Chaetoceros gelidus

Forms irregular colonies. The cells have one chloroplast and thin setae without chloroplasts. The window between cells is narrow, in contrast to C. socialis which has wide windows. Several short chains are organized into irregularly rounded colonies. Three setae of two adjacent valves are short, the fourth one is long and straight. Colonies are established by this fourth seta pointing towards the center of the colony. The resting spore has smooth valves in contrast to C. socialis where it has short spines.