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BC Conservation Data Centre: Species Summary


Azolla microphylla
Mexican mosquito fern

 
Scientific Name: Azolla microphylla Kaulf.
Scientific Name Synonyms: Azolla mexicana
English Name: Mexican mosquito fern
 
Classification / Taxonomy
Scientific Name - Concept Reference: Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Classification Level: Species
Species Group: Vascular Plant
Species Code: AZOLMEX
Kingdom Phylum Class Order Family
Plantae Filicinophyta Filicopsida Salviniales Salviniaceae
   
Conservation Status / Legal Designation
Global Status: G5 (Nov 2011)
Provincial Status: S3 (Apr 2019)
BC List: Blue
Provincial FRPA list:   
Provincial Wildlife Act:
COSEWIC Status: Threatened (Nov 2008)
SARA Schedule: 1  -  Threatened (Jun 2003)
General Status Canada: 1 - At Risk (2010)
   
Ecology & Life History
General Description:
Technical Description:
Subspecies Comments: There are no subspecies of Azolla mexicana recognized in BC.
Identification Comments: Azolla mexicana is a tiny floating aquatic fern under 3 cm in diameter (usually 1 to 1.5 cm) with an intricately-branched (crystalline) form (Brunton 1983; Douglas et al. 2000). The fern is pinnately branched from a central axis and dichotomously branched at its periphery. Leaves are numerous, tiny (< 1 mm), bi-lobed (upper and under-lobes) and densely over-lapping like shingles. Sporangia on short-stalked sori on lower surface of under-lobes, enclosed by an indusium, with either one megasporangium (rare) or several microsporangia. Megaspores are shallowly pitted at the basal region. Roots are simple, short and trailing. Azolla mexicana is pale green in the summer when colonies increase to cover the water surface, and becomes brick red in the fall.
Similar Species: Two other Azolla species occur in BC: A. filiculoides and A. caroliniana, both of which are likely introduced from eastern North America. Azolla caroliniana is distinguished from the other two species by its smaller size (less than 1 cm in diameter), its dichotomously-branched form throughout, and its leaves that are scarcely, or not at all, overlapping. Azolla mexicana can be distinguished from A. filiculoides by its less imbricate leaf form, smaller size (< 3 cm vs. 2 to 6 cm), many crosswalls (vs. no crosswalls) on the hooked hairs of microspore masses, and the megaspores shallowly pitted (vs. net-veined) in the basal region (Brunton 1983; Douglas et al. 2000).
Provincial Reproduction Comments: As an annual species, Azolla mexicana depends on successful sexual reproduction for its persistence. The sporangia that form on the under-lobes of Azolla mexicana can be either one megasporangium or several microsporangia. Megaspores and microspores are released when ferns senesce. There are far greater numbers of microspores than megaspores (Brunton 1983). The spores sink to the bottom of the pond and remain in the sediment until favourable conditions arise (e.g. appropriate water chemistry, water levels, temperature). When conditions are favourable, spores float to the pond's surface and germinate. Spores of Azolla species may be dormant for several years. Once spores break dormancy and germinate, plantlets undergo asexual reproduction and can rapidly form large blankets of thousands of individuals across a pond's surface. Large blankets are not formed in stream habitats (Martin 1998). Brunton (1983) suggested that very few sexually-produced sporophytes can 'explosively' fill in available growing surfaces. The amount and rate of gene exchange among populations or sub-populations via spore dispersal or plant dispersal is not known. Water transport of entire plants is one mode of dispersal for Azolla mexicana. Alternatively, sporangia or fern fragments may be carried to new sites over short distances in debris during spring floodwaters and/or by animal agents such as waterfowl, amphibians, or painted turtles (Brunton 1983). Martin (1998; 2005) proposed that waterfowl were the most likely agent of dispersal and that the fall red colouration may be an artifact to attract waterfowl for dispersal. It is not known if A. mexicana is able to persist in the long term in stream sites. In fact, it is not known if it is a permanent resident of any sites within its range or if it acts everywhere as an opportunist, dependent on chance introduction or re-introduction every few years (Martin 2005).
Provincial Ecology Comments: Of importance, is the nitrogen-fixing capability of Azolla mexicana through its symbiotic relationship with the cyanobacterium, Anabaenae azollae, which grows in the cavities of Azolla leaves. The relationship continues throughout the life cycle of Azolla mexicana and enables it to thrive in nitrogen-poor sites, which lack competition from other aquatic plant species (Brunton 1983). However, if dense coverage of the water surface by Azolla fronds prevents competition from other habitat users, a break in continuity of Azolla at the site by a year or two of unfavourable water or other conditions could ultimately lead to loss of that particular site to invasive species like Phalaris arundinacea by allowing their establishment (Martin 2005). Brunton (1983) suggested that Azolla mexicana requires specific combinations of water temperature, chemistry and available light. Suitable characteristics for optimal growth of Azolla mexicana include partially-shaded light environments (50% full sunlight), cool water temperatures, slightly acidic water (low pH), adequate levels of phosphorous, iron, manganese, cobalt and molybdenum (Brunton 1983). These characteristics are also believed to promote nitrogen fixation by Anabaenae azolla (Brunton 1983). Azolla species are susceptible to high salinity, ceasing to grow at levels of 1.3% salt and dying at higher levels. Brunton (1983) characterized A. mexicana as an early successional species, which thrives on disturbance to the shoreline and changing water characteristics (e.g. water levels), but which functions within a narrow chemical tolerance. Invasive plants, diseases and pests appeared to have little or no effect on BC populations of Azolla mexicana during field sampling by Brunton (1983).
Habitats:
(Type / Subtype / Dependence) 		Lakes / Pond/Open Water / Obligate
Riparian / Riparian Shrub / Facultative - frequent use
Provincial Habitat Comments: In BC, Azolla mexicana occurs in the Interior Cedar Hemlock biogeoclimatic zone (ICHmw2) and the Interior Douglas-fir zone (IDFxh1a, IDFxh2) (Douglas et al. 2002). Within this zone, Azolla mexicana has been found in a range of aquatic environments from standing and sluggish water (ponds, sloughs, backwaters, oxbow lakes, human-made ditches) to slow and strongly flowing streams (Martin 1998). There is little known about the habitat requirements of A. mexicana in streams. In 2004, A. mexicana was not found in stream habitats, only in oxbow lakes and shallow ponds. Azolla mexicana often grows in shallow slow-moving or standing water, with fluctuating water levels, over sandy alluvium which is covered with organic debris from various aquatic plants. It prefers cool, slightly acidic, partially shaded, phosphorous-rich, iron-rich, manganese-rich, nutrient-poor, still waters with low salinity (Brunton 1983). Most sites in BC occur in broad flat river floodplains that were initially formed by post-glacial outwash deposited in broad mountain valleys (Brunton 1983). Plants can be free floating or on logs and rotting vegetation (Brunton 1983). Mats of Azolla mexicana are often associated with other aquatic floating species such as Lemna minor, Utricularia macrorhiza, and the liverwort, Riccia fluitans as well shrubs and herbs such as Salix exigua, Typha latifolia, Potamogeton spp., Glyceria borealis, and Phalaris arundinacea (Brunton 1983).
Provincial Phenology:
(1st half of month/
2nd half of month)		 Apr: Germinating / Germinating
May: Germinating / Germinating
Jun: Vegetative / Vegetative
Jul: Vegetative / Vegetative
Aug: Vegetative / Vegetative, Fruiting
Sep: Vegetative, Fruiting / Vegetative, Fruiting
Provincial Phenology Comments: There is little specific information on the phenology of Azolla mexicana. In the spring (or when conditions are suitable), microspores and megaspores float to the surface after over-wintering in the pond sediment. They germinate, forming gametophytes which produce gametes. Sexual fusion of the gametes occurs and young plantlets (sporophytes) are formed. During the summer, these pale green plantlets undergo rapid asexual reproduction, forming large mats. By late summer, the entire pond or backwater can be filled with Azolla mexicana. In the fall (late August to early September), the sporophytes turn red, senesce and spores drop to the bottom of the pond (Brunton 1983; Martin 1998). For the purposes of the seasonal phenology (illustrated above), it was inferred that spring (and germination) was April and May, but no specific dates are available. Brunton (1983) examined two localites of Azolla mexicana in BC in mid-June, mid-July and mid-September of 1982. One of the sites showed no evidence of Azolla plants until mid-July and the other site showed no sign of plants until mid-September. This information suggests that phenology may be linked to water conditions that are not necessarily linked to seasonal conditions. Brunton (1983) also suggested that develoment was more dependent on water-levels than on seasons.
Elevation (m) (min / max): Provincial:  340 / 550
Known Pests:
Pollen Vector:
Pollinator:
Dispersal:
   
 
Provincial Inventory
Inventory Priority: A - Highest
Ownership of occurrences (Known locations): Mostly private
Inventory Need: Monitoring of known sites and other suitable habitats (oxbows, ponds and backwaters) in the regions surrounding the Azolla mexicana populations is strongly recommended. It is also recommended that sites that harboured populations in the past (and have not been infilled or permanently damaged) should be routinely surveyed because A. mexicana can remain dormant for many years. The populations may re-appear under favourable conditions in future years.
Inventory Comments: Brunton (1983) surveyed twelve likely-looking oxbow ponds between Sicamous and Craigellachie siding along the Trans-Canada Highway but found no new occurrences. Brunton (1983) suggested that the Eagle River floodplain as well as areas near known populations in Salmon Arm and Tappen could support new populations. More recent studies do not discuss how many sites were surveyed unsuccessfully during Azolla inventories (Martin 1998).
 
Economic Attributes
Provincial Economic Comments: There are no known economic uses for Azolla mexicana in BC. Other species of Azolla have been used as green manure (fertilizer) in rice paddies in southeastern Asia, as an industrial fish food in China, as fodder for farm animals in Asia and Africa and for mosquito control in western Europe (Brunton 1983). Applications for A. mexicana have not been specifically studied but it likely has potential for at least some of these uses.
 
Distribution
Endemic: N
Disjunct, more common elsewhere: Y
Peripheral, major distribution elsewhere: Y
 
Authors / Contributors
Global Information Author:
Last Updated:
Provincial Information Author: Bartemucci, P.
Last Updated: Jul 03, 2005
Last Literature Search:
   
References and Related Literature
B.C. Ministry of Environment. Recovery Planning in BC. B.C. Minist. Environ. Victoria, BC.
Brunton, D.F. 1984. Status Report on Mexican Mosquito Fern, Azolla mexicana, in Canada. Unpubl. rep. submitted to the Comm. on the Status of Endangered Wildl. in Can. Ottawa. 36 pp.
Douglas, G.W., D. Meidinger, and J. Penny. 2002. Rare Native Vascular Plants of British Columbia, 2nd ed. B.C. Conserv. Data Centre, Terrestrial Inf. Branch, Victoria. 358pp.
Douglas, G.W., D. Meidinger, and J. Pojar, eds. 2000. Illustrated Flora of British Columbia, Vol. 5, Dicotyledons (Salicaceae through Zygophyllaceae) and Pteridophytes. B.C. Minist. Environ., Lands and Parks, and B.C. Minist. For., Victoria. 389pp.
Douglas, G.W., G.B. Straley, and D. Meidinger, eds. 1998. Rare Native Vascular Plants of British Columbia. Conserv. Data Centre, Resour. Inventory Branch, B.C. Minist. Environ., Lands and Parks, Victoria, and B.C. Minist. For., Victoria.
Flora of North America Editorial Committee (FNA). 1993a. Flora of North America north of Mexico. Vol. 2. Pteridophytes and gymnosperms. Oxford Univ. Press, New York. xvi + 475 pp.
Lumpkin, T. A. and D.L. Plunkett. 1980. Azolla: botany, physiology, and use as a green manure. Economic Botany 34: 111-153.
Martin, M. 2005. Mexican Mosquito Fern (Azolla mexicana), ecology, habitat requirements and sites. May 24, 2005 e-mail to Jenifer Penny, BC Conserv. Data Centre.
Martin, M.E. 1998. Update COSEWIC Status Report on Mexican Mosquito Fern, Azolla mexicana, in Canada. Unpubl. rep. submitted to the Comm. on the Status of Endangered Wildl. in Can. Ottawa. 17 pp.
Southern Interior Rare Plants Recovery Team. 2008. Recovery Strategy for the Mexican Mosquito Fern (Azolla mexicana) in British Columbia. Prepared for the B.C. Ministry of Environment, Victoria, BC. 16 pp.
 

Please visit the website Conservation Status Ranks for definitions of the data fields used in this summary report.

Suggested Citation:

B.C. Conservation Data Centre. Species Summary: Azolla microphylla. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jun 5, 2026).