Key to Heteranthera in the Mexican Yucatan Peninsula

1. Leaf-blade clearly differentiated from petiole, variously ovate, elliptical to orbicular; the pseudo-petiole longer than the blade; flowers always purple or purple-violet............................................................................................................... Heteranthera limosa

- Leaf-blade not differentiated from petiole, linear; flowers purple-violet, lilac or yellow..................................................................2

2. Inflorescence sessile, axillary, with a solitary flower; flowers yellow, actinomorphic; all three anthers of the same size and shape. ............................................................................................................................................................................. Heteranthera dubia

- Inflorescence pedunculate, terminal, generally with several flowers, rarely only a single one; flowers lilac, zygomorphic; anthers of different sizes and shapes........................................................................................................................... Heteranthera yucatana