A tale of two biodiversity levels inferred from DNA barcoding of selected North Atlantic crustacean

Biodiversity is the variety of life and can be studied at different levels (genetic, species, ecosystems) and at different scales (spatial and temporal). The past decades have shown that marine biodiversity has been severely underestimated. To study the characteristics of the great diversity of marine species and the underlying processes of formation and maintenance of marine biodiversity, it is obvious and necessary to know what lives out there. We are now faced with the highest extinction rates since the formation of the human society ("biodiversity crisis") and only a fraction of species was formally described (1.9 million of 11 million), because of a shortage of trained taxonomists available for this immense work, among other things. All these factors have led to the proposal of molecular tools to enable and facilitate the identification of species including DNA barcoding. This method uses a DNA fragment of the mitochondrial gene cytochrome C oxidase subunit 1 (COI) as a fast, accurate and cost effective tool to identify species. Thus, each species can be defined by a unique identification tag that will not be changed during taxonomic revisions. In addition to the assignment of unknown specimens to species identified a priori by taxonomists, data generated through barcoding studies will be very useful for comparative phylogeographic studies of multiple taxa, phylogenetic studies at different taxonomic levels and for studies on evolutionary patterns between groups of organisms.;Chapter 1 provides some background on DNA barcoding with a review on studies that were published on the subject, especially those focusing on the identification of marine species.;Chapter 2 develops a reference library for marine crustaceans from the Estuary and the Gulf of St. Lawrence. All data (taxonomy, collection information, images, DNA sequences and chromatograms) are stored online in the Barcode of Life Data Systems (BOLD) and are available for general use. Specimens used for barcoding are kept as "vouchers" in public institutions for future use. The results showed the presence of an invasive amphipod in the estuary (mentioned previously in the Great Lakes and near Montreal, with impact on the native fauna of amphipods), and the existence of potential cryptic species in amphipods, mysids and decapods.;Chapter 3 focuses on the use of COI sequences provided through DNA barcoding as a complementary tool for taxonomy and phylogeny of the amphipod family Talitridae in the North Atlantic. The current distribution and diversity of species is the result of evolutionary processes and interaction with the environment across a geographic region . Phylogenetic studies can investigate this issue by developing evolutionary scenarios on the relationships between taxa. The results show the existence of cryptic species in three morphological species. In addition, older genera do not cryptic species in three morphological species. In addition, older genera do not appear to be monophyletic, suggesting the need for taxonomic revisions in this family.;Chapter 4 addresses the issue of genetic diversity which enables the persistence of populations and species over time, allowing continuous adaptation to environ mental changes. At large spatial scales, diversity within species can be structured in genealogies according to geography, thus defining phylogeographic patterns, which may coincide or not with biogeographic divisions. COI sequences generated by DNA barcoding were used to infer phylogeographic patterns in an amphipod species with amphi-Atlantic distribution, Gammarus oceanicus . This species is very abundant and an important part of the intertidal communities and coastal food webs. The results showed a deep division within this species with two divergent groups corresponding to a latitudinal segregation (temperate region of Atlantic Canada versus the subarctic Hudson Bay and Europe), indicating the presence of two potential cryptic species.;This research showed that marine biodiversity, as seen in marine crustaceans from North Atlantic, was underestimated. Potential cryptic species were found in eight morphological species, knowing that only the most common species were sampled for this study. The level of diversity will certainly increase with the addition of different taxa, different types of habitat and distinct marine regions.;Keywords: marine biodiversity; DNA barcoding; species identification; Crustacea; cryptic species; North Atlantic.