Water Ferns
Order Salviniales
The "Water Ferns" are a monophyletic group of true ferns, that do not have a a stereotypical "fern frond" appearance. They have evolved aquatic emergent forms (Marsilea, Pilularia, and Regnellidium) and floating forms (Azolla and Salvinia). This is the only group of living ferns that is heterosporous, with both megaspores and microspores. Azolla is known to form a symbiotic relationship with nitrogen-fixing bacteria.
Around 50 million years ago, during the peak warming of the Earth after the demise of the dinosaurs, huge areas of the Arctic Ocean were covered with Azolla. The photosynthesis and death of these plants is thought to have contributed to the steady decrease in carbon dioxide levels on Earth and global cooling, ushering in the glaciations in the past tens of thousands of years.
Above (clockwise): Salvinia (top left), Azolla (top right), Pilularia (bottom right), and Marsilea (bottom left)
Ecology & Form
Growing emergent in damp soil, sometimes submerged, or floating forms
Azolla is known to form a symbiotic relationship with the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen
This feature has been advantageous to rice farmers in China for thousands of years
Sporophyte (=spore-bearing phase)
Vegetative features
Stems
Some forms possess rhizome roots in saturated or submerged soil
Floating forms possess thin stem that connects leaflets
Leaves
Few or no leaflets
Leaf form is atypical for ferns
Roots
Marsilea, Pilularia, and Regnellidium have adventitious roots that emerge from the rhizome to anchor the plant in saturated soils
Since Azolla and Salvinia are floating, they exhibit roots that hang in the water column
Reproductive features
Heterosporous
Sporangia are found inside a sporocarp, a water resistant structures that contain mega- and micro-sporangia
The sporocarp is hard and pit-like, which opens after a few years of wetting and drying
When the sporocarp opens, a gelatinous frond, called the sorophore, emerges with sporangia
Gametophyte (=gamete-bearing phase)
Female and male gametophytes are endosporic, never emerging from the spore wall.
Each megasporangium only produces a single megaspore, similar to seed plants
Below: Marsilea sporocarps
Above: Sporocarp of Marsilea "germinating" showing gelatinous frond with sporangia
Diversity
The water ferns are represented by two families (below)
Marsileaceae
Marsilea, Pilularia, and Regnellidium
Salviniaceae
Azolla and Salvinia
Geologic Range
Jurassic - present
Marsileaceae is first recorded in the Middle to Late Jurassic, and diversify in the Aptian–Albian of the Early Cretaceous (De Benedetti et al. 2021)
Diversity decreased during the Cenomanian–Santonian
Salviniaceae is first recorded in the Early Cretaceous, and diversify during the Campanian–Maastrichtian of the Late Cretaceous (De Benedetti et al. 2021)
Decrease in diversity for all Salviniales occurred during the Cretaceous–Paleogene mass extinction
From the Paleocene onwards there is a steady decline in the fossil record of the group, with most remains belonging to the extant genera
Above: Marsilea vegetative sporophyte, exhibiting four leaflets
Below: Regnellidium sporophyte, exhibiting two leaflets
Above: Pilularia sporophyte, exhibiting photosynthetic shoots without leaflets
Above: Salvinia, the velvetleaf fern
Below: Azolla, the mosquito fern
Additional Resources
Azolla could help reduce food insecurity (Phys.org 19Feb2024)
└ Winstead et al. (2024) Nutritional properties of raw and cooked Azolla caroliniana Willd., an aquatic wild edible plant
Scientists explore oil clean-up properties of aquatic ferns (EurekAlert 2016)
Azolla: the fern that cooled the planet - (Scientific American synopsis 2014)