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10 pages; National audience; Pinna nobilis is the largest bivalve in the Mediterranean Sea. This endemic species lives in the Posidonia meadows,in the form of small sparse populations. It is a heritage species: its byssus has long been collected to be wovenand used to make gloves, hats or stoles. Protected by a European Directive (1992), P. nobilis is closely monitored,nationally and internationally. But since the end of 2016, an epidemic, which origin seems to be a protozoan parasite,decimates populations (up to 100% mortality). This event began on the Spanish coast and spreads now all around theMediterranean. Although conservation measures have been taken, the next few years could see the extinction of thisemblematic bivalve. Apart from its symbolic value, P. nobilis is a species worthy of biotechnological interest. Its shellcomprises two mineralized layers, one of which, external, calcitic, consists of large monocrystalline prisms, consideredas a model in biomineralization. Understanding their formation and growth is a challenge in biomimetics. Its byssusforms very fi ne remarkably solid fi laments, with ultrastructural properties different from those of the mussel byssus.Until now, the chemistry of this byssus remains mysterious. Understanding it is also a challenge in material physics.These different points are discussed in this article.; La Grande nacre Pinna nobilis est le plus grand bivalve de Méditerranée. Espèce endémique, elle est inféodée auxherbiers de posidonies, vivant sous forme de petites populations clairsemées. C’est une espèce patrimoniale : sonbyssus a longtemps été collecté pour être utilisé à la confection de gants, bonnets ou étoles. Protégée par uneDirective Européenne (1992), P. nobilis fait l’objet d’une surveillance étroite au plan national et international. Or depuisfi n 2016, une épidémie, dont l’origine semble être un parasite protozoaire, décime les populations (jusqu’à 100 %de mortalité). Détectée au départ sur les côtes espagnoles, elle s’étend sur tout le pourtour méditerranéen.Bien que des mesures aient été prises, les prochaines années pourraient voir l’extinction de ce bivalve emblématique.Hormis sa valeur symbolique, P. nobilis est une espèce digne d’intérêt en science des matériaux et composites bioinspirés.En effet, sa coquille comprend deux couches minéralisées, dont l’une, externe, est constituée de grandsprismes calcitiques d’aspect monocristallin, simples en apparence, mais présentant une structuration multi-échelletrès complexe. Comprendre leur formation et leur croissance est un challenge en biomimétique. Son byssus formedes fi laments très fi ns remarquablement solides, aux propriétés ultrastructurales différentes de celles du byssus demoule. Jusqu’à présent, la chimie de ce byssus reste mystérieuse. L’appréhender est aussi une gageure en physiquedes matériaux. Ces différents points sont abordés dans cet article.

11 pages; International audience; Numerous mussel species produce byssal threads - tough proteinaceous fibers, which anchor mussels in aquatic habitats. Byssal threads from Mytilus species, which are comprised of modified collagen proteins - have become a veritable archetype for bio-inspired polymers due to their self-healing properties. However, threads from different species are comparatively much less understood. In particular, the byssus of Pinna nobilis comprises thousands of fine fibers utilized by humans for millennia to fashion lightweight golden fabrics known as sea silk. P. nobilis is very different from Mytilus from an ecological, morphological and evolutionary point of view and it stands to reason that the structure-function relationships of its byssus are distinct. Here, we performed compositional analysis, X-ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate byssal threads of P. nobilis, as well as a closely related bivalve species (Atrina pectinata) and a distantly related one (Pinctada fucata). This comparative investigation revealed that all three threads share a similar molecular superstructure comprised of globular proteins organized helically into nanofibrils, which is completely distinct from the Mytilus thread ultrastructure, and more akin to the supramolecular organization of bacterial pili and F-actin. This unexpected discovery hints at a possible divergence in byssus evolution in Pinnidae mussels, perhaps related to selective pressures in their respective ecological niches.