Species Profile

Mexican Mosquito-fern

Scientific Name: Azolla mexicana
Other/Previous Names: Mexican Mosquito Fern
Taxonomy Group: Vascular Plants
Range: British Columbia
Last COSEWIC Assessment: November 2008
Last COSEWIC Designation: Threatened
SARA Status: Schedule 1, Threatened

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Quick Links: | Photo | Description | Distribution and Population | Habitat | Biology | Threats | Protection | Recovery Initiatives | National Recovery Program | Documents

Image of Mexican Mosquito-fern

Mexican Mosquito-fern Photo 1



The Mexican Mosquito-fern is a tiny floating aquatic fern, 1 to 2 cm in diameter. This small annual is highly branched and its tiny, fleshy leaves overlap like shingles. It usually has three floats, small structures that allow the plant to float. Spore-bearing structures, which produce reproductive cells called spores, are borne below the water surface. The species forms extensive green- or red-coloured mats on the surface of water bodies or small wet areas. This fern may be confused with two other mosquito-fern species occurring in British Columbia: Pacific Mosquito-fern and Carolina Mosquito-fern. Mexican Mosquito-fern can generally be distinguished by its tiny size.


Distribution and Population

Mexican Mosquito-fern is found in disjunct patches in North, South and Central America. In the United States, it is found in several western and mid-western states. In South America, it is found in Peru, northern Bolivia and Brazil. In Canada, it is restricted in occurrence to British Columbia. The eight extant populations occur in three regions of south-central British Columbia: the Little Fort area, the Shuswap Lake area and Vernon. Since 1889, when the species was first observed in British Columbia, 10 populations have been reported. By 2007, 2 of these populations had become extirpated as a result of road construction and urban development. In spite of this, the number of known populations has increased since 1984, when only 4 populations were reported. The total Canadian population likely includes millions of tiny ferns in any one year. Since the species has a tendency to form extensive mats of variable thickness, it is not realistic to count the individuals in the mats to determine the absolute population total. Site-specific yearly monitoring of actual areas occupied would be an easier and more practical manner of determining population fluctuations. Populations fluctuate dramatically from year to year in areal extent; while in some years plants appear absent from a site, they can reappear in following years. Losses may be attributable to water chemistry and temperature fluctuations, precipitation and/or draw-down in creeks and ponds. Reappearance may be attributable to the germination of spores present at the site when conditions are favourable. Because of this, population trends are unknown, and further study over several years is needed. It is possible, though highly unlikely, that individuals or spores may be transported by waterfowl over the 300-km distance from Washington state.



Mexican Mosquito-fern is an aquatic species that grows in still waters. In British Columbia, it is specifically found along the shores of quiet lakes, and in oxbows, ponds, wet pastures proximal to creek systems, and in ditches. The species prefers cool, slightly acidic, partially shaded, phosphorus-rich, but otherwise nutrient-poor still waters with low salinity. It grows well in shallow water with a depth of only a few centimetres, especially where the roots can touch the bottom. Mexican Mosquito-fern often occurs with other aquatic species that include Common Duckweed, Ivy-leaved Duckweed, Crystalwort and Reed Canary Grass. This fern is susceptible to changes in water levels and water chemistry, which may restrict its occurrence in what otherwise appears to be suitable habitat. Overall, suitable habitat for this species remains stable, since the large majority of it occurs in rural areas outside of any immediate development potential. However, as a result of road and highway construction, many of the small water bodies associated with the occurrence of Mexican Mosquito-fern in British Columbia have been eliminated.



Mexican Mosquito-fern is an aquatic fern that grows in a symbiotic relationship with a species of blue-green alga (a cyanobacterium), which lives in cavities on the upper side of the fern’s leaves. The fern provides protection and nutrients, while the alga provides fixed atmospheric nitrogen in a form that the fern can use. Primary reproduction is through fragmentation, where fragments break off and multiply. The species also reproduces sexually through spore production by structures called sporocarps, which occur in pairs in the leaf axils of older plants. Sporocarps produce two types of spores of different sizes and appearance. In British Columbia, sporocarp development is generally associated with mat development and summer months. Once produced, spores fall to the bottom of water bodies and eventually germinate, producing new plants. Dispersal of spores and fragments is by wind, water currents, waterfowl and, accidentally, by humans. The plant dies back or decomposes in the fall. As with all other Azolla species, this fern is preyed upon by many insect types, particularly butterflies, flies, beetles and aphids, as well by snails. Mexican Mosquito-fern can tolerate low levels of salinity but is killed by levels that are too high. It is poorly resistant to desiccation and high temperatures and, although it is generally resistant to cold, freezing of the water surface can result in die-off.



In British Columbia, road construction poses a major threat to the species. Most Canadian populations of Mexican Mosquito-fern are found along highways and in publicly accessible areas where the impact of adjacent roadwork, chemical spills or land development could potentially affect the wetland habitat and could alter water chemistry, conceivably destroying entire populations. One population was lost as a result of infilling related to road construction (south of Darfield). Additionally, sensitivity to high levels of salinity would make salting of roadways and associated runoff of salt and other toxic chemicals a threat. Urban development, more specifically housing development, is a direct threat to populations in the Sicamous and Vernon areas. In the Shuswap Lake sector, where there is moderate urban expansion occurring, one site for this species has been lost. Eutrophication represents another threat to several Canadian populations. This phenomenon, which results in algal blooms following excessive inputs of certain nutrients such as phosphorus, is apparent at the Salmon Arm foreshore site. The actual effect of a bloom on Mexican Mosquito-fern is, however, unknown. Invasive species are another threat. Encroachment of Reed Canary Grass is evident at the Salmon Arm foreshore site. Finally, while the chances of a railway spill adjacent to the two populations located along the railway at Salmon Arm are low, an occurrence would nevertheless have a significant impact on the adjacent wetlands and on Mexican Mosquito-fern.



Federal Protection

The Mexican Mosquito-fern is protected under the federal Species at Risk Act (SARA). More information about SARA, including how it protects individual species, is available in the Species at Risk Act: A Guide.

Mexican Mosquito-fern is not protected under any provincial legislation in British Columbia. However, one of the Canadian populations is located in Dunn Peak Provincial Park and is protected under the British Columbia Park Act, which prohibits certain activities, such as logging and mine development.

Provincial and Territorial Protection

To know if this species is protected by provincial or territorial laws, consult the provinces' and territories' websites.


Recovery Initiatives

Status of Recovery Planning

Recovery Strategies :

Name Recovery Strategy for the Mexican Mosquito-fern (Azolla mexicana) in Canada
Status First posting on SAR registry



PLEASE NOTE: Not all COSEWIC reports are currently available on the SARA Public Registry. Most of the reports not yet available are status reports for species assessed by COSEWIC prior to May 2002. Other COSEWIC reports not yet available may include those species assessed as Extinct, Data Deficient or Not at Risk. In the meantime, they are available on request from the COSEWIC Secretariat.

6 record(s) found.

COSEWIC Status Reports

  • COSEWIC Assessment and Update Status Report on the Mexican Mosquito-fern Azolla mexicana in Canada (2009)

    Mexican Mosquito-fern Azolla mexicana is a tiny floating aquatic fern found in wetlands and small wet areas. Individual plants range in size from 1-2 cm in length. Leaves are tiny and overlap like shingles. Spore-producing structures, borne below the water surface, are present in the Canadian populations. Roots are simple and short. Plants usually have three “floats”. This species forms extensive green or red mats on the water surface.

COSEWIC Assessments

  • COSEWIC Assessment - Mexican Mosquito-fern (2009)

    Mexican Mosquito-fern Azolla mexicana is a tiny floating aquatic fern found in wetlands and small wet areas. Individual plants range in size from 1-2 cm in length. Leaves are tiny and overlap like shingles. Spore-producing structures, borne below the water surface, are present in the Canadian populations. Roots are simple and short. Plants usually have three “floats”. This species forms extensive green or red mats on the water surface.

Response Statements

  • Response Statement - Mexican Mosquito-fern (2009)

    This tiny floating aquatic fern of south-central British Columbia is restricted to 8 small water bodies where its populations undergo periodic fluctuations in numbers of individuals. Two populations and their habitat have been lost in recent years due to construction activities with most of the extant populations occurring adjacent to major highways or a railway where they are at potential risk from maintenance activities, including the use of chemicals such as road salt.

Recovery Strategies

  • Recovery Strategy for the Mexican Mosquito-fern (Azolla mexicana) in Canada (2017)

    The Minister of Environment and Climate Change is the competent minister under SARA for the Mexican Mosquito-fern and has prepared the federal component of this recovery strategy (Part 1), as per section 37 of SARA. To the extent possible, it has been prepared in cooperation with the Province of British Columbia as per section 39(1) of SARA. SARA section 44 allows the Minister to adopt all or part of an existing plan for the species if it meets the requirements under SARA for content (sub-sections 41(1) or (2)). The Province of British Columbia provided the attached recovery plan for the Mexican Mosquito-fern (Part 2) as science advice to the jurisdictions responsible for managing the species in British Columbia. It was prepared in cooperation with Environment and Climate Change Canada.

COSEWIC Annual Reports

  • COSEWIC Annual Report - 2009 (2009)

    2009 Annual Report to the The Minister of the Environment and the Canadian Endangered Species Conservation Council (CESCC) from the Committee on the Status of Endangered Wildlife in Canada.

Recovery Document Posting Plans

  • Environment and Climate Change Canada's Three-Year Recovery Document Posting Plan (2016)

    Environment and Climate Change Canada’s Three-Year Recovery Document Posting Plan identifies the species for which recovery documents will be posted each fiscal year starting in 2014-2015. Posting this three year plan on the Species at Risk Public Registry is intended to provide transparency to partners, stakeholders, and the public about Environment and Climate Change Canada’s plan to develop and post these proposed recovery strategies and management plans. However, both the number of documents and the particular species that are posted in a given year may change slightly due to a variety of circumstances. Last update March 31, 2017