A tough nut to crack: multiple indirect pieces of evidence show a temporally and spatially complex invasion process of genus Carnicus in Argentine Patagonia

Marine invasive species pose serious threats to biodiversity and exert significant ecological and economic pressures. The genus Carcinus ranks among the most widespread marine invaders, with established populations on all continents except Antarctica. As a generalist and active predator, it jeopardizes native coastal ecosystems and the livelihoods of fishing communities. Despite its broad global distribution, invasion dynamics in some regions—such as the Southwestern Atlantic (SWA)—remain poorly understood. We aim to understand the dynamics of the Carnicus invasion in the SWA through analyzing morphological traits and mitochondrial DNA variation along the Patagonian coast of Argentina. Our results challenge the prevailing hypothesis of a single introduction event of C. maenas followed by a northward expansion. Instead, we detected an uneven geographical distribution of C. maenas and C. aestuarii, alongside higher genetic diversity in northern populations. Assuming no introgression between species, our findings support a more complex scenario involving multiple, spatially and temporally independent introduction events. These results call for a revision of existing invasion models for Carcinus in this region and underscore the need for further research into the origins of these introductions, the potential for hybridization, and the ecological factors shaping current distribution patterns.