The Squalus genus comprises a group of small demersal sharks occurring circumglobally, popularly known as dogfish sharks. This genus exhibits a conserved morphology, thus making correct morphological identification difficult. Considering these taxonomic problems and the scarcity of molecular data, the present study aimed to identify Squalus genus MOTUs, using DNA barcoding for species delimitation via ABGD (automatic barcode gap discovery), PTP (Poisson tree process), and GMYC (general mixed Yule coalescent) employing the mitochondrial COI gene. A total of 69 sequences were generated from samples obtained from the American coast in both the Atlantic and Pacific Oceans. The ABGD analysis was the most conservative among the three applied delimitations, indicating three taxonomic units, while the PTP analysis revealed nine MOTUs, with two conflicting units noted between S. clarkae + S. mitsukurii and S. albicaudus + S. cubensis. The GMYC analysis indicated an excessive division, with S. acanthias and S. mitsukurii subdivided into six MOTUs each and S. blainville, into four. These findings demonstrated that Squalus presents a complex of previously defined species, with misidentified samples deposited in databases leading to difficulties in analyzing the real distribution and diversity of species belonging to this genus. Thus, further efforts to highlight possible new species are recommended.

The guitarfishes Pseudobatos horkelii and Pseudobatos percellens meet the criteria for threatened status as Critically Endangered (CR) and Endangered (EN), respectively. Both species occur in the Southern Atlantic Ocean. Considering the lack of data on the genetic structure of these species, the present study evaluated the genetic variability and population structure of the P. horkelii and P. percellens in the southern region of Brazil and the northern coast of Argentina, based on sequences of mitochondrial DNA, Control Region (D-loop). Samples of P. horkelii (n = 135) were analyzed in six localities situated in Northern Argentina, along the Brazilian states’ coast. The mean of nucleotide diversity was 0.0053, the ΦST was 0.4277 and demographic analysis of P. horkelii suggests the existence of stability of the populations, with D = 0.9929, FS = 2.0155, SSD = 0.0817, R = 0.2153. In P. percellens (n = 101) were analyzed from six Brazilian localities along the coast of Santa Catarina, Paraná, and São Paulo. The mean nucleotide diversity was 0.0014 and ΦST value of 0.2921, the demographic analysis indicates a high migration rate of P. percellens among the localities evaluated, with D = 0.5222, FS = 0.3528, SSD = 0.01785, R = 0.3890.

In the present paper a specimen of the blue runner Caranx crysos, a thermophilic species commonly found in subtropical Atlantic waters, was newly reported from the Mar del Plata coast. The presence of the blue runner in Argentine waters could be explained by the warm drift theory.

The high intraspecific variation among and the conservative external morphology of Merluccius spp. have resulted in serious identification difficulties. Four hundred and twenty fresh and preserved specimens of Merluccius were analyzed, including the type series of Merluccius australis, M. tasmanicus and M. hubbsi; specimens of M. hubbsi from Argentina, Brazil and Uruguay, and individuals of M. australis from Argentina and New Zealand were examined. The nomenclatural status of the type specimens of M. australis is discussed and the designation of a lectotype and a paralectotype is proposed. The comparative study of morphology, meristic, traditional and landmark-based morphometry, both external and internal, and through DNA-based Barcoding molecular tools demonstrates that Merluccius tasmanicus is a junior synonym of Merluccius australis. Meristic and morphometric characters of types of M. tasmanicus completely overlap those of M. australis, whereas M. hubbsi show fewer scales along the lateral line, total vertebrae, second dorsal and anal-fin rays. A trend of a longer snout and wider head in M. australis and M. tasmanicus, and larger eyes and longer pelvic fins, in M. hubbsi was observed. While discriminant characters were found in the internal elements (hyomandibula, urohyal and sagitta otolith) between M. hubbsi and M. australis, none were observed between M. australis and those reported for M. tasmanicus. DNA barcoding analyses found no evidence of the existence of other species of Merluccius besides M. hubbsi and M. australis.