Subordinate Taxa
Scientific Name:
Hydatellaceae U.Hamann, New Zealand J. Bot. 14: 195 (1976)
Type Taxon:
Hydatella Diels = Trithuria Hook.f.

Annual or perennial herbs, sub-aquatic or aquatic; small tufts from short erect rhizomes. Co-sexual, dioecious, or apomictic. Stem crystals absent. Leaves spiral, lamina filiform-linear, margins entire, 1‑veined. Peduncles scapose, axillary. Reproductive unit capitate, unisexual or bisexual (with carpels surrounding stamens), subtended by bracts in whorls; perianth absent. Stamens, with long, fine, cylindrical filaments, anther dehiscence latrorse; pollination syndrome anemophilous. Carpels, single stipitate, ascidiate; post-genital carpel closure incomplete; apex bearing 3–many apical multicellular, uniseriate stigmatic hairs (rarely absent), stigma absent; ovules 1 per carpel. Fruit an indehiscent achene or a capsule dehiscent via 3 ribs.


A monotypic family of one genus Trithuria. Traditionally treated in the Centrolepidaceae (Diels & Pritzel 1904; Edgar 1966; Edgar 1970), until Hamann (1976) published the family Hydatellaceae to accommodate Trithuria (Hooker 1860) and Hydatella (Diels & Pritzel 1904), outlining the significant differences from the centrolepids in embryology, seed anatomy, and flower and pollen morphology (Bortenschlager et al. 1966; Hamann 1975, 1976), but he was not able to establish its affinities "…the affinities of Hydatellaceae are still obscure" (Hamann 1976).

Hydatellaceae had continued to be classified as a monocot in the Poales until multi-genic phylogenetic analyses placed it in the Nymphaeales and sister to Cabombaceae and Nymphaeaceae (Saarela et al. 2007). Embryological evidence, seed morphology and sieve-element plastids support this placement (Hamann 1976; Rudall et al. 2008; Friedman 2008; Tratt et al. 2009).

Species of Hydatellaceae are minute, tufted herbs with linear leaves and inconspicuous flowering heads with two to several bracts surrounding the carpels and/or stamens. They differ significantly from other Nymphaeales, which are broad-leaved with showy petaloid flowers. Particularly unusual is the arrangement of the bisexual "reproductive units", where stamens are surrounded by carpels leading to early interpretations of the flowering head being a pseudanthium or inflorescence, an aggregation of unisexual perianthless flowers (Hooker 1860; Diels & Pritzel 1904; Edgar 1966; Hamann 1976). Also, they are described as having an "inside-out flower" and a "none-flower", and not able to be recognised as a typical angiosperm flower due to secondary loss or a pre-floral condition (Rudall et al. 2007, 2009). Despite recent morphological and anatomical studies, there is still uncertainty about the nature of the flowering head, and whether it is a pseudanthium, a flower, or a flower homologue (Sokoloff et al. 2010). Therefore the term ‘reproductive unit’ (Rudall et al. 2007; Sokoloff et al. 2008) is used here for the flowering head, in place of capitula, pseudanthium or inflorescence.

Until recently, two genera, Trithuria and Hydatella, were recognised for Hydatellaceae (Diels & Pritzel 1904; Edgar 1966; Edgar 1970; Cooke 1987). Diels & Pritzel (1904) originally distinguished Hydatella from Juncella F.Muell. ex Hieron. (= Trithuria) based on separate male and female ‘capitula’, stipitate flowers, longer and more numerous stigmatic hairs and a lesser number of involucral bracts (two rarely four). Based on Diels’ circumscription and the additional feature of indehiscent fruits, Cheeseman (1907) transferred the New Zealand species T. inconspicua to Hydatella, and this was followed by Edgar (1966) and Edgar (1970). Cooke (1981, 1983, 1987) recognised Trithuria and Hydatella based on the correlation of unisexual reproductive units and non-ribbed (indehiscent) fruits versus bisexual reproductive units and ribbed three-angled fruits (usually dehiscent). Sokoloff et al. (2008), with new insights and the discovery of additional species, demonstrated the non-correlation of these features and merged the two genera, with Trithuria Hook.f. having nomenclatural priority.

Biogeographical analyses date the stem lineage of Hydatellaceae to the early Cretaceous but the crown clade to the early Miocene, an indication that extant species mostly represent dispersal rather than vicariance (Iles et al. 2014). They infer an exception to the earliest split being vicariant and equating to tropical (northern Australia and India) versus subtropical/temperate lineages (South East Australia, Western Australia, New Zealand) and correlation with aridification of central Australia in the early Miocene. This may also correlate with differences in cotyledon morphology (Friedman et al. 2012; Iles et al. 2012; Sokoloff et al. 2008, 2013).


Minute tufts with non-petiolate linear leaves. The inconspicuous flowering heads have two to several bracts surrounding the carpels and/or stamens.  A variable sexual system, either co-sexual, dioecious, or apomictic. Crystals are absent. Fruit an indehiscent achene (NZ) or a capsule dehiscent via three ribs.


Australia, India, New Zealand.

Indigenous (Non-endemic)
Number of species in New Zealand within Hydatellaceae U.Hamann
Indigenous (Endemic)1
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Cheeseman, T.F. 1907: Notice of the occurrence of Hydatella, a genus new to the New Zealand flora. Transactions and Proceedings of the Royal Society of New Zealand 39: 433–434.
Cooke, D.A. 1981: New species of Schoenus (Cyperaceae) and Trithuria (Hydatellaceae). Muelleria 4: 299–304.
Cooke, D.A. 1983: Two Western Australian Hydatellaceae. Muelleria 5: 123–125.
Cooke, D.A. 1987: Hydatellaceae. In: Hydatellaceae to Liliaceae. Vol. 45. In: George, A.S. (ed.) Flora of Australia. ABRS and CSIRO, Melbourne. 1–5.
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Edgar, E. 1966: The male flowers of Hydatella inconspicua (Cheesem.) Cheesem. (Centrolepidaceae). New Zealand Journal of Botany 4: 153–158.
Edgar, E. 1970: 12. Centrolepidaceae. In: Moore, L.B.; Edgar, E. Flora of New Zealand. Vol. II. Indigenous Tracheophyta: Monocotyledones except Gramineae. Botany Division DSIR, Wellington. 79–85.
Ford, K.A.; Champion, P.D. 2019: Nymphaeales. In: Breitwieser, I.; Wilton, A.D. (ed.) Flora of New Zealand - Seed Plants. Fascicle 5. Manaaki Whenua Press, Lincoln.
Friedman, W.E. 2008: Hydatellaceae are water lilies with gymnospermous tendencies. Nature 453: 94–91.
Friedman, W.E.; Bachelier, J.B.; Hormaza, J.I. 2012: Embryology in Trithuria submersa (Hydatellaceae) and relationships between embryo, endosperm, and perisperm in early-diverging flowering plants. American Journal of Botany 99: 154–191.
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Iles, W.J.D.; Lee, C.; Sokoloff, D.D.; Remizowa, M.V.; Yadav, S.R.; Barrett M.D.; Barrett, R.L.; MacFarlane, T.D.; Logacheva, M.D.; Rudall, P.J.; Graham, S.W. 2014: Reconstructing the age of the ancient flowering-plant family Hydatellaceae (Nymphaeales). BioMed Central Evolutionary Biology 14: 102.
Iles, W.J.D.; Rudall, P.J.; Sokoloff, D.D.; Remizowa, M.V.; MacFarlane, T.D.; Logacheva, M.D.; Graham, S.W. 2012: Molecular phylogenetics of Hydatellaceae (Nymphaeales): sexual-system homoplasy and a new sectional classification. American Journal of Botany 99: 663–676.
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Rudall, P.J.; Remizowa, M.V.; Beer, A.S.; Bradshaw, E.; Stevenson, D.W.; Macfarlane, T.D.; Tuckett, R.E.; Yadav, S.R.; Sokoloff, D.D. 2008: Comparative ovule and megagametophyte development in Hydatellaceae and water lilies reveal a mosaic of features among the earliest angiosperms. American Journal of Botany 101: 941–956.
Rudall, P.J.; Remizowa, M.V.; Prenner, G.; Prychid, C.J.; Tuckett, R.E.; Sokoloff, D.D. 2009: Nonflowers near the base of extant angiosperms? Spatiotemporal arrangement of organs in reproductive units of Hydatellaceae and its bearing on the origin of the flower. American Journal of Botany 96(1): 67–82.
Rudall, P.J.; Sokoloff, D.D; Remizowa, M.V.; Conran, J.G.; Davis, J.I.; Macfarlane, T.D.; Stevenson, D.W. 2007: Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early-divergent Angiosperm lineage. American Journal of Botany 94(7): 1073–1092.
Saarela, J.M.; Rai, H.S.; Doyle, J.A.; Endress, P.K.; Mathews, S.; Marchant, A.D.; Briggs, B.G.; Graham, S.W. 2007: Hydatellaceae identified as a new branch near the base of the angiosperm phylogenetic tree. Nature 446: 312–315.
Sokoloff, D.D.; Remizowa, M.V.; Macfarlane, T.D.; Conran, J.G.; Yadav, S.R.; Rudall, P.J. 2013: Comparative fruit structure in Hydatellaceae (Nymphaeales) reveals specialized pericarp dehiscence in some early-divergent angiosperms with ascidiate carpels. Taxon 62(1): 40–60.
Sokoloff, D.D.; Remizowa, M.V.; Macfarlane, T.D.; Rudall, P.J. 2008: Classification of the early-divergent angiosperm family Hydatellaceae: one genus instead of two, four new species and sexual dimorphism in dioecious taxa. Taxon 57(1): 179–200.
Sokoloff, D.D.; Remizowa, M.V.; Yadav, S.R.; Rudall, P.J. 2010: Development of reproductive structures in the sole Indian species of Hydatellaceae, Trithuria konkanensis, and its morphological differences from Australian taxa. Australian Systematic Botany 23: 217–228.
Tratt, J.; Prychid, J.; Behnke, H.-D.; Rudall, P.J. 2009: Starch-accumulating (S-type) sieve-element plastids in Hydatellaceae: implications for pastid evolution in flowering plants. Protoplasma 237: 19–26.