume that a similar fatty acid level in the cell membranes of adults and in the food (relative proportion :S 1) confirms that the nutritional requirements for this fatty acid were largely satisfied. For ail treatment combinations, AA and EPA ratios were generally close to 1, signifying that the food fulfilled the nutritive necessities for those EF A. For DHA, which higher ratios were observed in the LF treatment (up to 4.7; Fig. 12), ratios were mostly higher than 2, suggesting an important selective incorporation. A triple interaction among time, food and thermal stress was detected (PERMANOV A, pseudoF = 5.06; P = 0.0031). An important decrease of the three EFA ratios was observed at T2 in the control for the HF treatment compared to the LF treatment, especially for DHA (ratio of 1.3 and 4.7, respectively). Despite this diminution, those ratios stay close to 1 for the LF treatment.
Impact of food supply
The tirst hypothesis stipulated that high food supply wou Id enable adult barnacles to produce high quality larvae. Yet, our barnacle populations maintained ad libitum did not produce larger numbers of nauplii, neither affect their neutral fatty acid profile, their size, or their survival (Fig. 9A and IOA). FA composition in the algal mix could explain the lack of adults FA content and mortality differentiation observed between populations maintained under the two different food conditions. Furthermore, no difference was observed in adult growth in ail treatment. What we defined as a low-food treatment might be high enough for this species, ev en if a ratio of 3 to 6% of microalgae by biomass of adults has been shown necessary to support reproductive needs in bivalves hatcheries (Helm et al. 2004). Our results could suggest that this barnacle species in this oligotrophic environment is very weil adapted to the low food availability to support reproduction with food availability lesser than 1 % of the adults’ biomass Larvae and adult barnacles feed on the natural seston to obtain their food, including lipid resources (Whyte et al. 1989, 1991 , Soudant, Marty, Moal, Robert, et al. 1996, Nevejan et al. 2003, Pernet et al. 2005). A part of those fatty acids could be used for energetic need (p-oxidation), stored as energetic reserve (neutral fraction) or used for ce ll membranes structure (polar fraction; Waldock & Holland 1978, Gallager & Mann 1986, Spector & Yorek 1985, Pernet et al. 2003). So, the analyses of lipid profiles of adu lts, larvae and their food provided information on preferentially fatty acids ingested and retained. It’s important to notice that fatty acids incorporation in neutrallipids reflect more closely the food composition ingested as there are less regulated than fatty acids in polar lipids (Delaunay et al. 1993, Caers et al. 2002). The comparison between the natural seston and the algal mix informs on resource available for adults. Barnacles FA profiles resulting from the different trophic resources (before and after the 22 d acclimation) show potential modifications by acclimation to experimental diets. The low ratio «1) of 16: 1116:0 and EPAIDHA indicated that planktonic composition should be mainly dominated by dinoflagellates comparatively to diatom (Napolitano et al. 1997, Budge & Parrish 1998, Dalsgaard et al. 2003, Wilson et al. 2010). Despite the low amount of DHA and EPA « 5%) in the natural seston, adults sampled at T-l accumulated those EFA over th an 35% in the neutral fraction. Thus, in this . environment generally considered as oligotrophic, Chthamalus bisinuatus succeeded to selectively accumulating essential fatty acids (DHA, EPA and AA) from phytoplanktonic species (Martfnez-Pita et al. 2005).
|
Table des matières
REMERCIEMENTS
RÉSUMÉ
ABSTRACT
LISTE DES TABLEAUX
LISTE DES FIGURES
INTRODUCTION GÉNÉRALE
CHAPITRE 1 Apport maternel chez la balane tropicale: Plasticité de l’allocation de la ressource trophique pour la production larvaire en conditions de limitation nutritionnelle et de stress environnemental
RESUME EN FRANÇAIS DU PREMIER ARTICLE
MATERNAL INPUT IN A TROPICAL BARNACLE: PLASTICITY IN TROPHIC RESOURCE ALLOCA TION FOR LARV AL PRODUCTION IN NUTRITIONAL LIMITATION CONDITIONS AND ENVIRONMENT AL STRESS
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
Sampling of adult populations and natural seston
Experimental setup
Response variables
Statistical analyses
RESULTS
Effect of laboratory acclimation
Impact on mortality and larval productivity
Impact on fatty acids profiles
DISCUSSION
Impact of food supply
lmpact of thermal stress
CONCLUSION
ACKNOWLEDGMENTS
CHAPITRE 2 CONCLUSION
RÉFÉRENCES BIBLIOGRAPHIQUES
Télécharger le rapport complet
