K. mikimotoi has been classified under many names and has been mis-assigned to different species and genera in the North Atlantic and Pacific because of its morphological similarities to other Gymnodinoid species. It is now known to be widely distributed, but there remain unresolved questions about whether K. mikimotoi was introduced into the North Sea from Japanese waters, or whether it has always inhabited this region and been erroneously classified as Gymnodinium spp. or has been a part of the hidden flora prior to be recognised in a bloom off the Norwegian coast in 1966. To address questions about geographical genetic variation within K. mikimotoi and broader issues about its biogeography it was deemed important to develop a suitable diagnostic molecular marker that could then be used to monitor the presence/absence of different K. mikimotoi ecotypes over long time scales in European waters. This study showed that the partial rDNA LSU (D1-D2) was too conserved to separate the different strains of K. mikimotoi, while, the ITS region was better able to discriminate between the different strains. However, the rbcL gene was the most informative gene and contained sufficient substitutions to separate the different strains of K. mikimotoi. Specific PCR-primers were designed to amplify a variable region of the rbcL gene able to distinguish differences between K. mikimotoi isolates from the different regions. The innovative high resolution melting temperature (HRM) technique based on specific primer set allowed rapid discrimination of K. mikimotoi from distinct geographic localities (= sequence variants) that differed by only a single nucleotide. Moreover, this study used archival environmental samples collected from the Celtic Sea shelf-break region. The high resolution melting temperature assay successfully detected the European K. mikimotoi isolate within the south-western English Channel in a 1963 sample, which is prior to thefirst report of a K. mikimotoi bloom in Norwegian waters in 1966 and in the south-western English Channel in 1975 and in western Japan in 1965. HRM observations were further validated using clone libraries and sequencing. In summary, this data provided more information about the genotypes present over the analysed timescales, revealing that K. mikimotoi sub-species 2 (European and New Zealand strains) was present in south-western English Channel and south-west Ireland for over 47 years, with sub-species 1 (the Japanese isolate) being absent from all examined samples. This finding supports the hypothesis that K. mikimotoi isolates within Europe are not of Japanese origin and suggests that they are native species to the region.

Karenia brevis, le dinoflagellé produisant des brevetoxines (PbTx), est la principale espèce d’efflorescences d’algues toxiques dans le Golfe du Mexique. Les effets de cette algue sur Mercenaria mercenaria et Crassostrea virginica sont méconnus tandis que les efflorescences coïncident avec la période de reproduction de ces espèces. Ce projet avait pour but de déterminer les effets i) d’une exposition à K. brevis en laboratoire et naturelle de terrain sur les processus physiologiques associés à la reproduction de M. mercenaria et C. virginica, et ii) d’une exposition à K.brevis sur la qualité et le développement des gamètes, embryons et larve de ces espèces. Suite à l'exposition des adultes de clams et d’huîtres à K. brevis, les paramètres physiologiques de la reproduction ont été affectés. La présence de PbTx dans les tissus des gamètes et le potentiel transfert maternel de PbTx à la progénie via les ovocytes, pourraient avoir entraîné les effets négatifs observés lors du développement larvaire.Les effets négatifs similaires causés par l'exposition des stades précoces à différentes préparations de cellules de K. brevis suggèrent que d'autres composés toxiques, en plus de PbTx, pourraient être impliqués dans la toxicité et, que la majorité des effets délétères se produisent durant les divisions embryonnaires.Le clam et l’huître américaine sont sensibles à K. brevis. Les effets négatifs sur les adultes et jeunes stades de vie, combinés à une exposition quasi-annuelle aux efflorescences de K. brevis, pourraient engendrer des perturbations majeures sur le recrutement des populations de ces espèces importantes, et avoir des répercussions environnementales et économiques. / The brevetoxin (PbTx) producing dinoflagellate, Karenia brevis is the most prevalent harmful algal bloom species in the Gulf of Mexico. The effects of this alga on Mercenaria mercenaria and Crassostrea virginica are poorly understood yet, blooms typically overlap with periods of reproduction and spawning in these species.The aims of this project were to determine the effects of i) laboratory and field exposure of K. brevis on the reproductive and related physiological processes of adult M. mercenaria and C. virginica and the quality of the offspring that were produced and ii) K. brevis exposure on gamete, embryo and larval development in these species.Following exposure of adult clams and oysters to K. brevis, negative effects were recorded on reproductive and physiological parameters. PbTx was recorded in gamete tissues, and maternal transfer of this PbTx to the offspring via the oocytes, may have resulted in the significant negative effects recorded on larval development up to the end of the lecithotrophic phase.The similar dose-dependent negative effects caused by direct exposure of gamete and early life stages to different cell preparations of K. brevis suggests that other toxic compounds in addition to PbTx may be involved in toxicity and, that the majority of negative effects occur during embryonic divisions.Hard clams and eastern oysters are susceptible to K. brevis exposure. The negative effects on adult and early life stages combined with the near- annual exposure to blooms of K. brevis could cause significant bottle-necks on the recruitment and population dynamics of these important species and, have wider reaching environmental and economic impacts.