This treatment includes two genera, Ruppia and Zannichellia, treated as their own families in Michigan Flora.
1. Submersed leaves opposite or whorled, floating leaves absent.
1. Submersed leaves alternate, floating leaves (sometimes present) alternate or opposite.
2. Flowers 2, at first enclosed in sheathing leaf base, the peduncle elongating and often spiraled or coiled at its base; fruit long-stalked; stipular sheath lacking free ligule at summit (the stipule wholly adnate to the leaf blade and merely rounded at the summit); leaf blade terete.
2. Flowers several to many in a peduncled head or spike; perianth of 4 tepals; fruit ± sessile; stipular sheath absent (stipules entirely free from leaf) or with a short ligule-like extension if stipules fused to the leaf blade.
3. Stipules adnate to the leaves for 10–30 mm or more (at least on the larger leaves), adnate for ca. 2/3 of the length of the stipule; leaves all submersed, filiform to narrowly linear (up to 2 (–2.5) mm wide).
3. Stipules free from the leaves or adnate for less than half the length of the stipule (adnate for 5 mm or less except in P. robbinsii); leaves submersed or floating, filiform to ovate, oblong, or elliptic.