To Be, or Not to Be: That Is the Hamletic Question of Cryptic Evolution in the Eastern Atlantic and Mediterranean Raja miraletus Species Complex

Abstract

Despite a high species diversity, skates (Rajiformes) exhibit remarkably conservative morphology and ecology. Limited trait variations occur within and between species, and cryptic species have been reported among sister and non-sister taxa, suggesting that species complexes may be subject to stabilising selection. Three sibling species are currently recognised in the Raja miraletus complex: (i) R. miraletus occurring along the Portuguese and Mediterranean coasts, (ii) R. parva in the Central-Eastern Atlantic off West Africa and (iii) R. ocellifera in the Western Indian Ocean off South Africa. In the present study, the genetic variation at mitochondrial and nuclear markers was estimated in the species complex by analysing 323 individuals sampled across most of its geographical distribution area to test the hypothesis that restricted gene flow and genetic divergence within species reflect known climate and bio-oceanographic discontinuities. Our results support previous morphological studies and confirm the known taxonomic boundaries of the three recognised species. In addition, we identified multiple weakly differentiated clades in the Northeastern Atlantic Ocean and Mediterranean, at least two additional cryptic taxa off Senegal and Angola, a pronounced differentiation of ancient South African clades. The hidden genetic structure presented here may represent a valuable support to species’ conservation action plans.

I Like This New Me: Unravelling Population Structure of Mediterranean Electric Rays and Taxonomic Uncertainties within Torpediniformes

“…Similarly, the NADH2 sequence JQ518930 [ 16 , 91 ], from a Senegalese specimen provisionally identified as T. torpedo , was recognized as distinct (MOTU 10) with respect to the ‘true’ Mediterranean ocellate torpedoes, potentially representing a ‘still undescribed’ new species. These results confirm the need to further investigate the Senegalese waters, where additional undescribed elasmobranch species are possibly reported to occur [ 7 ]. Finally, sequences from North-Western Atlantic specimens, originally attributed to T. nobiliana (NADH2: JQ518931, COI: KC015969), were identified as a different MOTU; they should be attributed to T. occidentalis as suggested in [ 92 ].…”

“…In particular, the role of oceanographic discontinuities has been invoked in several elasmobranch studies as the main factor responsible for geographically isolating and hence genetically differentiating the populations. This pattern has been reported for the differentiation of the easternmost Mediterranean locations of the Levantine Sea, with the Strait of Sicily potentially representing a barrier limiting the genetic exchanges between the Eastern and the Western Mediterranean basins [1,7,[84][85][86][87].…”

“…As concerns the quite divergent sequences retrieved in public repositories, obtained from specimens caught outside the Mediterranean Sea, they were analyzed using several species delimitation methods, recently applied to other elasmobranch species to address identification issues and solve taxonomic uncertainties [ 3 , 6 , 7 , 89 , 90 ].…”

“…It is for this very reason that traditional morphological approaches may be ineffective in identifying specimens from cryptic, rare, or elusive species [ 1 , 3 ]. In recent years, molecular tools have been proposed to complement the traditional methods; on several occasions, they have helped to fill gaps of knowledge and resolve taxonomic uncertainties [ 2 , 4 , 5 , 6 , 7 ]. Moreover, they have allowed the early detection of invasive or new species or cryptic species [ 8 , 9 , 10 ] due to their ability to overcome morphological similarities and related specimen misidentifications.…”

DNA barcoding and morphological characterization of marine stingrays (genus Fontitrygon) from the African Gulf of Guinea

“…DNA barcoding involving mitochondrial cytochrome C oxidase subunit I (mt COI ) has been successfully implemented for various taxonomic groups and has been used in identifying rays from several locations, including Taiwan (Chang et al, 2017), Indonesia (Madduppa et al, 2016), Australia (Ward et al, 2008), India (Bineesh et al, 2017) and Northeastern Atlantic Ocean and Mediterranean Sea (Ferrari et al, 2023). To date, a limited number of mt COI sequences of African stingrays are publicly available in global databases.…”