Morphological and genetic characteristics of the anisakid nematode Raphidascaris acus from the southwest Caspian Sea: evidence for the existence of sibling species within a species complex

Recently, it has been shown that many nematode species are in fact species complex, using exact morphological and genetic studies. In this case, there are no such studies related to the genus Raphidascaris Railliet & Henry, 1915. Herein, the morphological and genetic variations among the Iranian population of the species Raphidascaris acus (Bloch, 1779) Railliet & Henry, 1915 and the other allopatric populations with morphological and genetic information were compared to show whether this species can be considered as a species complex. R. acus is an anisakid species and has been frequently reported from different host species from the Caspian Sea. Nonetheless, there are no morphological and genetic information for this species from the region. In the present study, a total of 20 specimens of R. acus were collected from Esox lucius Linnaeus, and the morphology of the Caspian population of this species was surveyed for the first time using both light and scanning electron microscopy. Meanwhile, some parts of ribosomal DNA (rDNA) including internal transcribed spacer 1 (ITS1), 5.8 s, and ITS2 were sequenced and presented as the genetic marker for this species. To understand whether R. acus can be considered as a species complex, the Caspian population of this species was compared morphologically with the allopatric populations of Czech and Canada and genetically with the allopatric population of Poland (Vistula lagoon). Morphologically, there was no difference between the Caspian and Czech populations, but the Caspian and Canadian populations differed in the length of ejaculatory duct and the presence of small triangular elevation between the bases of subventral lips. The nucleotide difference between the Caspian and Polish populations was 4.48 %. In comparison with the interspecific genetic distances in the genus Raphidascaris, this value is notable. In conclusion, based on morphological and genetic differences among the allopatric populations of R. acus, this species is probably a species complex. Nonetheless, the definitive taxonomic decision in recognizing R. acus as a species complex and the description of its sibling species depend on surveying other allopatric populations morphologically and genetically accompanied by an evaluation of reproductive isolation among them.