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1. INTRODUCTION 1
1.1. Ecological aspects of brackish-water habitats 1
1.2. Colonization of brackish waters 2
1.3. Intraspecific genetic diversity 5
1.4. Phylogeography 7
1.5. The mitochondrial DNA (mtDNA) 12
2. THE SPECIES 13
2.1. Family Nereididae (Annelida, Polychaeta) 13
2.1.1. Genus Hediste 14
2.1.2. The species Hediste diversicolor 14
2.2. Family Mytilidae (Mollusca, Bivalvia) 17
2.2.1. The genus Mytilaster 18
2.2.2. The species Mytilaster minimus 18
2.2.3. The genus Xenostrobus 20
2.2.4. The species Xenostrobus securis 20
3. MATERIALS AND METHODS 23
3.1. Sampling 23
3.1.1. Hediste diversicolor 23
3.1.2. Mytilaster minimus 24
3.1.3. Xenostrobus securis 25
3.2. DNA extraction 26
3.3. DNA amplification 26
3.4. PCR products purification and sequencing 28
3.5. Data treatment 28
3.5.1. Alignment of sequences 28
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3.5.2. Within-population genetic diversity 29
3.5.3. Among-population genetic diversity 30
3.5.4. Inferences on demographic history and neutrality tests 32
4. RESULTS 34
4.1. Hediste diversicolor 34
4.1.1. Within-population genetic diversity 34
4.1.2. Among-population genetic diversity 35
4.1.3. Inferences on demographic history and neutrality tests 40
4.2. Mytilaster minimus 42
4.2.1. Within-population genetic diversity 42
4.2.2. Among-population genetic diversity 43
4.3. Xenostrobus securis 47
4.3.1. Within-population genetic diversity 47
4.3.2. Among-population genetic diversity 47
5. DISCUSSION 52
5.1. Genetic diversity in Hediste diversicolor 52
5.2. Genetic diversity in Mytilaster minimus 55
5.3. Genetic diversity in Xenostrobus securis 56
6. CONCLUSIONS 60
7. LITERATURE CITED 61
8. ACKNOWLEDGEMENTS 75
APPENDIX CD
