The most distinctive morphological character of this species is its very broad costa, as observed by A.J. Grout in 1933 and later confirmed by H. Crum in 1969, J.T. Wynns and others in 2009 and M.S. Ignatov in 2015. South Llano Springs moss resembles Hygroamblystegium noterophilum, which is another relatively large aquatic moss with broad costa, but its linear-flexuose upper median leaf cells, short apical cells and serrulate leaf margins distinguish it from the latter species, as documented by H. Crum in 1969.

At the Edwards County spring, water flows from a large number of cavities at the base of a limestone bluff which is at the edge of the South Llano River. When last observed, the moss grew in the spring outflow, partially submerged in both shaded and unshaded niches and within a 33 feet (10 meters) zone, between the springs and the river below, as documented by R. Wyatt and A. Stoneburner in 1980. The water temperature was consistently 70.7° F (21.5° C) in June, and the pH ranged from 7.0 to 7.2. Associated vascular plant species included maidenhair fern (Adiantum capillus-veneris), southern shield fern (Thelypteris kunthii), water cress (Nasturtium oficinale), as well as members of the mint family (Lamiaceae) and composite family (Asteraceae). Associated moss species included Hygroamblystegium tenax and Eucladium verticillatum - the former species occurred in more exposed, less submerged situations, and the latter, in more protected areas like the small caves in which springs originate. Hyalella azteca, a common, widespread amphipod, occurred at very high densities within the moss mats.

Our knowledge of the Kimble County habitat is limited to two specimen labels, which state: On dam (Redfearn 27198) and Limestone, edge of creek (Redfearn 27208). Both labels state, “Rocky limestone slopes and narrow canyons with flowing water. Ca. one mile south of Telegraph.” This is near the confluence of a large, spring-fed tributary, Paint Creek, and the South Llano River, and could be on either stream or both. The individual plants at both sites are attached to submerged limestone and form partially submerged mats that are constantly wet. Thus, it is likely that individuals of this species must be continuously immersed in spring water. It is also possible that they may be able to endure brief dry periods, but we have no evidence to support this. Our 2017 South Llano River Aquatic Moss Survey documented that rocks and plants that are immersed in the upper South Llano River quickly become encrusted with travertine- or tufa-like mineral deposits, to an unusual degree not seen in most springs in the Edwards-Trinity Aquifer. Thus, it is possible that high mineral concentrations, or the precipitation of minerals from solution, could be requirements for the establishment and growth of individuals of this species.

This description is adapted from A.J. Grout’s observations in 1933, as well as H. Crum and L.E. Anderson’s in 1979, R. Wyatt and A. Stoneburner’s in 1980, J.T. Wynns and others in 2009 and M.S. Ignatov’s in 2015. This species is an aquatic moss that grows on submerged or partially submerged rocks. The deep, loosely interwoven mats are blue-green to blackish-brown when shaded and yellow-green when exposed to full sun. The curving, rigid stems of gametophytes reach 1.2 to 5.5 inches (3 to 14 centimeters) in length, with irregular branches up to 0.4 inches (10 millimeters) long. Leaves are loosely erect and spreading when moist, dark green to brownish, 0.02 to 0.03 inches (0.4 to 0.8 millimeters) wide by 0.04 to 0.07 inches (0.9 to 1.8 millimeters) long, oblong-lanceolate to oblong-ovate, bluntly acute, obtuse, rounded, or notched at the apex. The margins serrulate in the distal half and the lamina, with multiple-layered streaks up to five cells thick. Pseudoparaphyllia are foliose, broadly rounded, with entire to irregularly serrulate margins. The strong, thick 0.006 to 0.007 inches (160 to 200 microns) costa is about one-third the width of the leaf base, elliptical in cross-section, often laterally spurred, and may terminate just before reaching the apex or extend into a cuspidate point. Up to four perigonia per centimeter occur in the axils of vegetative leaves; perigonial leaves are 0.02 to 0.04 inches (500 to 900 microns) long, ovate, acuminate, with the costa lacking or extending only to the midpoint. From eight to 12 antheridia arise from the base of perigonial leaves. Antheridia are about 0.01 inches (300 microns) long and are surrounded by filiform paraphyses seven to eight cells long. Archegonia and sporophytes have not been observed this species.

Life cycle information is adapted from observations by B. Crandall-Stotler and S. Bartholomew-Began in 2007. Moss spores are haploid; that is, they possess a single set of chromosomes. Spores germinate to produce the green, photosynthetic moss plant that is familiar to most people, which is also haploid. Mosses may reproduce asexually through budding and branching of new stems, or by the dispersal and establishment of various types of tissue fragments. Mosses that are capable of sexual reproduction produce specialized sexual structures called gametangia at the ends of stems or modified branches. The male inflorescence, or androecium, consists of numerous antheridia surrounded by a cluster of modified leaves, or perigonium. The antheridia produce biflagellate sperm through mitotic divisions. The female inflorescence, or gynoecium, consists of a cluster of modified leaves, or perichaetium, surrounding groups of long-necked, stalked structures called archegonia. Each archegonium produces a single egg cell through mitosis. Mature sperm swim though a film or droplets of water to reach an archegonium and fuse with the egg. The fertilized egg then produces a diploid sporophyte, which consists of a long stalk and a terminal capsule. Meiosis occurs within the capsule of the sporophyte to create haploid spores that are released to the environment.

Although specialized vegetative reproduction is absent in the family Brachytheciaceae, as noted by M.S. Ignatov in 2015, asexual reproduction may occur through the establishment of fragments of the protonema or stem. In 1980, R. Wyatt and A. Stoneburner observed antheridia formation when plants from Edwards County were propagated after four months, but no archegonia formed even after one additional year. Additionally, neither antheridia, archegonia, nor sporophytes have been observed in the wild population. This population may be a clone of a single individual, or only male individuals, and may be incapable of sexual reproduction. Since neither female plants nor sporophytes of this presumed dioicous species have been observed, we have no evidence that sexual reproduction occurs and no indication of the actual number of genetically distinct individuals present.

The only information we have on the life span of this species is that mats survived for at least 16 months in propagation, as documented by R. Wyatt and A. Stoneburner in 1980. As with other clonal organisms, the concept of moss life span is different from the life span of organisms with discrete individuals. A moss individual establishes from a single spore and grows into mats that are formed by the vegetative budding of new stems. These mats can expand to occupy new habitats, while the portion that established earlier may die. An individual remains alive if old stems die no faster than new stems develop. The same individual could migrate back and forth through available habitats for an unlimited period of time, and it is not inconceivable that the individuals we see today arose from spores that germinated many thousands of years ago.