[3] Leptosira forms cushion-like tufts of branched, erect and prostrate filaments.

The cell contains a single nucleus and a parietal chloroplast; pyrenoids are absent or masked by starch.

Similar genera (Leptosiropsis, Pleurastrum have been separated from Leptosira on the basis of having a pyrenoid).

However, because Leptosira apparently has a pyrenoid but is difficult to observe, the status of the genera need reexamination.

[2] Leptosira is placed within Trebouxiophyceae, but phylogenetic analyses show that it occupies a unique lineage not corresponding to existing clades or orders.