AQUATIC HORTICULTURE		BY KAREN RANDALL

Liverworts and Mosses — Our Most Primitive Aquarium Plants

Algae are among the most primitive organisms in which a nucleus is present. The first algae were free-floating, single-celled organisms, but with the passage of time, multi-celled, attached filamentous forms developed. These green algae were the ancestors of our terrestrial plants. As plants developed differentiated body parts, it gave them the ability to survive on land as well as in the water. Most of our modern aquatic plants have terrestrial ancestors. They have retained many of the anatomical structures that allowed them to adapt to life out of the water and many have continued to use these adaptations to lead an amphibious existence.

The next plants to evolve after the algae were the liverworts and mosses (phylum Bryophyta) and the horsetails, club mosses and ferns (phylum Pteridophyta). This month we will focus on the liverworts and mosses.

KAREN RANDALL

Riccia fluitans, the plant in the lower left of this photo, is one of only a handful of bryophytes we use in the aquarium. It can be grown either on the bottom attached to a stone or allowed to float on the surface.

Phylum Bryophyta

Although members of this phylum resemble vascular plants in a number of ways, one important difference is the way their cells divide. This is different than all but a very few forms of algae. Like vascular plants, the sporophytes of mosses have stoma some mosses also have a cuticle on the portions above the ground.

The first bryophytes appear in fossils from the Devonian era, about 350 million years ago. They were very similar to the species that survive today. Because the first known fossil evidence of vascular plants dates back 400 million years (the Silurian era) it is unlikely that vascular plants developed from bryophytes. Most likely, both bryophytes and vascular plants had a single common ancestor that would have been a complex, multicelled green alga and probably moved toward terrestrial life about 500 million years ago (Ordovician period). Because the function of stoma is gas exchange, and stoma are common to both bryophytes and vascular plants, the common ancestor of both types of plants must have evolved as the algae adapted to life on land, but before the bryophytes and vascular plants diverged.

Glossary of Terms Cuticle — A non-cellular, waxy material that protects the epidermis of the plant from water loss. Gamete — A mature, functional haploid cell. The nuclei of two gametes fuse (in fertilization) to form a diploid zygote that will then develop into a new individual. Gametophyte — An individual of the haploid gamete-producing generation, typically produced from a haploid spore. Protonema — An early stage in the development of a bryophyte. The word literally means “first thread.” Rhizoid — A hair-like structure of a single or several cells that serves as an anchor for the plant. Sporophyte — Spore-producing, diploid (2n) phase in the life of a plant having alternating generations. It is formed by the union of sexual cells produced by the gametophyte. Stoma — Pore in the epidermis of plants through which gaseous exchange takes place. Vascular plants — Complex plants that contain vessels for the transport of fluids through the plant.

If present at all, leaves of bryophytes are of simple construction. They may have a “midrib” that is used to transport water and dissolved minerals through the leaf. The gametophyte stage of a bryophyte is usually attached to the substrate by rhizoids, which are elongated cells or strings of cells. In most cases, these serve as an anchor for the plants, while the plants absorb water and nutrients directly through the leaves and plant axis. However, there are exceptions among the mosses. In some mosses the rhizoids also draw water up into the plant via capillary action. In these cases, the plants also have a cuticle on the leaves that helps to retain moisture.

The main difference between bryophytes and vascular plants is the relative development of the two alternating generations of sporophytes and gametophytes. The sporophyte is the conspicuous generation among vascular plants, while the gametophyte is much reduced. In bryophytes, the conspicuous form is the gametophyte. The sporophyte is attached to the gametophyte and is at least to some extent, dependent on it for nutrition.

Bryophytes can and do reproduce sexually. When the spores of bryophytes land on a suitable substrate, they germinate. The resulting “sprout” is called the protonema, which means “first thread.” Gametophytes of the mature form develop from the protonema,. This sexual reproduction is rarely encountered under aquarium conditions, although it may happen in a bog-type terrarium.

Bryophytes also reproduce asexually. In one method of asexual reproduction, a single protonema can give rise to a number of small individual plants. In both mosses and liverworts a small piece of a gametophyte can also branch to produce individual plantlets. At this point, the older parts disintegrate. Most aquarists are familiar with this form of reproduction. Even a small fragment of Riccia or Java moss (Vesicularia dubyana) can reproduce to populate an entire tank.

Liverworts (class Hepaticea)

Both plants are found world wide and are extremely hardy. While they can be ornamental in the aquarium and are also useful as fry cover and building material for bubble-nesting fishes, they can also be a nuisance because of their fast reproduction. Most aquarists simply allow Riccia to float on the surface of the tank.

Recently, following the release of the Takashi Amano books (1994a, 1994b) it has become popular to grow Riccia as a ground cover. Obviously, R. rhenana is a better candidate for this treatment, but R. fluitans can be used as well. It is easiest to allow the Riccia to form a thick mat on the surface of the water before attempting to attach it to stones for use on the bottom. The Riccia mat can be made more stable by allowing it to grow mixed with either Java moss or bladderwort (Utricularia). These plants with longer threads will hold the smaller pieces of Riccia together. Mats of Riccia will often form on the bottom in shallow, still water in a small natural or man-made pond. Often, these clumps will have enough gravel or soil attached to the undersides that they will drop to the bottom on their own in the aquarium.

Once a mat of Riccia (with or without other plants) has formed, it can be attached to a stone. It can be tied in place using thread or fishing line wound around the stone in many directions. Another even simpler method suggested by the folks at Tropica is to buy fine-mesh ladies’ hairnets at the pharmacy. These can be cut in half and each half used to make a “Riccia rock.”

Riccia used as a ground cover in the aquarium is not trouble-free. It must be pruned regularly or it will grow through its restraints. At this point, the inner plants will not get enough light and will begin to deteriorate. The Riccia will then peel away from the stone and pop to the surface. To avoid this situation, make sure you give your Riccia rocks regular hair cuts! Also, remember that although Riccia will happily grow on the surface in even moderately lighted tanks, it needs bright light to stay compact and grow well on the bottom of the tank.

Riccia can also be grown quite easily as a terrestrial ground cover in a very humid, moist-bottom terrarium. I find it very attractive around the bases of the larger plants in my carnivorous plant terrarium.

Mosses (class Musci)

Interestingly, both of the moss species commonly used in aquaria, Vesicularia dubyana and Fontinalis antipyretica exhibit both growth patterns. They grow into short, upright “cushions” when grown emersed, but the aquatic forms are many branched and feathery.

Mosses are capable of sexual and asexual reproduction, but in the aquarium, usually only asexual reproduction will be seen. In cases where the plants are allowed to grow emersed, you may see the development of the elongated sporophytes that, if allowed to mature, will release spores to start the next generation of gametophytes.

As with the liverworts, all that is needed for asexual reproduction is a small piece of gametophyte. From this small piece, an entire new individual can develop. Both of our aquarium species are tolerant of very modest lighting. Java moss (V. dubyana) is popular with killifish breeders, specifically because it does extremely will in small, dimly lit killie containers. Java moss is tolerant of a wide range of temperatures and will do well at temperatures from the low 70s (degrees Fahrenheit) up through the mid 80s. Fontinalis antipyretica, on the other hand, prefers cooler temperatures. It will do best with water no warmer than the low to mid 70s.

Both plants will grow well if they are left floating loose in the aquarium, but are more decorative if they are attached to some sort of structure, such as a rock or driftwood. It can be tied in place like Riccia, but a very simple way of attaching it to driftwood is to use a staple gun. The staples quickly rust and are all but invisible against the dark wood. When attached to a substrate in this way, both Vesicularia and Fontinalis will develop beautiful, long lacy fronds that sway in the water currents. They are often used as a spawning media for plant-spawning fishes in the tank.

REFERENCES Amano, T. 1994a. Nature Aquarium World. T.F. H. Pubs., Neptune City, NJ. Pp. 183. (This is the English translation. ISBN: 0-7938-0089-7.) Amano, T. 1994b. Nature Aquarium World. Book 2. Marine Planning Co., Ltd., Tokyo. Pp. 300. (Also in English. ISBN: 4-89512-150-X.).