Ecologie du système hôte - parasite, juvéniles G0 de sole (Solea solea) - métacercaires de Digènes : dynamique et effets de l'infestation

Coastal areas, such as shallow water bays and estuaries, serve as nurseries for juveniles of many marine fishes and especially flatfishes. Parasites play an important role in these marine ecosystems, but it remains poorly studied. Little attention has been paid to parasitism as a potential factor of variation in flatfish juveniles biological performances in these habitats. Sole juveniles, Solea solea (Pleuronectiformes, Soleidae), that is a species of primary importance for fisheries, are infected by digenean metacercariae. Thus, the aims of this PhD thesis were: i) to determine the spatio-temporal dynamics of the digenean metacercariae infection of 0-group sole juveniles at different scales; ii) to estimate the effects of this infection at different biological integrative levels (molecular immunity, condition, growth, behaviour). Both descriptive and experimental approaches were used. In coastal nursery grounds, 0-group sole locally accumulate different taxa of digenean metacercariae: Prosorhynchus spp. (Bucephalidae), Timoniella spp. (Acanthostomidae) and Podocotyle sp. (Opécoelidae). Spatial dynamics of the infection was studied through field sampling at meso-scale between nurseries (several tens of km) and at small scale (few km) in the Pertuis Charentais area; an in situ caging experiment allowed to analyse the dynamics at fine scale (hundreds of m). These studies showed that digenean metacercariae load in 0-group sole is controlled by the major local factors, i.e. proximity (probably also abundance) of the first intermediate hosts and cercariae dispersal capacities. Higher infection levels in bays (compared to large estuaries) highlight the success of the parasites life cycles in this habitats. Infection is characterized by a high heterogeneity at small spatial scale. Thus, digenean metacercariae have been used as biological tags of habitat use by 0-group sole juveniles in the Pertuis Charentais. 0-group sole juveniles show a high residency and a high variability of their growth performances at small spatial scale. The quality of the habitat is highest for 0-group sole in the Aiguillon Bay (compared to other part of the Pertuis Charentais): it is a typical nursery. Species of the genus Prosorhynchus (that infest 0-group sole), namely P. crucibulum and Prosorhynchus sp. A (similar to P. aculeatus) were distinguished combining morpho-anatomical and molecular analysis (ITS 1 sequencing). Life cycle of P. crucibulum was confirmed and the impact of mussel farming (mussel = first intermediate host) in the transmission of this parasite was highlighted. At around 1 month post-infection (when metacercariae lie free in host tissues), a positive relationship between the abundance of P. crucibulum and the expression of C3 gene (measured by RT-real time qPCR) of 0-group sole was shown. This might indicate a generalized immune response where the complement system is involved. A 3 months growth experiment was realized in semi-natural conditions on experimentally in situ infected individuals (with a control group). The growth and condition of 0-group sole are not affected which show the compensation capacities of sole for the cost of parasitism in non-restricted trophic and thermal conditions. A positive relationship between the abundance of P. crucibulum and the swimming activity (quantified by video-tracking) was detected. This result might indicate a behavioural strategy of energetic compensation of the host for the cost of parasitism, leading also to a potentially higher exposure of highly infected individuals to predators. In the field, only a negative correlation between the abundance of P. crucibulum metacercariae and the Fulton's condition index (K) of 0-group sole was detected at the onset of winter. The strong water temperature decrease during this time and the very low nutritional status (lipid index TAG / ST) of the individuals in this part of the habitat, likely conduct to diminish their compensation capacities against high P. crucibulum infection levels, which might lower their first winter survival. Parasitism (i.e. digenean metacercariae) is a factor that has effects on immunity, condition and behaviour of 0-group sole juveniles. These impacts are modulated by the development of the parasite, the physiological status of the host and the environmental factors. Furthermore, these parasites are particularly efficient biological indicators to study habitat use of their host. From now, parasitism appears essential in the study of the functioning of flatfish coastal nurseries.