Brachysira serians is greater in length and broader than B. neoacuta. In addition, the apices of B. neoacuta are distinctly apiculate. Large valves of B. superserians (known from Florida) can be confused with B. serians, but B. superserians has more more narrowly pointed apices. Brachysira subrostrata (known from Brazil) has a similar lanceolate valve to B. serians", but is smaller in length (<70 um) with distinct protracted apices. *Brachysira serians var. rostrata is smaller than B. serians with apiculate to subcapitate apices.