Morphological variation and phylogeny of Karenia selliformis (Gymnodiniales, Dinophyceae) in an intensive cold-water algal bloom in eastern Hokkaido, Japan in September-November 2021

Abstract

Harmful algal blooms responsible for mass mortalities of marine organisms have so far been rare in Hokkaido, northern Japan, although fish killing blooms have been frequently reported from western Japanese coasts. In September-November 2021, a huge and prolonged cold-water bloom occurred along the Pacific coast of eastern Hokkaido, Japan, and was associated with intensive mortalities of sea urchin, fish, octopus, shellfish, etc. In this study, morphology and phylogeny of the dominant and co-occurred unarmored dinoflagellates of the Kareniaceae in the bloom were examined by using light microscopy, scanning electron microscopy and molecular phylogeny inferred from ITS and LSU rDNA (D1-D3) sequences. Morphological observation and molecular phylogeny showed that the dominant species was Karenia selliformis, with co-occurrences of other kareniacean dinoflagellates, Kr. longicanalis, Kr. mikimotoi, Karlodinium sp., Takayama cf. acrotrocha, Takayama tuberculata and Takayama sp. The typical cell forms of K. selliformis in the bloom were discoid, dorsoventrally flattened, and larger than the cell sizes in previous reports, 35.3-43.6 (39.4{+/-}2.1) m in length. Transparent cells of Kr. selliformis lacking or having several shrunken chloroplasts and oil droplets were also found. Cells of Kr. selliformis had morphological variation, but the species could be distinguished from other co-occurred Karenia species by its numerous (46-105) and small granular (2.9-4.6 m in diameter) chloroplasts and the nucleus positioned in the hypocone. Cell density of Kr. selliformis exceeding 100 cells/mL was recorded in the range of temperature 9.8-17.6{degrees}C. The rDNA sequences determined from Kr. selliformis in the blooms of Hokkaido, Japan in 2021 were identical to those from another bloom in Kamchatka, Russia in 2020. HighlightsO_LIA marine fauna-destructive harmful algal bloom in the Pacific coast of eastern Hokkaido, Japan in September-November 2021 was dominated by Karenia selliformis. C_LIO_LICells of Karenia selliformis typical in the bloom were discoid and possessing numerous small chloroplasts, approximately 70 in number. C_LIO_LICells of Karenia selliformis showed morphological variation in size and shape, and transparent motile cells lacking or having degraded chloroplasts were also present. C_LIO_LICo-occurred kareniaceans in the bloom were Karenia longicanalis, Karenia mikimotoi, Karlodinium sp. and Takayama spp. C_LIO_LIrDNA sequences of Karenia selliformis in the blooms of Hokkaido in 2021 and Kamchatka in 2020 were identical, which belong to the group I of Kr. selliformis. C_LI