noun
Closterium (genus of algae)
A genus of desmid algae with crescent-shaped cells, commonly found in freshwater.
池の水を顕微鏡で見ると、三日月藻がたくさんいた。
When I looked at the pond water under a microscope, there were many Closterium.
From 三日月 (mikazuki, 'crescent moon') + 藻 (mo, 'alga'), referring to the crescent shape of the cells.