Comparative analysis using complete ITS2 and partial 28S rDNA sequence data revealed that cercariaeum developing in rediae in Lithoglyphus naticoides represent two different lissorchiid species. One morphotype of cercariaeum is conspecific with adult Palaeorchis incognitus from European roach, Rutilus rutilus. The other cercariaeum is attributable to the genus Asymphylodora, but the species identity is not yet determined. We also generate the first rDNA sequences for Asymphylodora progenetica based on new collections from Bithynia tentaculata from Lithuania. Phylogenetic analyses of the newly generated sequences, together with information for other lissorchiids available on GenBank, showed that all representatives of Lissorchiidae form a strongly supported clade. Three monophyletic lineages, Asymphylodora, Palaeorchis and Lissorchis, were recognized at the generic level. Karyological analysis of the chromosome set of larval P. incognitus revealed a diploid number of 2n = 20. Its karyotype with subtelocentric chromosomes prevailing can be regarded as comparatively ‘primitive’, which is consistent with the basal position of P. incognitus in the 28S tree relative to the representatives of the genus Asymphylodora. The present study adds significant new information for establishing species-specific markers for the confident characterization of different developmental stages of lissorchiid species and clarification of their life cycles.

Populations of generalist species often comprise of smaller sub-sets of relatively specialized individuals whose niches comprise small sub-sets of the overall population niche. Here, the role of parasite infections in trophic niche specialization was tested using five wild fish populations infected with the non-native parasite Ergasilus briani, a copepod parasite with a direct lifecycle that infects the gill tissues of fish hosts. Infected and uninfected fishes were sampled from the same habitats during sampling events. Prevalence in the host populations ranged between 16 and 67%, with parasite abundances of up to 66 parasites per fish. Although pathological impacts included hyperplasia and localized haemorrhaging of gill tissues, there were no significant differences in the length, weight and condition of infected and uninfected fishes. Stable isotope analyses (δ13C, δ15N) revealed that the trophic niche of infected fishes, measured as standard ellipse area (i.e. the isotopic niche), was consistently and significantly smaller compared with uninfected conspecifics. These niches of infected fishes always sat within that of uninfected fish, suggesting trophic specialization in hosts. These results suggested trophic specialization is a potentially important non-lethal consequence of parasite infection that results from impaired functional traits of the host.

The prevalence and relative density of the monogenean Paradiplozoon homoion, were monitored over one year in populations of roach, Rutilus rutilus, captured in a thermally raised artificial lake located outside a nuclear power station and in a nearby reference locality. Recruitment of P. homoion started earlier in the warmer lake. A temporal shift of the life-cycle was thus observed but there were no differences between the total prevalence or the total relative density of the parasite in the two areas. Since there is no evidence of parasite-induced host mortality, or for the development of protective immunity, the main regulatory process in the heated area are most likely an increased mortality due to reduced thermal tolerance of the adult parasite.

Patterns of parasite infestation by plerocercoid forms of the tapeworm Ligula intestinalis (Cestoda, Pseudophyllidea) in the second intermediate host, the roach Rutilus rutilus, were analysed in three fish populations of South-Western France during 1998. In all three studied sites, i.e. Pareloup Lake, Lavernose-Lacasse and Muret gravel pits, roach were commonly parasitized with plerocercoids. A strong increase in prevalence, mean number of parasites and parasitic load was observed in the youngest hosts with a maximum peak reached in the medium-age (2+, 3+)roach classes. Such trends as a function of host fish age would indicate the occurrence of parasite-induced host mortality, with the most heavily infected fish specimens being removed from the three roach populations. The three roach populations infected by L. intestinalis exhibit different patterns of parasite-induced mortality which could correspond to different possible ecological and evolutionary responses associated with local variation in host-parasite interactions. The parasite-induced host mortality observed, i.e. direct mortality caused by pathogenic effects and /or enhanced fish predation by piscivorous birds to increase parasitic transmission is discussed within the context of the adaptative parasite-induced manipulation hypothesis.

Experimental exposure to bleached kraft pulp and paper mill effluent (BKME) at a concentration of 10% significantly reduced the abundance and mean number of species of Dactylogyrus on the gills of naturally infected roach, Rutilus rutilus, over a 3 week period. Seven Dactylogyrus species were recorded which differed widely in their susceptibility to the effluent. The experiment coincided with a natural spring peak in dactylogyrid infections on roach. BMKE exposure did not prevent parasite reproduction but post-larval abundance was significantly higher on control than effluent-exposed hosts. There was no evidence of a BKME-induced shift in microhabitat distribution of dactylogyrids. Elevated levels of infection with Dactylogyrus spp. have been recorded on roach from a Finnish lake containing relatively low concentrations of BMKE. The high exposure concentration in this experiment produced a similar reduction in dactylogyrid infections to that reported in separate studies in close proximity to Swedish pulp and paper mills. The discrepancy between the results of the Finnish and Swedish field investigations is therefore considered to be due to differences in BKME concentration between the study areas.

Les études menées au cours de quatre années sur la composition corporelle de la perche (Perca fluviatilis, L.) et du gardon (Rutilus rutilus, L.) de la retenue de Pareloup ont révélé certaines années, pendant la période estivale et essentiellement chez la perche, une diminution des lipides somatiques accompagnée soit d'un ralentissement soit d'un arrêt de l'accrétion protéique. En fin d'automne, on a pu observer une croissance compensatrice chez les animaux qui auraient subi des conditions nutritionnelles défavorables en été.La composition biochimique du plancton au cours du cycle saisonnier montre que la valeur nutritive du phytoplancton et du zooplancton est maximale en été. Par contre, on a observé une diminution de la quantité disponible au cours de l'automne, ce qui implique un élargissement du spectre alimentaire des deux espèces. La variabilité interannuelle de la structure du peuplement et de la biomasse planctonique disponible en été pourrait expliquer l'utilisation des réserves énergétiques chez les poissons à cette saison.

Les préférences alimentaires des gardons du lac de Pareloup ont été mises en évidence par l'analyse de leurs contenus intestinaux de janvier à juillet 1990. Les spectres alimentaires ont été établis grâce à l'indice de poids qui a permis de visualiser une hiérarchie des proies en fonction de la classe de taille et de la saison. Ces spectres montrent que les gardons de 10 à 21 cm ont une alimentation à base de détritus et de zooplancton alors que les individus plus âgés sont détritivores.Il existe une variation saisonnière du régime des gardons : les détritus dominent en hiver ; ils sont progressivementremplacés par le zooplancton (Cladocères) et, en quantité moindre, par les macroinvertébrés (Chironomidae) à partirde mars.Le phytoplancton (Diatomophycées) est ingéré en grande quantité, mais sa contribution pondérale est toujours faible.Les gardons de Pareloup se nourrissent activement pendant toute la période d'étude et de façon assez opportuniste.

As in the female (Breton et al., 1988), plasma gonadotropin levels (GtH) in male roach were always the lowest the day of aggregation on the spawning areas, eventhough they were elevated 3 daysearlier. The levels increased during spawning and remained at a high level in most of the animalscaptured out of the visible spwaning areas, 3 weeks after the beginning of the spawning activity. During the same period the GnRH contents did not significantly vary in the hypothalamic lobes, the di-met-and myelencephalon. On the contrary there were marked variations in the pituitary gland and the telencephalon in which the GnRH levels were the highest the day of aggregation when GtH plasma levels were the lowest. In these two structures they decreased during spawning in correlation with the increase of the plasma GtH levels. These results are discussed in comparison with those obtained in the female and in relation with the possible endocrine and paracrine roles of the GnRH in these structures and mainly in the telencephalon.

Plasma and pituitary gonadotropin (GtH) levels, and the gonadotropin releasing hormone (GnRH) in differents areas of the brain, were measured using radioimmunoassays during the prespawning and the spawning period in a wild population of roach (Rutilus rutilus). GtH pituitary levels did not vary. On the contrary GtH plasma levels showed variations during the prespawning period and were minimum on the day of arriva1 on the spawning areas, Do. During spawning they increased in ovulated females but also in unovulated animals and returned to low values, 1 month after. During this period GnRH varied inversely in all brain structures and was always negatively correlated with the blood plasma GtH levels. This suggests that all the brain GnRH would be involved in the control of the GtH secretion during spawning. This was not the case during the prespawning period, during which only hypothalamic and pituitary GnRH levels were correlated with GtH. There were also marked variations in the telencephalic GnRH extra hypothalamic GnRH. It is hypothetized that it could be a link between environmental factors and the hypothalamo-pituitary cornplex.