Mexican Mosquito Fern

Photo of Mexican mosquito fern plants on a pond surface, surrounded by duckweed
Scientific Name
Azolla mexicana
Salviniaceae (watermosses and mosquito ferns) (sometimes Azollaceae)

Water ferns float in velvety green or purplish-red masses on the water surface. Each mass is composed of many tiny fern plants, each smaller than a dime (at most, ¾ inch in diameter). Each plant has many small, overlapping leaves on branches radiating outward, like a rosette, from a central stalk, with a single rootlet dangling from the plant base. The velvety-seeming leaf surfaces repel water.

If you look very closely, you will see that each leaf is 2-lobed. The floating lobe is smaller, green with chlorophyll (turning dark red later in the season), lacking venation, and often with a nubby texture. The submerged lobe is larger, clear to translucent, lacking chlorophyll, and repels water, helping the plant to float. Ball-shaped, spore-producing structures are attached at the base of the submerged leaf-lobe, usually in groups of 2–4. Most reproduction, however, is vegetative, by the plants breaking into pieces and being distributed elsewhere by waterfowl.

Similar species:

  • Eastern mosquito fern (Azolla cristata, formerly A. caroliniana), a close relative, occurs scattered in eastern Missouri, where a few, vegetative specimens have been collected floating on ponds or stranded on mud. Its plants are smaller, usually less than ⅜ inch long, and the tiny, floating leaf-lobes are smaller and smoother than those of A. mexicana and do not overlap each other so much. It is difficult to tell Missouri's two mosquito ferns apart.
  • Duckweeds and watermeal (family Lemnaceae) are also small floating pond plants, but their oval or circular, leaflike bodies are usually larger than those of mosquito ferns and are not connected in elongated, branching stems.
  • Fringed heartwort (Ricciocarpos natans) is a species of floating liverwort with a fairly widespread distribution in Missouri. Fringed heartwort has a Y-shaped, flattened, leafy form that looks something like floating green hearts, or a fan composed of two heart-shapes joined at their tips. These plants can form rosettes to about 1½ inches in diameter.
Other Common Names
Water Fern
Fairy Moss
Water Velvet

Plant length: approx. ¼–¾ inch (0.7–2.0 cm). Floating leaf-lobe length: approx. 0.02–0.03 inch (0.6–0.8 mm).

Where To Find

Scattered, mostly in counties bordering the Mississippi, Missouri, and St. Francis rivers.

Mosquito fern floats on the water in ponds, sloughs, swamps, ditches, marshes, and still backwaters of rivers and streams. You may also see it stranded on mud by receding waters.

A pond covered with Azolla seems to have a severe algae problem. Huge floating mats of this plant can completely cover a pond with a velvety green or purplish-red carpet.

In autumn, the overwintering bodies of mosquito fern fall to the bottom of the pond; they return to the surface in late spring and summer.

Mexican mosquito fern (Azolla mexicana), the subject of this page, is a native Missouri species, but it can be a nuisance aquatic plant. It can completely cover a pond, lagoon, or slow-moving river, and it should never be introduced from one location to another.

Eastern mosquito fern (Azolla cristata, formerly A. caroliniana), Missouri's other native mosquito fern, is listed as a state species of conservation concern. There is uncertainty about its status, but experts have ranked it from vulnerable to critically imperiled in our state.

Life Cycle

Mosquito ferns most commonly spread via vegetative reproduction, when side branches break off and form new plants.

As true ferns, they do not flower or produce seeds. Instead, they have a two-part life cycle. Generally speaking the plants we see are called sporophytes, and they produce spores. The spores germinate to become gametophytes, which create eggs and sperm, which unite to become a new sporophyte.

But it’s much more complicated than that. Mosquito ferns have what botanists have called an “extraordinarily complex” set of reproductive structures. Most peoples' eyes will glaze over at just the names of these structures: sporocarps, two kinds of spores (megaspores and microspores), plus structures called massulae, sporangia, and glochidia. The tiny gametophytes live within the walls of the megaspores, which are rounded at the base and have a triangular cap with 3 saclike floats. If this sounds crazy, don't worry, we won't quiz you on any of this. You're doing well if you just remember their anatomy is "complicated"!

The name “mosquito fern” arose from the belief that populations can grow so densely on the water surface that mosquitoes are unable to breed. Mosquito fern species in Asia have been used for this purpose, but although it might make it more difficult for mosquito larvae to breathe, it does not completely suppress them.

Mosquito ferns can become overabundant and should not be introduced. In some areas of the southern United States, mosquito ferns have interfered with livestock watering, blocked pump inlets, and affected commercial fishing. Also, because this species can take nitrogen from the atmosphere and convert ("fix") it to a form available as plant food, it can enrich the water and encourage overgrowth of algae.

Elsewhere, mosquito ferns of various species have a tremendous value in agriculture. Because these aquatic ferns can capture nitrogen from the air and convert it to a form plants can use for nutrition, and because it can grow so abundantly, some species of Azolla are cultured as a “green manure” in rice paddies in southeast Asia. The paddies are flooded and the mosquito ferns introduced. Then, the fast-growing, thick mat of mosquito ferns suppresses weeds, and when the mosquito ferns rot, the nitrogen fertilizes the soil. In China, this practice has been used for more than 1,000 years.

The relatively high protein content of dried mosquito fern has inspired people to investigate using it as a component in commercial livestock feed.

Various types of mosquito ferns are sometimes used in aquariums. However, they are not as commonly used as duckweed, hornwort, crystalwort (Riccia, a type of liverwort), and several other floating plants that require less light. It is sometimes sold as a floating plant for decorative outdoor goldfish and koi ponds, but it can easily get out of hand. Remember, never release aquarium plants, fish, or invertebrates into natural waters.

The complex reproductive biology and structures of mosquito ferns can seem either mind-numbingly tedious or intensely fascinating, depending on your perspective. Many people love solving riddles and learning about weird, offbeat, nerdy topics.

Mosquito ferns apparently played an important role in creating the relatively cool climate Earth has enjoyed for millions of years. For information about people using mosquito ferns to help reverse human-caused climate change, read the section on Ecosystem Connections.

Mosquito ferns have chambers in the floating leaf lobes that contain a symbiotic blue-green alga (cyanobacterium), Anabaena azollae. This filamentous organism can fix nitrogen from the air into a nitrate form that can be used as a nutrient by the water fern. The blue-green alga benefits from the protective chamber in the fern, which also provides some mineral nutrients. This is similar to what happens in the roots of legume plants (peas, beans, clover, alfalfa, and so on).

About 49 million years ago, during the Eocene epoch and at a time when Earth’s atmosphere was about 41–46 F higher than it is today, tremendous mats of mosquito ferns apparently “bloomed” in the Arctic Ocean. When the plants sank to the bottom of the sea and were covered by sediment, they took with them large amounts of carbon dioxide, removing it from the global nutrient cycle. This contributed to the formation of polar ice sheets and helped to cool the atmosphere to how it is today. Scientists call this the “Azolla event.”

In addition to research on Earth’s climate, there is also currently a lot of interest in exploring the Arctic for oil deposits. Much of this centers around the ancient, buried, fossilized Azolla beds. Burning this petroleum for energy would release the carbon — that the Azolla mats had sequestered millions of years ago — back into the atmosphere.

On the other hand, researchers are exploring the possibility of using Azolla plants once again as a way to trap large amounts of atmospheric carbon and "sink" it back under the Earth's surface. By replicating the climate-changing event that happened naturally in the Eocene, humans might be able to slow or reverse global warming.

Media Gallery
Similar Species
About Wildflowers, Grasses and Other Nonwoody Plants in Missouri
A very simple way of thinking about the green world is to divide the vascular plants into two groups: woody and nonwoody (or herbaceous). But this is an artificial division; many plant families include some species that are woody and some that are not. The diversity of nonwoody vascular plants is staggering! Think of all the ferns, grasses, sedges, lilies, peas, sunflowers, nightshades, milkweeds, mustards, mints, and mallows — weeds and wildflowers — and many more!
Reviewed On