DNA barcoding of the intertidal chitons of the genera Stenoplax Dall, 1879 and Callistochiton Carpenter in Dall, 1879 from the Mexican Tropical Pacific with the description of two new species

The Mexican Tropical Pacific (MTP) is rich in chitons, mainly represented by the genera Callistochiton Carpenter in Dall, 1879 and Stenoplax Dall, 1879 . Despite the widespread presence of these genera in the region, few species have been studied at a molecular level. Using partial cytochrome oxidase I ( cox1 ) gene sequences, we analyzed the genetic relationships of four Callistochiton and four Stenoplax species (including Caribbean species), along with two putative new species. Each genus was analyzed separately. The Bayesian Inference (BI) and neighbor-joining analyses recovered two main clades in both genera and support the existence of two new species. Stenoplax quetzalcoatli sp. nov. was recovered sister to Stenoplax petaloides Gould, 1846 and this clade was recovered sisters to Stenoplax mariposa (Dall, 1919 ). Stenoplax limaciformis (Sowerby in Broderip and Sowerby, 1832 ) was recovered sister to Stenoplax purpurascens C.B. Adams, 1845 and both nested with Stenoplax floridana Pilsbry, 1892 . Callistochiton guerreroensis sp. nov. and Callistochiton cf. expressus are sister species of Callistochiton colimensis (A.G. Smith, 1961 ). In addition, to solve the identity of two new taxa, we inspected various morphological characters using scanning electron microscopy (SEM). We identified differences between Stenoplax quetzalcoatli sp. nov. and species of the Stenoplax rugulata (Sowerby, 1832 ) complex characterized by small size and irregular sculpturing. Moreover, Callistochiton guerreroensis sp. nov. differs from Callistochiton expressuss (Carpenter, 1865 ) by a higher number of ribs in the head valve, minor lateral teeth with a bilobed blade directed inwards, and dorsal girdle scales with rounded spherules.