Herbs aquatic, annual or perennial(?), attached to rocks in River-rapids and waterfalls (occasionally trunks of rheophytic trees/shrubs). Stems (shoot system) prostrate, tightly attached to substratum throughout, (C. noveloi with stems arising from leaf petioles; C. multipartita f. pendulosa with elongate, pendulous stems that attach via expanded holdfasts), cylindrical to flattened, repeatedly dichotomously, subdichotomously, or irregularly branched, with swollen side-branches (C. fluitans and C. monandra) or side-branches lacking (other species), becoming whitened when dry. Leaves distichous, arising from stem margins, monothecous (with a single sheath) or dithecous (with two sheaths); simple (entire, pinnately or variously lobed) or dichotomously or pinnately compound; petiolate or not, petiole rounded, elliptical or flat in cross-section, 1-several veined; rachis distinctly widened and flattened or elliptical to round. Spathella clavate, oriented horizontally, papillate apically, rupturing apically or subapically into numerous linear to triangular segments, lower region remaining covered by stem tissue. Flowers solitary, numerous per stem, embedded in cavities in main stem or 1–4 per branch and surrounded by fused leaf sheaths, oriented horizontally, hermaphroditic, zygomorphic, short pedicellate, covered by a sac-like spathella; pedicel not elongating during anthesis, attached to ovary perpendicular to ovary axis; receptacle asymmetrically expanded on side opposite stamen attachment (asymmetry becoming obscured in fruit as receptacle dries). Tepals 0–3, linear, hair-like to flattened, one on each side of fused stamen filaments, an additional tepal arising between two filaments in some populations of C. multipartita, apex acute. Stamens 1–2, filament flattened, wider at base than apex, attached to anther in pocket-like area, elongating during anthesis and projecting from ruptured spathella, persisting after anthesis; anthers basifixed, triangular, thecae fused apically, apex blunt, dehiscing introrsely and longitudinally. Pollen in monads, isopolar, oblate spheroidal to prolate spheroidal, tricolpate, planaperturate, spiculate, colpus short or long, colpus membrane with conspicuous spicules. Ovary 2-carpellate, unilocular at maturity, oriented horizontally or nearly so, strongly anisolobous; dorsal (upper) carpel markedly smaller than ventral (lower) carpel, papillate, papillae absent from ventral (larger) carpel; remaining inside spathella during anthesis; suture line between carpels diagonal relative to ovary axis. Ovules numerous, placentation axile, placenta thick. Stigmas 2, free, linear, papillose, offset toward one side of ovary apex, projecting from ruptured spathella at anthesis. Capsules oriented horizontally, partially covered by surrounding stem tissue and persistent spathella, markedly anisolobous; dehiscing by 2 valves; upper (dorsal) valve deciduous, 3–5 ribbed (nonsuture), suture margins thickened and rib-like, papil late on upper region or papillae lacking; lower (ventral) valve persistent, cup-like, 3–7 ribbed, suture margins thickened and rib-like, lacking papillae. Seeds orange-brown, obovate.
Previous authors proposed infrageneric classifications for Castelnavia (Van Royen 1954; Tulasne 1849; Weddell 1873; see above). Because only six taxa are recognized herein, and phylogenetic analyses thus far provide little insight into species relationships, a formal infrageneric classification is not proposed.