Molecular and morphological data support the synonymy of Muricanthusradix Gmelin, 1791 and Muricanthusambiguus Reeve, 1845 (Gastropoda, Muricidae)

Abstract

The Muricanthusradix/ambiguus/nigritus complex includes species with a great diversity of shell shapes and shared habitats in various regions, which has raised questions and doubts about the current taxonomic classification of these species. Muricanthusnigritus, M.radix, and M.ambiguus are three similar-looking black and white murex found commonly on the west coast of North and South America. The wide variety of morphological patterns within and between these species makes the classification of specimens difficult by visual observation. To this day, controversy persists over whether M.radix and M.ambiguus are one or two distinct species. Molecular genetic data have helped clarify the taxonomic classification of many mollusk species in recent decades, contributing to a more accurate understanding of biodiversity and ecosystems. In this study, DNA barcoding and double digest restriction-site associated DNA sequencing (ddRAD-seq) methodologies were applied to complement morphological data, establishing for the first time the phylogenetic relationships between M.nigritus, M.ambiguus and M.radix. The classic mitochondrial and nuclear barcodes obtained from 80 specimens collected from three different geographic locations differentiated only two phylogenetic clades (M.nigritus and M.radix/ambiguus from Mexico differentiated from M.radix/ambiguus from Mexico and Panama). High levels of mitochondrial DNA introgression have been observed between M.nigritus and M.radix/ambiguus. The deep-level approach performed using 3692 loci obtained from ddRAD-seq also differentiated only two genetic clusters (M.nigritus and M.radix/ambiguus). Our results clearly support the proposal that M.ambiguus should be synonymized with M.radix.

Keywords: Muricanthus; ddRADseq; mithocondrial genes; morphometrics; nuclear genes; phylogeny; taxonomy.

Figure 1.

Geographic representation of sampling locations with some of the specimens collected at these sites (the dimensions are not to scale). Map was generated with ArcGIS 10.5 software (ESRI, Redland, USA, https://desktop.arcgis.com/en/).

Figure 2.

Dorsal and ventral views of shells of M.nigritus (1N and 23N), M.radix (2A, 6A, V5, C1 and C4), M.ambiguus (5A, 12A, MR3, V6, C5, C6, C8, C12) and individuals that are difficult to classify M.radix/ambiguus (V4 and V11). The measurements given are shell lengths. More specimens and different views of the shells are presented in Suppl. material 2: figs S1–S4.

Figure 3.

Median-joining networks of all haplotypes identified for the four gene regions studied. Haplotypes are represented by different circles with a size proportional to their frequency (see Suppl. material 1). All cases in which more than one nucleotide substitution occurs between haplotypes are indicated, and all other haplotypes are separated by a single nucleotide change. Solid black circles represent unsampled haplotypes.

Figure 4.

Bayesian maximum clade credibility tree of the concatenated sequences (COI, 12S rRNA, 16S rRNA, 28S rDNA). Posterior probabilities for the nodes of the most divergent clades are presented.

Figure 5.

Characterization of genetic clusters obtained using SNP data a PCoA plot of genetic distances estimated using the method of Smouse and Peakall (1999)b Clusters obtained from the Bayesian clustering analysis without prior information on sample location. Each vertical line corresponds to one individual. The cluster assignments obtained for each K tested are represented with different colors, where the optimal number of clusters was estimated for K = 2.