2022 Volume 38 Pages 20-33
The ubiquitous diatom Skeletonema costatum (sensu lato) has been subdivided into several species based on genetic analyses with rDNA and fine morphological analyses in 2005 and 2007. In this report, most of the identification methods were the same as above, but a new method using mtDNA was used for some isolates. 1057 Skeletonema vegetative strains and 493 germinated strains were isolated from 11 coastal waters in Japan ranging from subtropical to subarctic zones from 2007 to 2015. The aim of this study is to clarify the ecological characteristics of every Skeletonema species.
The Skeletonema marinoi-dohrnii complex was the most abundant species. It was distributed broadly throughout the year, except in the subtropics, formed blooms even at low temperatures; and germinated over a wide range of temperatures. The next abundant species was S. japonicum. It is adapted to temperate, cool waters, where vegetative cells appeared at temperatures <28°C. It never germinated at 30°C. Skeletonema costatum (sensu stricto), S. menzelii, and S. psedocostatum are adapted to warm temperatures in a year. Three species—S. ardens, S. grevillei, and S. tropicum—are adapted to conditions during late summer and autumn at temperate sites. Skeletonema grevillei and S. menzelii were observed at subtropical sites. Skeletonema potamos occurred in brackish waters near fresh waters; S. costatum (s.s.) was able to grow even at salinities near zero; and the remaining seven species occurred in brackish and marine waters. Based on their distributions, S. costatum, and the other seven species were classified as a brackish water species, and marine species respectively. Skeletonema species except for S. grevillei and S. potamos were observed to form blooms and S. grevillei was not confirmed to germinate in Japanese coastal waters. Each Skeletonema species was associated with a different niche characterized by water temperature and salinity.