Azolla filiculoides (water fern)

A. filiculoides is a small fern native to the Americas which has spread widely throughout the world by a variety of mechanisms, of which man has become the most significant (Lumpkin and Plucknett, 1982). Man has introduced A. filiculoides into Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean and Hawaii. In eutrophic water systems, A. filiculoides grows rapidly, easily outcompeting indigenous vegetation. Decaying root and leaf matter below a mat of A. filiculoides, coupled with the lack of light penetration, creates an anaerobic environment which can reduce the quality of drinking water and make survival for other organisms in the water impossible.

The origin of the generic name Azolla is thought to be derived from a conjugation of two Greek words meaning 'to dry' and 'to kill', inferring that the fern is killed by drought (Moore, 1969; Lumpkin and Plucknett, 1980).

The genus Azolla has no generally accepted taxonomic framework, primarily as a result of the plant's diminutive structure, morphological and phenotypic plasticity (Stergianou and Fowler, 1990; Saunders and Fowler, 1992) resulting in numerous synonyms. Currently, the genus is divided into three sections (Saunders and Fowler, 1992; Saunders and Fowler, 1993): section Azolla, which includes A. filiculoides Lam., A. rubra R. Br. (now more commonly regarded as a subspecies of A. filiculoides), A. caroliniana Willd., A. microphylla auct. non Kaulf, A. mexicana Presl and A. cristata Kaulf.; section Rhizosperma, which includes A. pinnata R. Br. var. africana (Desv.) R.M.K. Saunders and K. Fowler, stat. et comb. nov., A. pinnata R. Br. var. asiatica R.M.K. Saunders and K. Fowler, subsp. nov., and A. pinnata R. Br. var. pinnata; and section Tetrasporocarpia, which includes A. nilotica Decne. ex Mett. Taxonomic problems have primarily centred around the closely related species in section Azolla (Nayak and Singh, 1989; Stergianou and Fowler, 1990). A study of specimens by Evrard and Van Hove (2004) concluded that only two species exist in America and, according to the priority rule for nomenclature, they should be named A. filiculoides and A. cristata. 

A. filiculoides is a small aquatic heterosporous fern, rarely larger than 25 mm (O'Keeffe, 1986). The genus is unique in that it grows in association with a heterocystous cyanobacterium (blue-green alga), Anabaena azollae Strasburger (Nostocales: Nostocaceae), which is located in cavities in the dorsal leaf-lobes (Ashton and Walmsley, 1984). This symbiotic association is the only one known between a pteridophyte and a cyanobacterium (Ashton and Walmsley, 1984). The Azolla macrophyte consists of a main rhizome, which branches into secondary rhizomes. These all bear alternately arranged small leaves. Ventrally, unbranched adventitious roots hang down into the water from nodes. Nutrients are absorbed directly from the water by the roots. In very shallow water, however, the roots may touch the soil thus deriving nutrients from it (Wagner, 1997). Rao (1936) reported that in A. pinnata, once the roots attain a length of 40-50 mm they drop off. Root hairs found along the length of the root provide accommodation for a large number of protozoa, algae and soil particles (Rao, 1936).

According to Lumpkin and Plucknett (1980), A. filiculoides is native to the Rocky Mountain states of the western USA and Canada, through Central America and to most of South America. It has been introduced to Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean and Hawaii.

Further spread is likely as A. filiculoides continues to be sold in nurseries as a fish pond plant. In Africa and Europe, dispersal between countries will no doubt continue due to the movement of waterfowl, which can spread plant fragments between bodies of water. A. filiculoides will continue to invade countries where the presence of eutrophic waters, lack of natural enemies and an unregulated nursery trade will contribute to its status as a weed.

Natural Dispersal

A. filiculoides propagates both asexually and sexually. Both spores and plant fragments are dispersed long distances along river systems.

Vector Transmission

Waterfowl, amphibians and rodents are thought to spread small fragments of the plant that adhere to their bodies.

Agricultural Practices

A. filiculoides has been introduced in tropical regions worldwide as a green manure in rice cultivation.

Intentional Introduction

A. filiculoides was intentionally introduced into South Africa as a fish pond plant in 1948 (Jacot-Guillarmod, 1979) and by the early 1980s it was thought to have invaded every river system in South Africa. The plant was also deliberately introduced into south-east UK at the end of the nineteenth century as an ornamental plant (Janes, 1998a) and is now naturalised in numerous still and slow-flowing waters (Preston and Croft, 1997). There are also instances of research collections that have escaped from culture; A. filiculoides has been spread around the world as a model plant to study the Azolla-Anabaena symbiosis (Carrapiço, 2010)

A. filiculoides is not generally considered a weed of crops. It is commonly grown in conjunction with rice, as A. filiculoides has the ability to fix nitrogen via an endosymbiotic blue-green alga (Wagner, 1997), acting as a green manure in rice cultivation. However, in its introduced range, A. filiculoides may impact upon trout and other fish farming (McConnachie et al., 2003). It has also been recorded to affect the growth of Potamogeton crispus L. (Janes et al., 1996).

A. filiculoides is difficult to distinguish from A. microphylla, A. mexicana, A. caroliniana and A. cristata due to the variable morphology of the plant modified by interacting environmental influences (Ashton, 1982). Cytological studies are the most reliable method of separating these species (Stergianou and Fowler, 1990; Saunders and Fowler, 1992; 1993). Under field conditions, however, the most reliable measure of separating these species would be by means of their sporocarp structure (Svenson, 1944; Sweet and Hills, 1971). A. filiculoides: glochidia not septate, or rarely with 1 or 2 septae at the apex, microsporangia 35-100 in an indusium, massulae 4-6, megasporangia with raised, irregular hexagonal markings; A. caroliniana: glochidia not septate, microsporangia 8-40 in an indusium; A. mexicana: glochidia multi-septate, microsporangia usually with 4 massulae, megaspore pitted; A. microphylla: glochidia multi-septate, megaspore smooth. For comparisons including A. cristata, see Evrard and Van Hove (2004).

A. filiculoides in its native areas (South America and western North America) is a plant of slow flowing streams and rivers, ponds and lakes (Reed, 1962; Lumpkin and Plucknett, 1980; Ashton, 1982). Its native range is characterised by warm, tropical climates with humid summers and mild winters. It has commonly been utilised as an ornamental in fishponds and tanks and has spread from these foci, exhibiting a weedy phenology in nutrient enriched reservoirs and roadside canals (T. Center, Senior Researcher, Aquatic Weeds, United States Department of Agriculture, personal communication).

Genetics

The chromosome number of A. filiculoides is 2n=44 (Stergianou and Fowler, 1990). This number is shared by A. pinnata, A. caroliniana, A. microphylla and A. mexicana. Hybridization has been recorded with A. microphylla under laboratory conditions (van Cat et al., 1989), the artificial hybrid having the same chromosome number as its parents (Stergianou and Fowler, 1990). Hybrids were found to produce only microsporocarps, had intermediate stem lengths, and grew better than A. microphylla in the field (van Cat et al., 1989).

Physiology and Phenology

Megaspores are produced in spring and summer, and can overwinter and survive extreme desiccation (Lumpkin and Plucknett, 1982). Megaspores germinate on the mud surface at the bottom of a waterbody, often in very shallow (0.1-0.2 m) areas. Ashton (1982) conducted detailed studies regarding A. filiculoides spore germination. The first visible sign of germination is the upward displacement of the indusium cap, due to the expansion of the first leaf. Eventually, as the second and third leaves start pushing upwards, the indusium cap is lost and the new sporophyte floats to the water surface where further development takes place. The first root appears from the base of the first leaf, and at this point the remains of the old megaspore break away from the sporophyte. Continued development results in the rapid elongation of the rhizome and the production of further leaves and roots.

Reproductive Biology

A. filiculoides is able to undergo rapid vegetative reproduction throughout the year by the elongation and fragmentation of the small fronds, and under ideal conditions, the daily rate of increase can exceed 15% with the doubling time being every 4-5 days (Lumpkin and Plucknett, 1982). Under favourable environmental conditions, A. filiculoides undergoes sexual reproduction. Pairs of sporocarps are formed from a ventral lobe initial of a lateral branch. The sporocarps are of two types, male microsporocarps and female megasporocarps (Ashton, 1982; Wagner, 1997). There is usually a pair of either microsporocarps or megasporocarps, but one of each may be present (Moore, 1969). Microsporocarps are approximately 1.5 mm in diameter, containing 8-130 microsporangia (Wagner, 1997). Within each microsporangium there are 64 microspores, which are, in turn, aggregated into 3-10 massulae (Moore, 1969). Megasporocarps are approximately 0.5 mm in diameter, each producing a single megasporangium (Wagner, 1997). A single megaspore, which contains a small colony of Anabaena azollae is contained within the megasporangium (Wagner, 1997). Upon reaching maturity, both micro- and megasporocarps dehisce. Microsporangia release spongy masses of massulae into the water, which attach to megasporocarps via barbed, protruding appendages (glochidia) (Lumpkin and Plucknett, 1980). These entanglements usually sink to the bottom of a waterbody and, after a period of dormancy, the micro- and megaspores will germinate to form prothalli (Wagner, 1997). Ciliated, male gametes (antherozoids) develop in antheridia on the male thallus and female gametes (oospheres) develop in archegonia on the female thallus. After fertilization of the oospheres by the antherozoids, an embryo develops (Moore, 1969). Ashton (1982) found megaspore germination to be affected by desiccation (desiccation for >40 days greatly reduced germination); turbulence (severe turbulence lowered germination levels to a low 6%); photoperiod; light intensity; and pH (no germination occurred at pH levels <4.5 or >9.5).

Associations

A. filiculoides grows in association with the heterocystous cyanobacterium (blue-green alga) Anabaena azollae (Nostocales: Nostocaceae), within the dorsal leaf lobe cavities (Ashton and Walmsley, 1984). The alga has the ability to fix atmospheric nitrogen and is able to fulfil the nitrogen requirements of the fern making it successful in nitrogen-deficient waters (Ashton, 1982).

Environmental Requirements

A. filiculoides is an obligate aquatic plant, whose growth is phosphorous limited. Climatic requirements include suitably warm months for sporocarp development, adequate radiation and light intensity for vegetative growth, and adequate amounts of rainfall to prevent its aquatic habitat from drying up. This species of tropical origin is thought to have evolved a cold-tolerant strain since its introduction into Britain (Janes, 1998b) and South Africa (McConnachie, 2003). A. filiculoides may be able to survive temperatures as low as -10ºC before death occurs.
 

Host records from around the globe show that the genus Azolla is attacked by generalist herbivores and that very few specialist insect species have evolved on these plants (Hill, 1997). However, four beetle species, the weevils Stenopelmus rufinasus and S. brunneus and the two flea beetles Pseudolampsis guttata and P. darwinii, appear to have specialized on the genus Azolla (Richerson and Grigarick, 1967; Habeck, 1979; Hill, 1999) and were identified as potential biological control agents for A. filiculoides in South Africa (Hill, 1997). Following host range testing, Stenopelmus rufinasus was released in 1997 as a biocontrol of A. filiculoides in South Africa (McConnachie et al., 2004).

The economic impact of A. filiculoides in South Africa was examined by McConnachie et al. (2003). Thick mats on reservoirs and slow-moving waterbodies caused economic losses to water-users. Among those water-uses most seriously affected were farming (71%), recreational (24%), and municipal (5%). On average, A. filiculoides was found to cause on-site damages of US$589 per hectare per year.

In eutrophic water systems, A. filiculoides grows rapidly, easily outcompeting indigenous vegetation. Decaying root and leaf matter below a mat of A. filiculoides, and the lack of light penetration, creates an anaerobic environment. Not only can very little survive under such conditions, but the quality of drinking water is reduced, caused by bad odours, colour and turbidity (Hill, 1997). Cases have been reported where both livestock and game farmers have lost animals due to them refusing to drink from infested waterbodies or drowning as a result of mistaking the mat for solid ground. The weed also reportedly increases water loss through evapotranspiration and promotes the development of waterborne, water-based and water-related diseases (Hill, 1997).

Primarily, social impacts of A. filiculoides have centred around the reduction of useful water surface area for recreation (fishing, swimming and water skiing) and water transport.

Members of the genus Azolla are utilized throughout the world for a wide variety of purposes besides its widespread uses as an ornamental in fish ponds and tanks (Lumpkin and Plucknett, 1980; 1982). A. filiculoides is used as a green manure in rice paddies, mainly in Asia, as an inhibitor of weed growth in rice cultivation in China and Vietnam (Kröck and Alkämper, 1991), and as an alternative high protein fodder for cattle, swine, poultry and fish, and possibly as an alternative food source for humans, again, mainly in Asia. It has also been used as a nitrate-rich compost which potentially increases soil organic nitrogen levels and cation exchange capacity. It is used for purification of water, removal of heavy metals (Sanyahumbi et al., 1998) and removal of nitrogen and phoshorous from wastewater (Forni et al., 2001). It has also been used variously as an ingredient in soap production, a cure for sore throats and as a control for mosquitoes in southern India as complete mats disrupt larval development (Rajendran and Reuben, 1991).

Cultural Control

Even though A. filiculoides has been used as fodder for pigs, poultry and cattle (Lumpkin and Plucknett, 1982), control through grazing may not be achieved because of its aquatic habit. However, A. filiculoides may actually be a preferred forage for herbivorous fish, even over most other aquatic weeds (Edwards, 1974).

Mechanical Control

Small infestations of the weed in accessible areas may be removed using rakes and fine meshed nets (Hill, 1997). The disadvantage of mechanical control, however, is that under ideal conditions, the weed can double itself every 4-5 days (Lumpkin and Plucknett, 1982). Hence a concerted effort would be required just to keep up with the daily production of even a small infestation, and even if eradication were achieved, re-establishment from spores resident in the waterbody substrate would be inevitable. Ashton (1992) noted that A. filiculoides was susceptible to fragmentation from physical disturbance, and that the fragments were highly sensitive to high light intensity and were killed by direct sunlight. He thus suggested the use of a mechanical agitator or stirrer to provide enough turbulence to break up the plants. The cost, however, of such an approach (even on a small scale) would be prohibitive.

Chemical Control

Chemicals proposed to control A. filiculoides include glyphosate (Steyn et al., 1979; Ashton, 1992), paraquat and diquat (Axelsen and Julien, 1988), and kerosene mixed with a surfactant (Diatloff and Lee, 1979). However, paraquat is now banned in the EU, Switzerland and a number of other countries. Diquat use in the EU is restricted to terrestrial treatments and can no longer be authorised for aquatic weed control. Glyphosate is toxic to fish and algae and the water cannot be used for irrigation or stock until the herbicide has broken down.

Biological Control

South Africa is the only country that has initiated a classical biological control programme against A. filiculoides. Four insect species were identified as potential biological candidates - all frond-feeding beetles: Pseudolampsis guttata Leconte (Chrysomelidae), P. darwinii Sherer, Stenopelmus rufinasus Gyllenhal (Curculionidae) and S. brunneus Hustache . All species do extensive damage to the plants in the country of origin (Hill, 1997). The biology and host range of P. guttata was investigated by Hill (2002), and although it was found to be fairly damaging, the weevil S. rufinasus was identified as the most suitable of the four for release in South Africa, and was imported into quarantine for host-specificity screening. The weevil was released in 1997 and results have been dramatic, causing local extinction of A. filiculoides at the sites where it was released (McConnachie et al., 2004). The surface area of weed controlled totalled 203.5 ha and infested sites were controlled in approximately seven months on average (in a range of 3-11 months). Five years after the release of the weevil, A. filiculoides no longer poses a threat to aquatic ecosystems in South Africa and its effects on the utilization of water resources have been significantly reduced (McConnachie et al., 2003). S. rufinasus has been released in Mozambique and Zimbabwe with material provided by South Africa, has established, and is proving a successful biocontrol agent in these countries also (Cilliers et al., 2003).

S. rufinasus has been present in the UK since it was first reported there by Janson (1921) and was probably brought into Europe with the plant. Since then it has been recorded in Ireland, France, Belgium, the Netherlands and Spain (Pratt et al., 2013). This agent has been used as in an augmentive manner in England and Ireland and is under consideration for mass-rearing and releasing in the Netherlands, France and Belgium (Pratt et al., 2013).

Integrated Control

Because of the success of the biological control of A. filiculoides, there has been no need to develop an integrated approach.

Further research on the taxonomy of the genus Azolla is on-going (Madeira et al., 2013, 2016).