Background Margaritifera auricularia is a large bivalve mollusc (measuring up to 20 cm). It belongs to the oldest family of freshwater mussels (also known as pearl mussels) and used to be common in all the large rivers of western Europe and northern Africa up to the 15th century. Since then its area of distribution has been drastically reduced to the point that it was considered largely extinct at the beginning of the 20th century. That is until some populations were rediscovered in the River Ebro, in Spain, in the mid-1980s. Its life cycle is associated with certain fish species such as the sturgeon or freshwater blenny, which it parasitises in the larval stage, fixing itself on to the fishes' gills. Once it has metamorphised into a small bivalve it falls to the bottom of the river, where it must find a suitable habitat and survive a barrage of predators. It is for this reason that each female releases millions of offspring, since only a small proportion will succeed in reaching maturity. Destruction of the habitat caused by canalisation and contamination of the rivers is the main cause of the species' disappearance. Today its situation is critical. Objectives The project aimed to ensure the long-term conservation of the species by managing its habitat appropriately and encouraging an expansion in its area of distribution. To do this the quality of the water was to be improved, impacts on the populations were to be minimised and reserves were to be set up. This work was to be accompanied by a captive breeding programme to reinforce and increase the existing wild populations. Finally, there was to be an awareness raising campaign to draw public attention to this curious but still obscure species and its conservation problems. Results The project had the objective of guaranteeing the long-term conservation of the freshwater bivalve Margaritifera auricularia, a mollusc considered endangered at the national level and included in Annex IV of the Habitats Directive. This objectives were achieved only partially. In effect, after the project began some external factors emerged hindering its development: the polemic Hydrologic Plan of Spain had the area as one of the points for water detraction; also the Ebro river was invaded by the zebra mussel (Dreissena polymorpha), an alien species that not only competes intensely with the target species but also changes deeply the physical and biological conditions of the waters where it thrives. This, together with the poor previous knowledge on the species –considered extinct until the mid 1980s–, caused the project to suffer important delays and prevented the completion of many of its actions. Nevertheless, this LIFE project has brought a significant increase in the knowledge on the actual wild populations of the species existing in the Ebro river, their population status and its biology and life cycle. The work done on the ex situ management and breeding of this naiad, despite some problems occurred in the beginning, has yielded many innovative results and has the potential to become a reference for the conservation of other mollusc species. New methods for feeding the Margaritifera and the blenny fish (Salaria fluviatilis) that hosts it have been developed as well as facilities for freshwater management the aquaculture research centre responsible for this set of actions works usually with marine species). A protocol for the capture (when the naiads are emitting larvae), transport, acclimatisation, keeping and analysis of the specimens was also elaborated and put into practice. Infestation with the larvae was tested in two species, the sturgeon and the blenny fish, and positive results were achieved. Still, the critical step is the survival of the juveniles as massive mortalities were recorded during the metamorphosis of the naiads (when they are 4 months old) and further work has to be develop in order to ensure that all this sound work results finally in an effective release of individuals in their natural habitat and the reinforcement of the existing populations. All this means an important contribution to the conservation of this naiad, although the research on the ecological requirements of the species in the different life stages was still ongoing by the time the project finished. Besides, the more intensive surveying done has resulted in the finding of several new nuclei and currently the estimated population is of 1,500-3,000 individuals (higher than expected). As a result, the enlargement of the limits of a pSCI to include the newly found colonies of the species was ongoing by the project end. The main pump priming effect has been the incorporation of the Aragón region to the conservation of the species through another LIFE project (LIFE04/NAT/ES/0033) that is also relying on the IRTA for the ex situ breeding of the species, thus assuring the continuity of the important work done by that research centre on both the species and its host fishes. Unfortunately, three foreseen results that would have had important consequences for the conservation of the species –an experience of habitat restoration, helping to increase the knowledge on the species and its conservation needs, and including measures for the prevention and mitigation of the impacts on the species in all the projects and actions to be undertaken in the relevant river stretches– were not achieved for the diverse reasons above explained as well as for internal problems.