Revisiting a Forgotten Invader: Discovery of a Third Corbicula in Illinois

The Asian Clam (Corbicula spp.) is a “hyper-invasive alien” with great biofouling capabilities. It was first recorded in North America in 1924 in British Columbia, and has since spread throughout the continent, first appearing in the Midwest in the 1960s. Corbicula taxonomy is muddled and unclear, as is the number of species that have become established. Literature reports vary from an invasion of but a single species to invasions of multiple species. The Midwest has long been recognized as having only Corbicula fluminea. However, in 2008, a tentative second species, Corbicula largilllierti, began appearing in the navigable rivers of Illinois. Then in 2015, a third Corbulid species was discovered in the Illinois River. Here, we report on these invasive Corbicula lineages. Our main objective was to document the occurrence of these new taxa and perform a preliminary analysis of its distinctiveness from sympatric taxa using shell phenotype characteristics and mitochondrial (mt) and nuclear DNA markers. Results showed that the three Forms were distinguishable using shell phenotype and nuclear 28S ribosomal DNA sequences. Building on previous Corbicula studies, individuals were unambiguously assigned to one of three discrete shell phenotypes, Form A (=C. fluminea), Form B (=C. largilllierti), or Form D (=unidentified taxa discovered in 2015). 28S genotypes identified three distinct morphs, with Form D differing from Forms A and B by 2-6 base pairs. In contrast, Form D individuals were distinguishable from Form B via mitochondrial markers but shared an identical mtDNA haplotype with sympatric Form A individuals. This latter result could stem from androgenetic capture of Form A eggs by invasive Form D sperm, a rare form of inheritance previously inferred for co-occurring North American Corbicula clones. Further morphological, ecological and genomic analyses characterizing the three morphotypes is required to establish the significance of our preliminary findings.