A community of benthic cladocerans (Crustacea) from the River Thames at Twickenham, comprising seven chydorid and two macrothricid species, was studied. The species present were Alona affinis, A. quadrangularis, A. rectangula, Disparalona rostrata, Leydigia leydigi, Pleuroxus uncinatus, P. trigonellus, Iliocryptus sordidus and Macrothrix laticornis. A general trend, among the chydorid species, of a midsummer peak in abundance followed by a rapid decline was noted. Alona affinis, A. rectangula and Pleuroxus uncinatus also exhibited an autumn peak in abundance. The Iliocryptus sordidus (Macrothricidae) population did not follow this pattern. Birth and death rates were calculated for the populations of Alona affinis, Disparalona rostrata and Leydigia leydigi. Results obtained from qualitative sampling of rivers in Southeast England, and from a survey of the literature, indicated the presence of a taxocene of benthic cladocerans characteristic of the unvegetated substrate of lakes and rivers. The life histories of Disparalona rostrata and Leydigia leydigi were examined in detail. The number and duration of adult and juvenile instars, duration of egg development and mean length of life were determined at four temperatures. A study of the relationship between egg volume and parent length for Alona affinis, Disparalona rostrata and Leydigia leydigi revealed a highly significant positive regression of egg volume on parent length. The life cycle strategy of the Chydoridae as a whole was examined and found to differ from that of both the large and small planktonic Cladocera. Like the small planktonic cladocerans, the Chydoridae produce large young relative to their size at maturity. However, unlike these, the Chydoridae exhibit curtailment of growth after maturity and in this resemble the large planktonic Cladocera. The annual production of the benthic chydorid community in the River Thames, Twickenham was calculated and was found to be 0.876g C m^-2 in 1982.

Life histories are known to vary across geographic ranges in response to a number of factors, both biotic and abiotic. Environmental calcium availability has been shown to affect freshwater gastropod life histories due to its fundamental requirement in shell formation. Adaptation of life histories to local environmental conditions may cause the response to novel pollutants to vary across populations within a species due to trade-offs between and among traits but very few studies have examined wide scale variation in life history response to stress across geographic ranges. A long term study was conducted and aimed to expand on current understanding by rearing populations of the great pond snail, Lymnaea stagnalis sampled from across the UK, for two generations in high and low calcium environments. A comprehensive suite of life histories was recorded throughout the study and traits were compared between populations and treatments, and across generations to distinguish between environmental, plastic and heritable sources of variation. Further work focussed on life history trade-offs under different environmental conditions before utilising a stage-classified matrix model to derive population growth rates (λ) and compare effects across populations and calcium treatments. Finally, acute and chronic effects of exposure to nanoparticulate carbon black were assessed before using matrix models to investigate the combined effects of environmental calcium with nanoparticulate carbon black on λ for three populations of snails. Significant intra-specific variation was recorded in the majority of life history traits, which were shown to display high levels of phenotypic plasticity as the norm. Intergenerational comparisons revealed that traits more directly linked to fitness, such as size at reproduction and reproductive output, showed higher heritabilities than those pertaining to growth, such as growth rates and age at first reproduction. Both generalised and population specific responses to calcium availability were shown in life history traits across the study populations. These effects tended to be subtle but suggest that environmental calcium plays a role in shaping life history strategies across the UK distribution. Life history trade-offs between traits tended to be conserved across populations, and showed little response to environmental calcium, although differential investment in life history traits across calcium treatment was detected in some cases. A strong trade-off between age and size at first reproduction was detected across all generations and calcium treatments. Size at first reproduction was also shown to correlate with reproductive output, wherein a trade-off between eggs per mass and number of egg masses was detected. Traits involved in trade-offs appeared to more strongly associate with fitness and these findings suggest that trade-offs between key life history traits are of importance in understanding population specific life history strategies. Stage-classified matrix modelling showed a trend towards reduced λ in low calcium but this trend was not significant. A significant reduction in λ across generations was recorded which was most likely to be the result of inbreeding. Local adaptation to calcium availability was shown to influence the life history response to nanoparticulate carbon black, and was mirrored in predicted population growth rates obtained from matrix models. Intraspecific differences in response to carbon black nanoparticles only became apparent when calcium concentrations were low. These findings would support the view that in order to be better able to predict the response of species to the presence of novel stressors such as nanoparticles, it is necessary to account for intraspecific adaptation of life history traits as well as geographical variation in the environmental context.

Increased releases of cadmium to the aquatic environment have raised concern over the potential for adverse impacts on freshwater organisms in affected aquatic bodies. This thesis explored the responses of a common freshwater amphipod, Gammarus pulex (G. pulex) to sublethal concentrations of cadmium under different environmental conditions and at various stages of the lifestyle. Endpoints studied encompassed molecular, cellular, physiological and behavioural changes, to enable a comprehensive analysis of the effects of the organism. Exposure to sublethal concentrations of cadmium (Cd) (0.001, 0.005 and 0.01 mg Cd L–1) for 7 and 14 days, influenced the percent survival, induced lipid peroxidation and damaged DNA in haemolymph cells of G. pulex. These concentrations also reduced feeding and ventilation rates as well as the motility, in particular of the females, with increasing Cd concentration and time of exposure. Furthermore, Cd accumulation from water in the body tissues of the amphipods was lower than in their food, with increasing Cd concentrations. These results illustrate how concentrations of Cd below the Environmental Quality Standard for European waters can cause sublethal molecular and cellular damage after relatively short periods of exposure. Histopathological effects of Cd on the gills and hepatopancreas (mid-gut gland) were examined using light, scanning and transmission electron microscopy. Multiple effects were observed on the tissues and cell organelles including external alternations in the muscular fibres of the hepatopancreas, lysis of microvilli and morphological changes in mitochondria. Exposure to 0.01 mg Cd L–1 altered the epithelial layer of the gill, causing vacuolation and lysis across the whole gill structure. Mitochondria showed damage to the inner membrane, shortened cristae and swelling, with an increase in apoptosis at concentrations of 0.005 and 0.01 mg Cd L–1. Collectively, these results document the adverse effects of Cd on target organs at concentrations within the range found in freshwater bodies. An investigation of the effects of water hardness on bioaccumulation and toxicity showed that hardness of water inhibited Cd toxicity and protected the juveniles during long term exposures, without affecting growth rate and food consumption. Soft water reduced the rate of survival, growth rate and food consumption during chronic exposure to low Cd concentrations, and facilitated Cd accumulation in the body parts compared with juveniles exposed in hard water. The results show that water quality plays a vital role in reducing or increasing detrimental effects of low Cd concentrations on the early life stage of amphipods which are a source of food for many species in aquatic environments. Exposure to Cd led to an increase in metallothionein concentrations in the amphipods in both hard and soft water. It also caused disruption to ion/osmoregulation, which may represent one mechanism of compensation for the ions lost in the amphipods exposed to Cd in soft water. In conclusion, these results add to the body of evidence describing the sublethal toxicity of Cd, a priority pollutant, to a common freshwater sentinel species. These results are of relevance for future environmental management and remediation approaches, because they provide scientific data to help in assessing, interpreting and understanding the effects of the heavy metal Cd in freshwater environments.

Les cténophores représentent l’un des quatre embranchements animaux extérieurs aux Bilateria. La majoritédes espèces sont planctoniques et gélatineuses, et sont reconnaissables à leurs huit rangées de peignes dont lebattement permet la nage. Leur systématique est encore de nos jours mal comprise. L'anatomie descténophores offre peu de caractères aussi bien pour placer la lignée dans l’arbre des métazoaires, que pourétablir les relations de parenté au sein de l’embranchement et délimiter les espèces, et jusqu’à présent lesdonnées moléculaires n’ont pas permis de résoudre ces problèmes de manière satisfaisante. L’objectif de cetravail de thèse est de contribuer à améliorer notre compréhension de l'évolution des cténophores àdifférentes échelles taxonomiques. A l’échelle des métazoaires, la position phylogénétique des cténophores aété abordée par une approche phylogénomique. Un effort significatif a été réalisé pour améliorerl’échantillonnage taxonomique à travers le séquençage et l’assemblage des transcriptomes de 22 espèces denon-Bilateria (cténophores, cnidaires, spongiaires). Deux jeux de données indépendants ont été analysés, lepremier représentant une mise à jour d’une supermatrice existante de 128 gènes ; le second (4235 gènes)ayant été entièrement construit de novo via un protocole original comportant la mise au point de nouvellesméthodes pour traiter de manière semi-automatisée les principales sources potentielles d’artéfact(contaminations, paralogies, données manquantes). Les résultats contredisent certaines études récentes enmontrant que les spongiaires et non les cténophores représentent le groupe-frère des autres métazoaires. Laposition exacte de ces derniers reste à ce stade incertaine (trois options se présentant suivant les analyses). Al’échelle intra-phylétique, l'analyse d'un jeu de données comprenant les marqueurs ADNr 18S et InternalTranscribed Spacers (ITS), associée à des analyses de gènes dupliqués chez les cténophores et aux analysesphylogénomiques précédentes, a permis de résoudre une grande partie des relations phylogénétiques entre lesordres et les familles de cténophores, tout en permettant de préciser la position de la racine. Enfin, à uneéchelle taxonomique plus fine, une comparaison approfondie entre deux espèces du genre Pleurobrachia aumoyen de marquages immunohistochimiques montre le potentiel de ces techniques comme source denouveaux caractères structuraux « micro-anatomiques » à valeur diagnostique pour la délimitation etl’identification des espèces de cténophores. En conclusion, ce travail se veut une contribution au progrès dela systématique d’un embranchement encore méconnu et d’une grande importance pour la compréhensiondes évènements anciens de l’évolution animale. / Ctenophores are one of the four animal phyla positioned outside from the Bilateria. Most ctenophore species are planktonic and gelatinous, and are easily recognisable by their eight comb rows used for swimming. Ctenophore systematics remains nowadays poorly understood. Anatomical characters do not help much in placing them within the animal tree, and similarly the grounds for establishment of their internal phylogeny as well as delimitation of species are notoriously weak due to a paucity of informative morpho-anatomical characters. Until now, the use of molecular data has failed to improve significantly this situation. The aim of this PhD thesis was to bring a contribution to our understanding of ctenophore evolution at different taxonomic scales. At the metazoan level, the position of ctenophores was addressed using a phylogenomic approach. Taxonomic sampling was significantly improved through sequencing and assembly of transcriptomes of 22 non-bilaterian species (belonging to ctenophores, cnidarians and sponges). Two independent datasets were analysed, one consisting in an update of an existing supermatrix of 128 genes, the other one (4235 genes) having been entirely built de novo, thanks to a newly-devised semi-automated protocol intended to eliminate all major potential causes of artefacts (contaminations, paralogies, missing data). Results clearly contradict recent phylogenomic studies which claimed Ctenophora to be the most early-diverging animal lineage, our analyses instead supporting Porifera as the sister-group to other metazoans. The exact position of ctenophores remains however uncertain at this stage, different conditions of analyses yielding three contradictory hypotheses as open possibilities. At the intra-phyletic level, analyses of a ADNr 18S and ITS (internal transcribed spacers) dataset, together with study of duplicated genes and the results of phylogenomic analyses, allowed resolving most phylogenetic relationships between ctenophore orders and families, including the placement of the ctenophore tree root. Finally, at smaller taxonomic scale, in-depth comparison between two species of the genus Pleurobrachia using immunohistochemistry demonstrates the potential of these techniques for uncovering new structural “micro-anatomical” characters useful for diagnosis and identification of ctenophore species. It is our hope that this work will contribute to improving the systematics of a poorly known phylum of great importance for understanding early animal evolution.