Azolla pinnata

Species

Azolla pinnata R. Brown

Family

Salviniaceae

Common names

mosquito fern

Disseminule

spores, whole plant

Description

A free-floating annual water fern 0.8–2.5 cm long, with tiny scale-like 2-lobed leaves; lobes each 1–2 mm long; leaves green or red, often giving water surface reddish appearance. Main stem with pinnate branches; branches longer towards base giving plant a triangular shape. Roots with fine lateral rootlets, appearing feathery. Produces both male and female spores.

Identification considerations

Diagnostic characteristics:

• vegetative: triangular shape of plants; feathery appearance of roots
• sexual: megasporangium has 9 floats; microsporangial massulae have no glochidia, but have vacuolated trichomes

Similar species

Azolla caroliniana Willd. (non-FNW)

Azolla filiculoides Lam (non-FNW)

Distribution

native to tropical Africa, South Africa, India, China, Japan, Malaysia, Philippines, Vietnam, New Guinea, Australia

also found in Bangladesh, Pakistan, Sri Lanka, Korea, Southeast Asia, New Zealand

Habitat

subtropical and tropical; in lakes, slow moving streams and rivers, marshes, ponds, wetlands, paddy fields, ditches

General information

Azolla pinnata has been used for centuries in Southeast Asia as a fertilizer in rice production. The rice benefits from A. pinnata's symbiotic relationship with the cyanobacteria Anabaena azollae, which fixes nitrogen and grows in a cavity in the dorsal lobe of the fern's leaves. Azolla pinnata dies in the summer heat and the decaying plants release nitrogen to the soil. This fern is also considered a weed, however. It can quickly spread to cover open areas of water. It forms dense surface mats that impede water flow and navigation, and clog irrigation pumps. The mats also reduce oxygen levels and light available to other aquatic organisms. Vegetative reproduction is by dispersal of basal branches that themselves form pinnate branches and break off the main stem.