Abstract
The actual occurrence of Squalus megalops in the Mediterranean Sea has recently been questioned. Several research works which sought to assess available morphological and meristic features that differentiate S. megalops from other Squalus species in the Mediterranean Sea, revealed poor discriminatory power and high variability of the assessed characters, especially when comparing S. megalops and S. blainville. The application of molecular tools does not support the presence of S. megalops. In the present study, we screened spurdog species from the Strait of Sicily using a molecular taxonomy approach based on two mitochondrial DNA markers and we report the occurrence of two Squalus lineages characterizing specimens collected from the stretch of sea between Tunisia, southern Sicily, Malta and Libya. The results support the hypothesis that a common species, S. blainville, currently inhabits the Mediterranean Sea, while a second and rare species is probably an occasional visitor with high morphological similarity to the S. megalops and S. blainville but is genetically distinct from both. Within this perspective, the occurrence of S. megalops in the Mediterranean Sea is not confirmed and our study highlights the taxonomic uncertainties in relation to the occurrence and distribution of Squalus species in this region. We encourage the establishment of a coordinated international effort to implement a comprehensive and integrated taxonomic assessment on this genus which represents an irreplaceable component of the biodiversity of the area.
Highlights
The intrinsic low variation of morphological charac ters specific to elasmobranchs hinders their taxo nomic identification at the species level and undermines their conservation at dif ferent geographical scales (McEachran & Dunn 1998; Quattro et al 2006; Aschliman et al 2012)
Since previous studies revealed that nominal species char acterized by a wide geographical distribution share mitochondrial lineages, we evaluated the relation ship between the genetic lineages retrieved in the Strait of Sicily and other Clades previously identified within the genus Squalus
Considering the congruence among the NJ, ML and Bayesian Inference (BI) tree topologies obtained using the concate nated dataset (Table S2), all reconstructions were summarised in one topology (Figure 1), which assigned most of the individuals collected off the southern coast of Sicily, Malta and Tuscany to a bigger cluster including S. blainville from the Mediterranean Sea and Squalus sp
Summary
The intrinsic low variation of morphological charac ters specific to elasmobranchs hinders their taxo nomic identification at the species level and undermines their conservation at dif ferent geographical scales (McEachran & Dunn 1998; Quattro et al 2006; Aschliman et al 2012). Most spurdog shark species are included in the IUCN Red List of Threatened Species and are currently classified in categories from Data Deficient to Endangered (i.e. the Greeneye spurdog S. chloroculus Last et al 2007; IUCN 2019). For this reason, highlighting the need to increase our knowledge about taxonomy, biology and ecology of these species appears essential. Difficulties due to the problems of distinguishing between morphologically similar species and to the lack of effective and user-friendly identification field guides have been commented upon by many authors (Garrick 1960; Compagno 1984; Munoz-Chapuli & Ramos 1989; Veríssimo et al 2017). The well-established DNA barcoding techni que based on the mitochondrial DNA (mtDNA) cytochrome c oxidase subunit I (COI) has been coupled with analysis of faster-evolving mitochon drial genes or long ribosomal DNA to improve the resolution and support of inferred phylogenetic rela tionship, up to the description of the evolutionary history of species (Avise 2004; Naylor et al 2012; Krehenwinkel et al 2019)
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