Species identification of two Loliginidae cryptic species in China Seas with morphological and molecular methods

Abstract

Uroteuthis chinensis and Uroteuthis edulis are morphologically similar squids and commercially important for Indo-Pacific Ocean fisheries. Hence, species identification is vital for them because of their similarity. In order to distinguish the two species, two methods (teeth shape of arm sucker ring and DNA sequence) were adopted. In the study, 341 individuals (210 U. chinensis and 131 U. edulis) from different locations of China Seas were used. The two identification methods (teeth shape of arm sucker ring and DNA) yielded inconsistent identification results for 14% of specimens. The high matching rate to DNA results indicated that the identification method utilizing the teeth shape of arm sucker ring was an ideal method. The ratio of Mantle Width (MW) to Mantle length (ML) was not suitable for identification because of the overlapping ratio range between the two species. The ratio of MW to ML showed sharp variations among different ML groups, indicating the allometric growth in both species. The Principal Component Analysis (PCA) and Step-wise Discriminant Analysis (SDA) results showed that the morphological similarity between the two species from the same area was greater than that of the species from different areas. SDA results also showed that the validation rates of three groups (U. edulis in East China Sea, U. edulis in South China Sea, and U. chinensis in South China Sea) were above 75%. Phylogenetic analysis based on COI showed that U. edulis in the study formed two clades, whereas U. chinensis just formed one clade. U. chinensis was only found in the south of 25°N. The finding should be confirmed through a large amount of samples in East China Sea. The Haplotype network indicate that U. edulis in ECS and SCS were both independent evolutionary units. These cladogenetic events seem to be related to habitat discontinuities, oceanographic and geographic processes that build the barriers for gene flow of U. edulis.