Deep-sea demersal fishes of the Bothrocara hollandi species complex are distributed in the Japan Sea, the Okhotsk Sea, and the northwestern Pacific Ocean. Based on the nucleotide sequences of the nuclear Internal Transcribed Spacer 1 (ITS1) region and the microsatellite analysis, cryptic speciation resulting in the existence of two species (sp. 1 and sp. 2) in the Japan Sea was indicated for this species complex. In the Japan Sea off the San-in district, the westernmost part of the Japanese mainland, the frequency of sp. 2 individuals was highest at a depth of ∼400 m and decreased at both greater and lesser depths. Complete genetic deviation was observed between the individuals of the Japan Sea and the other sea areas, with the exception of a single sp. 2 individual, which shared an ITS1 sequence with an individual from the Pacific Ocean. Furthermore, a microsatellite analysis showed that the individuals of the other sea areas were more closely related to sp. 2 individuals. Two species were thought to have deviated from each other after their isolation from the individuals of the sea areas outside of the Japan Sea, through the occurrence of habitat fragmentation and bottleneck events in the Japan Sea during the glacial periods. Group A, one of two mitochondrial haplotype groups that were reported for the Japan Sea individuals in the previous studies, may have evolved within the lineages of sp. 2.
Keywords: Bothrocara hollandi; cryptic speciation; deep-sea demersal fish; internal transcribed spacer 1; microsatellite.