Morphological and Genetic Assessment of Invasive Corbicula Lineages in Southern South America: A Case Study in Argentina

Abstract

The broad global distribution of freshwater clams belonging to the genus Corbicula is driven by multiple hermaphroditic lineages. These lineages, characterized by shared morphological traits and phenotypic plasticity, pose challenges to morphological identification. Genetic markers, such as the mitochondrial COI gene, play a crucial role in delineating these lineages and their ranges. Morphotypes represent observed phenotypic variations, while lineages are defined based on genetic markers. Here, we comprehensively review Corbicula's distribution in Argentina, discriminate extant lineages based on both morphological and genetic (COI) data, and describe variations in internal and external morphologies using 15 Argentine populations. Genetic analyses identified two mitochondrial lineages: the AR morphotype (FW5 haplotype) and CS morphotype (FW17 haplotype). Strikingly, despite having similar vectors, origins, and invasive stages, Corbicula lineages exhibit virtually segregated distributions. However, mitochondrial haplotypes are found in sympatry mainly in northeastern Argentina where individuals with intermediate morphotypes exist, suggesting the presence of hybrids due to maternal genome retention. These findings contribute to the clarification of the identity and distribution of Corbicula lineages in Argentina, where the genus has been found for over half a century. Similar studies are needed in other areas to better understand the invasion patterns of this successful and adaptable group.

Keywords: Argentina; COI; Corbicula; distribution; morphology.

Figure 1

Internal and external views of the left valve of Corbicula form AR (C. fluminea sensu lato; Müller, 1774), form CS (C. largillierti sensu lato; Phillipi, 1844), and intermediate forms (Form I), along with the diagnostic characters utilized for morphological assignment of lineages in the present study.

Figure 2

Examples of images taken to measure the gill area: (A) external demibranch, (B) internal demibranch), palp area (C), and filament density (D) in Corbicula specimens collected in Argentina between 2015 and 2017.

Figure 3

Bayesian majority rule consensus tree obtained from the analysis of 268 sequences of the mitochondrial gene COI (683 bp), showing the relationships among estuarine and freshwater species and major haplogroups of Corbicula clams. Numbers above or below the branches indicate Bayesian posterior probability node support. Some clades were collapsed and a few support values were omitted for clarity and simplicity (see Figure S1 for the full topology with all tips shown). The tree was rooted with Neocorbicula limosa (Figure S1).

Figure 4

(A) Sampling locations and the geographic distribution of the COI haplotypes for Corbicula clams collected at 15 sites between 2015 and 2017. Pie charts depict the proportion of haplotypes observed at each site corresponding to the two freshwater lineages found within the country, with FW5 in black and FW17 in white. The border color of each pie chart indicates the identified form (see codes in panel (B)). Ellipses with dashed lines indicate distinct biogeographic zones covered in this study. (B). Unrooted statistical parsimony network of Corbicula clams based on 658 bp of COI, focusing on the 210 specimens from Argentina sequenced in this study and representative of the freshwater (FW) haplogroups FW5 and FW17. Solid lines represent single mutational steps, empty circles represent intermediate, unsampled haplotypes and circle sizes are proportional to the number of identical haplotypes. Haplotypes are color-coded based on the phenotypic form of each specimen (forms AR, CS, and I from Argentina were collected in this study, while the others come from the literature), and major FW haplogroups are indicated with dashed arrows and circles over the network. Codes and details regarding sampling sites can be found in Table 1.

Figure 5

Principal component analysis of morphometric variables of 225 Corbicula clams sampled from 15 populations in Argentina between 2015 and 2017. Each data point represents an individual, with colors indicating the probability of belonging to different clusters based on the fuzzy cluster analysis. Orange data points indicate individuals with mismatches between COI lineage and morphotype. Population centroids are indicated by triangles. Ellipses represent the morphospace containing 90% of individuals from each cluster (solid lines) and intermediate individuals (dotted line) obtained from fuzzy cluster analyses. Variable codes correspond to those in Table 3, and population codes correspond to those in Table 1.