Nitzschia clausii is comparatively narrower than N. brevissima and has finer striae than both N. filiformis and N. sigma. The valves of N. clausii are more sigmoid in shape and have narrower, more constricted apices than N. filiformis. Nitzschia clausii has a curved keel at the valve center, while that of N. sigma is straight at the valve center. N. clausii is narrower and has more narrowly rounded apices than N. scalpelliformis. Nitzschia clausii has apices that are more constricted than those of N. nana.