Eelgrass Sex is Strange

Photo by Fredlyfish4 licensed by CC BY-SA 4.0

Pollination may seem like a strange thing to us humans. Whereas we only require two of us to accomplish reproduction, plants have to utilize a third party. The most familiar cases include insects like bees and butterflies. Unique examples include birds, bats, and even lizards. Many plants forego the need of an animal and instead rely on wind to broadcast copious amounts of pollen into the air in hopes that it will randomly bump into a receptive female organ.

This has worked very well for terrestrial plants but what about their aquatic relatives? Water proves to be quite an obstacle for the methods mentioned above. Some species get around this by thrusting their flowers above the surface but others don't bother. One genus in particular has evolved a truly novel way of achieving sexual reproduction without having to leave its aquatic environment in any way.

Photo by eyeweed licensed by CC BY-NC-ND 2.0

Meet the Vallisnerias. Commonly referred to as tape or eelgrasses, this genus of aquatic plants has been made famous the world over by their use in the aquarium trade. In the wild they grow submerged with their long, grass-like leaves dancing up into the water column. Where they are native, eelgrasses function as an important component of aquatic ecology. Everything from fish and crustaceans all the way up to manatees utilize tape grass beds for both food and shelter. Eelgrasses stabilize stream beds and shorelines and even act as water filters.

All this is quite nice but, to me, the most interesting aspect of Vallisneria ecology is their reproductive strategy. Whereas they will reproduce vegetatively by throwing out runners, it is their method of sexual reproduction that boggles the mind. Vallisneria are dioecious, meaning individual plants produce either male or female flowers. The female flowers are borne on long stalks that reach up to the water surface. Once there they stop growing and start waiting. Because of their positioning, water tension causes a slight depression around the flowers at the surface. The depression resembles a little dimple with a tiny white flower in the center.

A female Vallisneria flower. Photo by eyeweed licensed by CC BY-NC-ND 2.0

Male Vallisneria flowers floating on the water surface. Photo by eyeweed licensed by CC BY-NC-ND 2.0

Male flowers are very different. Much smaller than the female flowers, a single inflorescence can contain thousands of individual male organs. As they mature underwater, the male flowers break off from the inflorescence and float to the surface. Similar to wind pollinated terrestrial plants, Vallisneria use water currents to disperse their pollen. Once at the surface, the tiny male flowers float around like little pollen-filled rafts.

If a male flower floats near the dimple created by a female flower, it will slide down into the funnel-like depression where it will contact with the female flowers. This is how pollination is achieved. Once pollinated, hormonal changes signal the stem of the female flower to begin to coil up like a spring, drawing the developing seeds safely underwater where they will mature. Eventually hundreds of seeds are released into the water currents.

After pollination, the stem of the female flower coils up, drawing the ripening ovaries safely underwater. Photo by Peter M. Dziuk [source]

The Vallisneria are incredible aquatic plants. Their bizarre reproductive strategy has ensured that these plants never really have to leave the water. The fact that they can also reproduce vegetatively means that many species are very successful plants. In fact, some species have become noxious invasive weeds where they have been introduced far outside of their native range. If you own these plants in any way, do take the necessary measures to ensure that they never have the chance to become invasive.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1]

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]