Northward expansion of the barnacle Fistulobalanus albicostatus in Japan
Fistulobalanus albicostatus Pilsbry, 1916 (Thoracica: Balanidae) is a tropical to temperate species distributed in the NW Pacific. The previously known northernmost record of this species in Japan was from Aomori Prefecture, at the northern end of Honshu Island, Japan. However, field surveys conducted in 2023 and 2026 confirmed the occurrence of F. albicostatus in Hakodate Bay, the southern end of Hokkaido, Japan, across the Tsugaru Strait, thereby extending the northern limit of its known distribution. A line transect survey conducted in May 2026 recorded seven living individuals within an area of 128 m. F. albicostatus was rare on the mid-high shores, accounting for only 2% of all barnacle individuals in a quadrat survey. The basal diameter of the living individuals ranged from 0.76 to 1.23 cm, and all individuals possessed ovaries. Based on characteristics of both morphological and COI gene, the specimens were identified as F. albicostatus, and belonged to the same haplotype of populations that are present in Honshu Island. The establishment of F. albicostatus in Hokkaido suggests an ongoing northward range shift of this warm-water species, with the potential for further expansion under continued ocean warming.