Süßwassertang: A Fern Disguised as a Liverwort

Photo by Rǫgn licensed under CC BY-SA 4.0

If you enjoy planted aquariums, you may have crossed paths with a peculiar little plant called Süßwassertang. It can be propagated by breaking off tiny pieces, which eventually grow into a tangled carpet of tiny green thalli. One could be excused for thinking that Süßwassertang was some sort of liverwort and indeed, for quite some time was marketed as such. That all changed in 2009 when it was revealed that this was not a liverwort at all but rather the gametophyte of a fern.

Despite its German name, Süßwassertang appears to have originated in tropical parts of Africa and Asia. It is surprisingly hard to find out any information about this plant outside of its use in the aquarium trade. The name Süßwassertang translates to “freshwater seaweed” and indeed, that is exactly what it looks like. The fact that this is actually the gametophyte of a fern may seem startling at first but when you consider what they must deal with in nature, the situation makes a bit more sense.

A Süßwassertang gametophyte. B An antheridium, showing a cap cell (cc), ring cell (rc), and basal cell (bc). Bar: 20 µm. C Developing lateral branches with rhizoids (arrowhead) and meristems (m) Bar: 0.2 mm. D Ribbon-like, branched gametophyte (g) of L. spectabilis bearing a young sporophyte (sp) Bar: 1 cm

Fern gametophytes are surprisingly hardy considering their small size and delicate appearance. They are amazing in their ability to tolerate harsh conditions like drought and freezing temperatures. Because of this, fern gametophytes sometimes establish themselves in places that would be unfavorable for their sporophyte generation. For some, this means never completing their lifecycle. Others, however, seem to have overcome the issue by remaining in their gametophyte stage forever. Though no sexual reproduction occurs for these permanent gametophytes, they nonetheless persist and reproduce by breaking off tiny pieces, which grow into new colonies.

The sporophyte of a related species, Lomariopsis marginata, demonstrating the usual epiphytic habit of this genus. Photo by Alex Popovkin, Bahia, Brazil licensed under CC BY-NC-SA 2.0

This appears to be the case for Süßwassertang. Amazingly, despite a few attempts, no sporophytes have ever been coaxed from any gametophyte. It would appear that this is yet another species that has given up its sporophyte phase for an entirely vegetative habit. What is most remarkable is what the molecular work says about Süßwassertang taxonomically. It appears that this plant its nestled into a group of epiphytic ferns in the genus Lomariopsis. How this species evolved from vine-like ferns living in trees to an asexual colony of aquatic gametophytes is anyones’ guess but it is an incredible jump to say the least.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Ferns Afloat

Photo by Le.Loup.Gris licensed under CC BY-SA 3.0

My introduction to the genus Salvinia was as an oddball aquarium plant floating in a display tank at the local pet store. I knew nothing about plants at the time but I found it to be rather charming nonetheless. Every time the green raft of leaves floated under the filter outlet, water droplets would bead off them like water off of a ducks back. Even more attractive were the upside down forest of "roots" which were actively sheltering a bunch of baby guppies. 

I grew some Salvinia for a few years before my interest in maintaining aquariums faded. I had forgotten about them for quite some time. Much later as I was diving into the wild world of botany, I started revisiting some of the plants that I had grown in various aquariums to learn more about them. It wasn't long before the memory of Salvinia returned. A quick search revealed something astonishing. Salvinia are not flowering plants. They are ferns! 

The genus Salvinia is wide spread. They can be found growing naturally throughout North, Central, and South America, the West Indies, Europe, Africa, and Madagascar. Sadly, because of their popularity as aquarium and pond plants, a few species have become extremely aggressive invaders in many water ways. More on that in a bit. 

Salvinia comprises roughly 12 different species. Of these, at least 4 are suspected to be naturally occurring hybrids. As you have probably already gathered, these ferns live out their entire lives as floating aquatic plants. Their most obvious feature are the pairs of fuzzy green leaves borne on tiny branching stems. These leaves are covered in trichomes that repel water, thus keeping them dry despite their aquatic habit. 

These are not roots! Photo by Carassiuslike licensed under CC BY-SA 4.0

Less obvious are the other types of leaves these ferns produce. What looks like roots dangling below the water's surface are actually highly specialized, finely dissected leaves! I was super shocked to learn this and to be honest, it makes me appreciate these odd little ferns even more. It is on those underwater leaves that the spores are produced. Specialized structures called sporocarps form like tiny nodules on the tips of the leaf hairs.

Sporocarps come in two sizes, each producing its own kind of spore. Large sporocarps produce megaspores while the smaller sporocarps produce microspores. This reproductive strategy is called heterospory. Microspores germinate into gametophytes containing male sex organs or "antheridia," whereas the megaspores develop into gametophytes containing female sex organs or "archegonia." 

As I mentioned above, some species of Salvinia have become aggressive invaders, especially in tropical and sub-tropical water ways. Original introductions were likely via plants released from aquariums and ponds but their small spores and vegetative growth habit means new introductions occur all too easily. Left unchecked, invasive Salvinia can form impenetrable mats that completely cover entire bodies of water and can be upwards of 2 feet thick!

Sporocarps galore! Photo by Kenraiz licensed under CC BY-SA 4.0

Lots of work has been done to find a cost effective way to control invasive Salvinia populations. A tiny weevil known scientifically as Cyrtobagous singularis has been used with great success in places like Australia. Still, the best way to fight invasive species is to prevent them from spreading into new areas. Check your boots, check your boats, and never ever dump your aquarium or pond plants into local water ways. Provided you pay attention, Salvinia are rather fascinating plants that really break the mold as far as most ferns are concerned. 

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

Floating Ferns

Photo by Jon Sullivan licensed under CC BY-NC 2.0

Not every tiny plant you see growing on the surface of ponds are duckweeds. Sometimes they are Azolla. Believe it or not, these are tiny, floating ferns! The genus Azolla is comprised of about 7 to 11 different species, all of which are aquatic. Despite being quite small they nonetheless exert a massive influence wherever they grow. 

Like all ferns, Azolla reproduce via spores. Unlike more familiar ferns, however, sexual reproduction in Azolla consists of two markedly different types of spores. When conditions are right, little structures called "sporocarps" are formed underneath the branches. These produce one of two types of sporangia. Male sporangia are small and are often referred to as microspores whereas female sporangia are, relatively speaking, quite large and are referred to as megaspores. The resulting gametophytes develop within and never truly leave their respective spores. Instead, male gameotphytes release motile sperm into the water column and female gametophytes peak out of the megaspore to intercept them. Thus, fertilization is achieved. 

Photo by Miguel Pérez licensed under CC BY-SA 2.0

Azolla are fast growing plants. Via asexual reproduction, these little floating ferns can double their biomass every 3 to 10 days. That is a lot of plant matter in a short amount of time. As such, entire water bodies quickly become smothered by a fuzzy-looking carpet. Depending on the species and the environmental conditions, the color of this carpet can range from deep green to nearly burgundy. They are able to float because of their overlapping scale-like leaves, which trap air. Below each plant hangs a set of roots. The roots themselves form a symbiotic relationship with a type of cyanobacterium, which fixes atmospheric nitrogen. Couple with their astronomic growth rate, this means that colonies of Azolla quickly reach epic proportions.

In fact, they can grow so fast that Azolla may have played a serious role in a massive global cooling event that occurred some 50 million years ago. During that time, Earth was much warmer than it is now. Global temperatures were so warm that tropical species such as palms grew all the way into the Arctic. There is fossil evidence that massive blooms of Azolla may have occurred in the Arctic Ocean during this time, which was a lot less saline than it is now.

Everything red in this picture is Azolla. Photo by Jon. D. Anderson licensed under CC BY-NC-ND 2.0

Though plenty of other factors undoubtedly played a role, it is believed that Azolla blooms would have been so large that they would have drawn down CO2 levels considerably over thousands of years. As these blooms died they sank to the sea floor, bringing with them all of the carbon they had locked up in their cells. In part, this may have led to a massive drop in atmospheric CO2 levels and led to a subsequent cooling period. Evidence for this is tantalizing, so much so that some researchers have taken to calling this "The Azolla Event." However, this is far from a smoking gun. Regardless, it is an important reminder than really big things often come in very small packages.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]