First record of a putative novel invasive Corbicula lineage discovered in the Illinois River, Illinois, USA

The Great Lakes watershed is one of the most invaded freshwater ecosystems, making early and rapid detection of new invaders critical to reduce their effects on this fragile system. The genus Corbicula, freshwater clams native to the temperate/tropical regions of Asia, Africa, and Australia, contains some of the most common and successful aquatic nuisance species in the New World. These invasive populations appear to exclusively comprise asexual clonal lineages, which hinder our taxonomic understanding. Previous work suggests three clonal morphotypes have invaded the New World-Forms A, B, and C, with Forms A and B co-occurring in North America. Here, we report on an apparently novel North American invasive Corbicula lineage recently discovered in the Illinois River, which has an artificial connection to the Great Lakes. This putative new morph was found co-occurring with previously described Corbicula morphotypes. Our main objective was to document the occurrence of this new morphotype (=Form D) and perform a preliminary analysis of its distinctiveness from sympatric Forms A and B using shell phenotype characteristics and mitochondrial (mt) and nuclear DNA markers. Results showed that the three co-occurring forms were distinguishable using shell phenotype and nuclear 28S ribosomal DNA sequences. Individuals were unambiguously assigned to one of three discrete shell phenotypes, Form A, B, or D, with Form D specimens characterized by fine rust colored rays and white nacre with purple teeth. Likewise, 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 are required to establish the significance of our preliminary findings.