Cutting through the Gordian knot: unravelling morphological, molecular, and biogeographical patterns in the genus Zapteryx (guitarfish) from the Mexican Pacific

Abstract

Defining species boundaries is important not only for the appropriate attribution of life history and ecological traits but also for sustainable fishery management and for the conservation of biodiversity. Problems arise from taxonomic uncertainty and incorrect species delineation leading to historical misidentification. This is the case of Pacific guitarfishes in the genus Zapteryx. We use a molecular phylogenetic approach combining mitochondrial and nuclear loci to investigate genetic variation in fish along the Mexican Pacific coast. Our analyses reveal a lack of nuclear and mitochondrial distinction between rays identified morphologically as banded guitarfish Z. exasperata and as southern banded guitarfish Z. xyster, casting doubts on the validity of their current systematics. However, we detected two mitochondrial lineages in accordance with the number of species described for the Pacific: a “northern” lineage corresponding to Z. exasperata and a “southern” lineage possibly attributable to Z. xyster. The poorly understood phenotypic plasticity in coloration and size of the evolutionary lineage of Z. exasperata and its apparently wider than currently thought geographic distribution (at least to Oaxaca) are the major sources of confusion regarding the taxonomic and geographic delineation of these nominal species. In light of our findings, eastern Pacific guitarfishes in the genus Zapteryx require a thorough taxonomic revision using morphological and genetic data to unveil what appears to be a complex pattern of diversification.

Updated checklist of the extant Chondrichthyes within the Exclusive Economic Zone of Mexico

“… laticeps (Gill, 1865) ( Myliobatiformes : Aetobatidae ) for the Atlantic and Pacific Oceans, respectively. Recently, the existence of genetic and morphometric variability in a latitudinal gradient for the guitar species of the Zapteryx genus has been identified and should be studied in more detail ( Castillo-Páez et al 2017 ).…”

Population structure of the critically endangered Brazilian guitarfish Pseudobatos horkelii (Rhinobatidae) revealed by double‐digest restriction site‐associated DNA sequencing

“…The genetic differentiation is surprising because the individuals are morphologically similar, the region is relatively small (<200 km), and there is no apparent geographical barrier present. Strong population structure between Rhinobatidae species has been reported previously (Castillo‐Páez et al, 2017), and two hypotheses have been postulated to explain this.…”

“…The STRUCTURE analysis points to a genetically structured P. horkelii population along the Brazilian coast, corroborating previous studies based on D‐loop mitochondrial markers in population analyses for the same species (Cruz et al, 2021). The presence of genetically structured populations is not new for Rhinobatidae species, as significant F ST values have been indicated within populations of Pseudobatos productus (Sandoval‐Castillo et al, 2004; Meyer, 2020) and Zapteryx exasperata (Castillo‐Páez et al, 2017). These studies were conducted at similar distances (approx.…”

Almost half of the Gymnura van Hasselt, 1823 species are unknown: Phylogeographic inference as scissors for cutting the hidden Gordian knot and clarify their conservation status

“…In recent years, several molecular reviews have contributed to the reorganization of the elasmobranch family groups (Last et al, 2016a; White et al, 2018), revealing the existence of new cryptic lineages (Arlyza et al, 2013; Castillo‐Páez et al, 2017; Petean et al, 2020) and contributing to the delimitation of species ranges (Palacios‐Barreto et al, 2017; Sales et al, 2019). Previous molecular inferences on the genus Gymnura (Smith et al, 2009; Jacobsen & Bennett, 2009; Naylor et al, 2012; Muktha et al, 2018) focused on a relatively reduced sample of species and localities in comparison with the present study, which was based on the most comprehensive data set analyzed to date, in terms of both taxonomic and geographic sampling.…”