Staurosira construens var. venter

Valves are lanceolate with rostrate ends in larger specimens and elliptical with broadly rounded ends in smaller valves, 4-5.5 µm wide and 5-26 µm long. The valve face is flat, or slightly undulate due to raised costae. The valve face/mantle junction forms a sharp angle. In girdle view, frustules are rectangular and form ribbon-like colonies joined by linking spines. One cell within a colony is attached to the substratum by a frustule attached by a mucilage pad, or cells may be planktonic. The axial area is narrowly to widely lanceolate. The striae are distinct, alternate, and composed of oval areolae decreasing in size from the valve face/mantle edge to both the central sternum and the valve mantle; 12-16 in 10 µm. Lineolae bear finely branched volae. Striae vary from parallel to radiate in the central area to radiate toward the valve ends. Striae extend nearly to the mantle edge (abvalvar edge) in smaller specimens. The costae are broad. Spines are spatulate, hollow and some possess terminal digitations that connect to neighboring valve lineolae. Spines are present along the valve face edge, except at the apices, and always located on the costae between striae. Well-developed, ocellulimbus type, apical pore fields with round poroids are present. Apical pore fields are located on the junction between valve mantle/mantle. Rimoportula are absent. Scab-like deposits are present in some specimens on the abvalvar edge of the mantle near the valve apices. Copulae (girdle bands) are open and lack perforations. Valvocopulae are wider than copulae.

Morphological variability of specimens within single samples is common in S. construens var. venter, as with other varieties. This variability is not only related to size diminution during asexual reproduction, which mainly affects valve outline and striae orientation, but is also due to differences in shape of areolae, length and thickness of striae and degree of development of apical pore fields. For example, Morales (2001) and Morales et al. (2001) reported extensive morphological variability in valve outline and striation pattern and density in a single lake population from Connecticut.