Cryptic, yet big and booming: the Central European naiad Anodonta cygnea (Linnaeus, 1758) (Bivalvia: Unionidae) in a Mediterranean marshland (Ebro Delta, Catalonia)

We report the arrival and spread in a Mediterranean wetland of a previously undetected large naiad, or freshwater mussel.Identification was based on morphological and anatomical traits, as well as on phylogenetic analyses of three gene fragments (2 mitochondrial and 1 nuclear).The new invader is Ano donta cygnea, originating from central Europe and now spreading throughout the Mediterranean region.Its introduction into the Ebro Delta probably occurred in 2012, but misidentification was caused by its unusually wide ecophenotypic plasticity, often being externally quite similar to native congenerics living in the same area, as well as to the also invasive Sinanodonta spp.In its warmer new habitat, A. cygnea exhibits an accelerated growth and a much shorter life span, as well as a reduced winter breeding season.Previously arrived invasive bivalves (Corbicula spp.) may be effectively masking the spread of A. cygnea through habitat modification, as the latter's developmental response in the altered habitat yields extreme phenotypes.This cryptic invasion poses a threat to already endangered native naiads through further habitat alteration and life history disruption.The foreseeable outcome in the Ebro Delta, probably applicable to freshwater molluscan assemblages worldwide, appears to be a dramatic simplification of community structure through accelerated hydrological dynamics, extinction of long-lived specialists and their substitution by weedy, short-lived opportunists facilitating further invasions and ecosystem meltdown.