, a colonial volvocid flagellate. T. socialis
was formerly known as Gonium sociale
until the monospecific genus Tetrabaena
was erected (Nozaki,H. and Itoh, M. J. Phycol. 30:353-365, 1994).The colonies are plate-shaped with four cells all oriented in the same direction. Extracellular matrix joining adjacent cells can be seen. The gelatinous envelope surrounding the whole colony is not seen in these images. The ovoid cells have two equal-length flagella, a small eyespot, large cup-shaped chloroplast and posterior round pyrenoid. From freshwater pond near Boise, Idaho. Oblique illumination.