Taxonomy and systematic positions of three new Callistoctopus species (Octopoda, Octopodidae) discovered in coastal waters of China

The genus Callistoctopus comprises 13 species, and has been reported mostly in the Western Pacific Ocean. Here, we described three new species from China, Ca. paucilamellus sp. nov., Ca. sparsus sp. nov., and Ca. gracilis sp. nov., based on morphometric and meristic characteristics. The diagnoses, descriptions and detailed morphometric data are provided for each species. The cytochrome c oxidase I (COI) genes of the three new species are sequenced, and compared with related species and analyzed for their systematic positions. Both phylogenetic trees constructed using three mitochondrial genes (12S rRNA, 16S rRNA, COI) and one nuclear gene (Rhodopsin) revealed that our new species formed into two distinct clades with strong support values. One clade included Ca. gracilis sp. nov., Ca. sp. 1, Ca. xiaohongxu, Ca. tenuipes and Ca. paucilamellus sp. nov., which clustered together. The other clade showed that Ca. sparsus sp. nov. was closely related to Ca. sp. 2 and Ca. sp. 3. Ca. luteus and Ca. macropus were located at the base of the Callistoctopus group. Based on our integrative studies, both morphological and molecular evidence suggested strongly that O. minor is more likely to be classified as a species of Callistoctopus. Morphological comparisons were made between the three new species and related taxa, which could be recognized based on the 7–8 gill lamellae of each demibranch, numerous small black spots on the subdermal layer of the arms, and an elongated body.