Although individuals from this species can exist on their own they tend to form chains (2, 4 or 8 individuals long). Cells are armoured, semicircular (longer in width than length), anterio-posteriorly compressed, with a rounded apex and a slightly concave antapex. The apical pore plate (po) houses the characteristic fishhook shaped foramen. A. catenalla has yellow green to orange-brown chloroplasts and a U-shaped nucleus (Whedon & Kofoid 1936). Forms a resting cyst as part of it?s lifecycle.