Recent studies highlighted the phylogeographic breaking point between the Lusitanian-Boreal provinces which is shown in the cryptic species on either side of the Iroise Sea for several invertebrate species with a bentho-pelagic cycle. This coexistence of cryptic species along the coasts of Brittany raises numerous questions about the processes involved in the separation of the two biogeographical provinces and the species distribution boundaries: (1) the physiological constraints that changes in environmental parameters like temperature impose or (2) the characteristics of ocean circulation which control the organisms' ability to disperse. Against this backdrop, the general objectives of the Larvasud mission were to assess the role of complex hydrodynamic processes (fronts, upwellings, plumes) presents along the southern coasts of Brittany on larval transport during the spring-summer hydrological transition period. More specifically, it is a question of: describing the horizontal and vertical distribution of larvae from some target species of polychaetes (i.e., Pectinaria koreni, Owenia fusiformis and Sabellaria alveolata) related to the hydrological structures of the coastal area of the north of the Bay of Biscay; have sufficient terrain data available to validate a larval transport model in the study zone; perfect molecular techniques to identify larvae of cryptic species of taxa of interest in order to highlight any differences in the distribution over time and space. The cruise took place in 3 legs, on the following dates: leg 1: from 23 to 29 April 2008, leg 2: from 10 to 19 May, leg 3: from 6 to 14 June 2008. The cruise was part of the EC2CO Continental and coastal ecosphere programme.