Morphological and molecular evidence support the intertidal barnacle Octomeris intermedia Nilsson-Cantell, 1921 (Thoracica, Chthamalidae) as a valid species in Indo-Pacific waters

Octomerisis a chthamalid intertidal barnacle with eight shell plates. There are currently two species of such barnacles:O. brunneaDarwin, 1854 (type locality in the Philippines), common in the Indo-Pacific region, andO. angulosaSowerby, 1825, only recorded in South Africa.Octomeris intermediaNilsson-Cantell, 1921, identified from the Mergui Archipelago in Myanmar, was considered to be conspecific withO. brunneaby Hiro (1939) based on samples collected in Taiwan. The morphological differences in shell and opercular plates betweenO. brunneaandO. intermediaare believed to be intra-specific variations due to different degrees of shell erosion. In the present study, the genetic and morphological differentiations ofOctomerisin the Indo-Pacific region were examined. This study found two molecular clades (with inter-specific differences) based on the divergence in the COI genes, and the species also have distinct geographical distributions. TheOctomeris brunneaclade covers samples collected from the Philippines and Taiwan waters and the other clade, which we argue isO. intermedia, is distributed in Phuket and Krabi, Thailand and Langkawi, Malaysia. Phuket and Krabi are located approximately 300 km south of the Mergui Archipelago, the type locality ofO. intermedia. The morphology of samples collected from Thailand fits the type description ofO. intermediain Nilsson-Cantell (1921). Our study concludes thatO. intermediais a valid species based on morphological and molecular evidence.