2022 年 69 巻 1 号 p. 1-10
In Hokkaido, Japan, the toxic dinoflagellate Alexandrium catenella (A. tamarense species complex Group I) is the source of frequent contamination of bivalves with paralytic shellfish toxins over the last 40 years, whereas A. pacificum (Group IV) has rarely been reported. Recently, A. pacificum cells were identified based on their morphology and DNA sequences in Hakodate and Funka bays, southern Hokkaido. To understand their seasonal occurrence, A. pacificum and A. catenella cells in the two bays were detected using microscopy and multiplex polymerase chain reaction (PCR) over a 2-year period (May 2018–May 2020). Microscopic observation showed that cells of A. pacificum, a species without the ventral pore between the 1′ and 4′ plates, occurred in Hakodate Bay from July to November 2018 and in July 2019, with a maximum cell density of 4450 cells L−1 in November 2018. It also occurred in Funka Bay in October 2018, with a maximum cell density of 50 cells L−1. Multiplex PCR using Alexandrium species-specific primers showed a similar seasonal occurrence of A. pacificum in Hakodate Bay. In contrast, A. catenella was found from February to May in Funka Bay but its occurrence was uncertain in Hakodate Bay because the microscopy and PCR tests were not simultaneously positive. The occurrence of A. pacificum was limited to the period (July to November) of optimum water temperature for growth (15–25℃), suggesting that the occurrence of motile cells was affected by water temperature. When A. pacificum bloomed at a relatively high density in Hakodate Bay during autumn 2018, warmer water temperature and lower salinity in the surface layer were observed compared to the previous 5 years. These environmental conditions were thought to be established due to warmer air temperatures, a longer sunshine duration, and a large amount of precipitation from October to November 2018. Such environmental and meteorological conditions were suggested to be suitable for the growth of A. pacificum in Hakodate Bay.
