There exists a sad retail phenomenon in the aquarium trade, of retailers mis-selling non-aquatic plants as though they are suitable for permanent aquarium use, claiming or implying that they can grow entirely and permanently submerged. Usually the plant in question, at least in my experience, is a variegated cultivar of the familiar spider plant; but there are certainly other species involved over the years. Needless to say that although many plants can be grown either underwater or on wet soil, the sorts of plants I am grumbling about, are emphatically unsuitable for growing as underwater plants.
Lately, I have seen two interesting spike-mosses incorrectly traded as though they can grow submerged; these are Selaginella uncinata and S. wildenowii - two remarkably blue hued, iridescent species of Asia, that are adapted to grow in the low lighting conditions of shaded forest understories. The blue iridescence of these species is not common to all of the spike-mosses, and it fades if the plant is exposed to too much strong sunlight, so they are best appreciated when cared for in low light jungle terrariums.
Botanists consider the shininess of these blue Selaginella species - and of some other, unrelated plants from similarly shaded habitats - as quite a scientific mystery. Nonetheless, the trait is definitely connected to the relatively low light available to them where they can grow. Because both species are associated with damp soil and humid air in the wild, they actually are well suited to life in aquaterrariums or paludariums - but in the soil, not in the water.
S. uncinata is known as the blue, spring blue, or peacock spike-moss, or peacock moss. It is assumed to be indigenous from northeast India to Japan, but human activities have spread this species as far away as Java, Louisiana, and Paraguay. This is a wiry herb that forms mats with its long, creeping stems, up to 75 centimeters or 30 inches. Whereas the related S. wildenowii has a stouter, vine- or shrub-like growth habit, and is more prone to climbing or hanging; it can grow longer than 300 or 400 centimeters, or 120 to 160 inches.
S. willdenowii is found from the Himalayan region to Indonesia and the Philippines; human activity has led to its further colonisation, of regions in North and South America, and of islands in the Caribbean region. It is also present in Queensland, Australia. This species is known as the vine spike-moss, and as Wildenow's spike-moss, and it is also known as the peacock fern, owing to its deceptively frondose appearance. But whatever we call them, the spike-mosses are neither ferns nor mosses.
Spike-mosses are something deeply ancient, and their fossil remains - usually described as the form genus Selaginellites - first appear in the Early Carboniferous. Since those days, they have existed much as they do today, since so long ago that our own ancestors were barely distinguishable from ancestral amphibians - that is, if we were already distinct from them at that time. In spite of this extreme antiquity, spike-mosses are usually regarded as so similar to one another, that they are all classified into the single genus, Selaginella.
These plants are most common in the humid tropics, but there are also species of Selaginella in some deserts, and others in alpine and high latitude tundras. The novelty 'resurrection plant', for example, is a Selaginella species from the Chihuahuan desert, but it is not a suitable plant for damp aquaterrariums. Curiously I am not aware of a single species of Selaginella, that is known to grow underwater in nature; but people have still been selling some of these plants - wrongly - as though they grow well underwater.
But what exactly is a spike-moss, anyway, if it is neither a fern, nor a moss? Spike-mosses are vascular plants, like the ferns and the seed-producing plants, but unlike the mosses. Vascular plants are a natural group of plants, that share woody internal tissues, that enable them to have their namesake vascular systems. Although other plants may possess vascular systems, they lack this potential for its woody support. Vascular plants also share true roots, which mosses do not have.
However the spike-mosses and their kin, do not have true leaves, as do the ferns or seed-producing plants. As such an ancient clade, spike-mosses and their kin appeared before leaves had properly evolved. Spike-mosses are related to the quillworts of the genus Isoetes; these are yet more 'living fossil' plants, in this case since the Early Triassic, which was before the first dinosaurs and mammals, though these rocks are not as ancient as the first Selaginella fossils. More distantly these plants are related to the true club-mosses.
All of these plants may, broadly, be referred to as club-mosses, or as lycopods, or lycopsids; this is justified because certain fossil plants belonging to the group, are still labelled club-mosses by paleontologists; and this is because the club-mosses are archetypal lycopods. (Very ancient lycopods are preserved in Silurian rocks.) Nowadays all of these plants are short, but in the past they also included more visually prominent members - the giant club-mosses or scale-trees of the vast, Carboniferous swamp forests.
In any case, both S. uncinata and S. wildenowii are two amazing plants from humid forest floors, and they appreciate the humidity and warmth of a heated jungle terrarium, as long as the lighting is not too sttong. Both of these spike-mosses grow only in damp, shady habitats, to which their iridescence is an adaptation. But these are definitely not aquatic plants, although they can flourish in the 'dry' part of an aquaterrarium or paludarium.