Family: Unicellular or Colonial; cell body spherical, with one or more spiny projections;
a single cup-shaped chloroplast with a pyrenoid; asexual reproduction by zoospore or autospore;
sexual reproduction by fertilization between egg and sperm.
Genus: Colonial; cell body with 2 or more spiny hairs (Illustrations of The Japanese Fresh-water Algae, 1977).
Micractinium pussillum ?, x 400, x 640, Karasu-ga-mori koen, Nasu-shiobara city (formerly Kuroiso city), Tochigi Pref., January 2001 by Y. Tsukii
31 μm 63 μm 94 μm; x 640
Micractinium pussillum Fresenius var. elegans G. M. Smith: a colony of 4-32 spherical cells, cell body 3-5 μm diam., with 2-3 needle-like projections 20-30 μm long, a single cup-shaped chloroplast with a pyrenoid (Illustrations of The Japanese Fresh-water Algae, 1977).