Cabomba
Cabomba is a genus of approximately five species of perennial, rhizomatous aquatic herbs in the family Cabombaceae, native to tropical, subtropical, and temperate regions of the Americas.[1][2] These rooted, submerged plants possess two leaf types—finely palmately dissected submerged foliage up to 5 cm across and peltate floating leaves—and produce small, white to pinkish emergent flowers on elongated stems that reach the water surface.[2][3] Valued in the aquarium trade for their feathery appearance and rapid growth, species such as Cabomba caroliniana, C. aquatica, and C. furcata have escaped cultivation to become aggressive invasives in non-native waters, forming dense canopies that displace indigenous flora, reduce biodiversity, and obstruct waterways.[4][5][6] For instance, C. caroliniana is classified as a noxious weed in regions like Australia, where it proliferates in shallow, slow-moving waters less than 3 meters deep, exacerbating ecological and economic disruptions through unchecked vegetative reproduction via fragmentation.[6][4]Morphology and Biology
Vegetative Characteristics
Cabomba species are rooted, perennial aquatic herbs characterized by slender, branching stems that range from grass-green to olive-green or reddish-brown in color and can extend up to 10 meters in length, though commonly 1 to 3 meters.[7][8] These stems often produce adventitious fibrous roots at lower nodes, anchoring the plant in substrates.[9] A defining feature is the pronounced foliar heterophylly, with submerged leaves arranged oppositely or in whorls of three, finely dissected into 3 to 5 palmate series of linear segments forming a fan-like structure typically 2 to 5 cm wide.[10][11][12] Floating leaves, emergent near the stem apices during certain conditions, are alternate, peltate, and entire-margined with elliptic to orbicular blades.[10][12] This dimorphism reflects adaptations to submerged versus surface environments, with submerged foliage optimized for underwater light capture and floating leaves for aerial exposure.[13][14]Reproductive Characteristics
Cabomba species produce solitary, emergent or floating bisexual flowers on elongated peduncles arising from stem apices or rhizomes, adapted for pollination above the water surface. The flowers are actinomorphic and hypogynous, featuring a dichlamydeous perianth with three free, green sepals and three clawed petals that are typically white, though varying to yellow or purple across species. The androecium consists of three or six stamens in one or two whorls, with filaments that are somewhat flattened or laminar; anthers are introrse and tetrasporangiate.[15][13] The gynoecium is apocarpous, comprising 2–18 carpels with superior ovaries and parietal placentation, though typically fewer carpels (3–6) are observed in most species. Each carpel contains 1–3 anatropous, bitegmic ovules. Flowers exhibit protogyny, opening over two consecutive days: on the first day, stigmas are receptive while anthers remain closed, and on the second day, anthers dehisce to release pollen, facilitating cross-pollination primarily by small insects such as flies and bees.[15][13][16] Mature fruits form as coriaceous, dehiscent follicles, measuring 4–7 mm in length for species like C. caroliniana, which split along the dorsal suture to disperse 1–3 seeds per fruit. Seeds are small (1.5–3 × 1–1.5 mm), ellipsoid, with a tuberculate testa featuring tubercles arranged in four longitudinal rows; internally, they possess a small straight embryo with two cotyledons, a plumule, and radicle, surrounded by thin lipid-containing endosperm and abundant starchy perisperm.[17][15][18]
Cytological and Physiological Traits
Cabomba species display cytogenetic variation, with the ancestral base chromosome number estimated at x=13 based on surveys identifying 2n=26 as the lowest observed count, interpreted as diploid.[19] Higher ploidy levels, including tetraploid (2n=52) and octoploid (2n=104) forms, have been documented in C. aquatica sensu lato, reflecting polyploidy as a mechanism for genetic diversity and adaptation in the genus.[20] Cytological studies indicate that such variation aligns with the genus's basal position in angiosperm evolution, though specific karyotype details like chromosome morphology remain underexplored beyond basic counts.[19] Physiologically, Cabomba exhibits an induction period in photosynthesis, characterized by initial low rates of oxygen evolution that increase over time upon exposure to light and CO₂, observable across low and high intensities of both factors in C. caroliniana.[21] This trait underscores efficient carbon fixation in submerged conditions, facilitated by finely dissected leaves that enhance light capture and gas exchange. Species like C. caroliniana demonstrate pronounced shade tolerance, achieving higher relative growth rates (RGR) and specific leaf area (SLA) under low light than competitors such as Myriophyllum spicatum, aiding invasion in turbid waters.[22] Broad thermal tolerance, spanning subtropical to temperate ranges, supports persistent growth and fragmentation-based reproduction, with optimal performance in waters from 15–30°C.[23] Nutrient responses include resilience to elevated urea, though high concentrations impair photosynthetic efficiency via photosystem II disruption in C. caroliniana.[24]Taxonomy
Recognized Species
The genus Cabomba contains five accepted species: C. aquatica Aubl., C. caroliniana A. Gray, C. furcata Schult. & Schult. f., C. haynesii Wiersema, and C. palaeformis Fassett.[25][26] This delimitation stems from a 1991 taxonomic revision emphasizing diagnostic traits including the degree of leaf dissection (e.g., quaternary branching in submerged leaves), flower color (white versus reddish), stamen number, and seed surface sculpturing, which distinguish the species while resolving prior synonymies.[26] Subsequent assessments by botanical authorities, including government invasive species profiles and global plant databases, uphold this five-species framework without proposing additional segregates or mergers.[27][28]- Cabomba aquatica Aubl. (1753): Features highly dissected submerged leaves with up to five orders of branching and white to purplish flowers; widely distributed in tropical South America.[25]
- Cabomba caroliniana A. Gray (1848): Characterized by green-tinged leaves and flowers, with two recognized varieties (var. caroliniana and var. pulcherrima differing in petal pigmentation intensity); native primarily to temperate and subtropical North America.[26][25]
- Cabomba furcata Schult. & Schult. f. (1831): Distinguished by forked (furcate) leaf segments and reddish flowers; occurs in central and northern South America.[25]
- Cabomba haynesii Wiersema (1987): Defined by intermediate leaf dissection and specific stamen filament coloration; restricted to Guyana and northern Brazil.[26][25]
- Cabomba palaeformis Fassett (1940): Notable for pale, less dissected leaves resembling fossil forms and nine stamens; found in southern South America.[26][25]