Swimming activities of zoospores in Laminaria japonica (Phaeophyceae)

ABSTRACT: The release of zoospores from zoosporangia in Laminaria japonica mainly occurred at night. The maximum swimming speed of the zoospores was observed immediately after release (~160 μm/s), and then decreased with time. The zoospores exhibited clear chemotaxis to high concentrations of nitrate-N and phosphate-P. Only half of the zoospores were observed to swim against seawater with a flow velocity of 124 μm/s. The zoospores became floating spores after they stopped swimming in the seawater. Low light and low water temperature conditions prolonged the swimming period of the zoospores. However, the photosynthesis of the zoospores was suppressed in low water temperatures, and was also limited under high light irradiance. These results suggest that photosynthesis in zoospores does not prolong their swimming period, and that when their swimming energy is exhausted, floating spores appear.