Reassessing the Invasion History of the Genus Carcinus in Argentine Patagonia: Morphological and Mitochondrial Evidence for Multiple Introduction Events

Abstract

Biological invasions are recognized as a major driver of global biodiversity change, prompting scientific efforts to understand the mechanisms shaping the establishment and spread of non-native species. Marine invasive species pose ecological and economic challenges, and the green crabs (Carcinus spp.) are among the most successful coastal invaders worldwide. In the Southwestern Atlantic (SWA), a single introduction followed by a northward expansion has traditionally been proposed, though recent evidence is beginning to challenge this hypothesis. Here, we reassess the invasion history and expansion dynamics of Carcinus along the Patagonian coast by integrating morphological assessments with mitochondrial cytochrome c oxidase subunit I (COI) sequences from individuals collected across four gulfs to clarify species identity, diversity, and potential invasion pathways. Most morphological traits lacked diagnostic reliability, and even the most consistent metric, carapace width-to-length ratio (CW:CL), showed 47% agreement with COI-based species identifications. We found four haplotypes in C. maenas and two in C. aestuarii. Contrary to expectations under a single southern introduction followed by northward expansion, C. maenas exhibited higher mitochondrial diversity in northern populations, whereas C. aestuarii displayed two deeply divergent haplotypes in northern Patagonia, associated with two native-range clades (i.e., I and III). These patterns are more consistent with multiple spatially and temporally independent introductions, though results require cautious interpretation because of limited sampling. Our findings highlight the need for nuclear genomic data to clarify invasion pathways, hybridization, and demographic history and to better understand the ecological factors shaping the current distribution of Carcinus spp. in the SWA.