Aulacoseira italica

Frustules are cylindrical and are connected face-to-face to form filamentous colonies. Valves are 4-24 µm in diameter, with a mantle height of 9-20 µm. The ratio of the mantle height to valve diameter usually ranges from 0.7 to 5. The mantle has almost straight sides, and the valve face is flat with slightly convex margins. The straight or slightly sinuous rows of mantle areolae are inclined to the left (sinistrorse) or almost parallel to the pervalvar axis, 17-25 rows in 10 µm. The mantle areolae vary in shape from circular to slit-like. Spines are large, spatulate or anvil-shaped and originate from two to four costae. One or more pervalvar rows of areolae terminate at the base of the spine. Separation valves are rare and have thinner, pointed spines than linking valves. The number of rimoportulae is most often two, but may be one or more per valve. Externally, the rimoportula openings are not distinguishable from areolae. The ringleiste is solid, shallow, and thick. Resting spores are often observed and produced as semi-endogenous pairs. Spore frustules superficially resemble those of vegetative cells, but conspicuously lack visible spines, have thicker cell walls, may have a convex valve face, and often have incomplete rows of mantle areolae. Not all populations of A. italica appear to form resting spores; spores are especially common in fossil deposits and in populations from shallow polymictic lakes.

There has long been confusion surrounding the identity of Aulacoseira italica. Crawford et al. (2003) studied type material of both A. italica (Gallionella italica) and A. crenulata, confirmed that they represented the same taxon, and that A. italica was the earlier and correct name. Note that in the type material, the rimoportulae (illustrated in Crawford et al. 2003) are positioned along a stria about 4-5 areolae from the ringleiste and appear internally as long, straight tubes integral with the valve wall. In material from Florida, rimoportulae appear internally as short, sessile processes. The spore forms have also been separately named as Melosira laevis and M. semilaevis (Van Heurck 1882)