Riccia sp. growing with Calandrinia sp. in the Pilbara, Western Australia. [Photo: J. Palmer]
Spores from South African specimens of Riccia limbata. [Photo: K. Beckmann]
Spores from Australian specimens of Riccia limbata. [Photo: K. Beckmann]
Spores of Riccia crinita [above and below]. [Photos: K. Beckmann]
Report from an ABRS grantee
The liverwort family Ricciaceae in Australia
Ricciaceae (Marchantiales) are a cosmopolitan family of liverworts. More than 200 species have been described worldwide (Seppelt, 1998). The family includes two genera: Riccia and Ricciocarpos.
Ricciocarpos is monospecific, based on Ricciocarpos natans, a predominantly aquatic plant. The remainder of the family Ricciaceae in Australia consists of small terrestrial, often xerophytic, plants belonging to the genus Riccia. These are widespread and diverse and are found in all climatic regions of Australia. They are particularly numerous throughout the drier regions wherever there are patches of occasionally moist soil, in the shelter of rocks and stones, around the edges of lakes or along intermittent water courses. Here Riccia plays an important role in the stabilisation of soils as a component of the fragile, biological soil crusts of arid and semi-arid zones, including Australia’s sensitive rangeland habitats. In recent years there has been a growing interest in biological soil crusts: their composition, ecology and function. Like many soil crust organisms Riccia presents a challenge as there is little known of its ecology and identification of species is often difficult.
Riccias are small thallose liverworts with simple sporophytes immersed in the thallus. Thalli usually bear ventral scales, sometimes evanescent but often reaching the thallus margin or extending beyond it. These scales vary in colour from hyaline and white to red, purple or black. When the thallus dries out and in-rolls or folds along the dorsal median groove, the scales form a protective armour against the elements, reducing water loss. Some species also have long marginal hairs. Scales, hairs, thallus colour and structure are all used as taxonomic characters but because of the considerable phenotypic plasticity of liverworts, and the loss of many useful characters in dried herbarium specimens, the most reliable taxonomic character is spore sculpturing, which is examined with both compound light and scanning electron microscopy.
The genus Riccia has been variously rearranged and subdivided by different workers worldwide into subgenera, sections, groups, species and subspecies but to date there has been no common agreement. While previous work on the Australian Ricciaceae (Na-Thalang, 1980; Seppelt, 1998; Jovet-Ast, 2000) provided a good taxonomic foundation, it was based on limited collections. The opportunity, in this study, to examine all available Australian material is unprecedented and has enabled clarification and expansion of understanding species of this often confusing family.
Fifty species are currently recognised for Australia (McCarthy, 2003), including widespread/cosmopolitan species and others believed to be Australian endemics, a few known from single collections from one locality. Australian endemic species seem most numerous, though poorly collected, in the arid regions (Jovet-Ast, 2000) and these regions remain under-studied.
Recent studies of Riccia in South Africa, which has a climate similar to that of Australia (numerous studies of Perold and Volk from 1984–1991; Perold, 1999), have resulted in the description of many new taxa and this situation appears likely in Australia.
To date, this study has identified what appear to be six undescribed species and a number of described taxa not previously recognised in Australia. Evidence supporting the long-suspected links between the Australian and South African Riccia floras is continually coming to light. A detailed analysis of the many variable plants with long cilia has led to the conclusion that the South African Riccia trichocarpa is synonymous with Australian R. crinita. While the gametophytes and the distal spore faces of Australian and South African material of Riccia limbata are indistinguishable, the ornamentation of the proximal spore face of Australian plants differs consistently from that of the South African type. The Australian plants will become a variety of R. limbata. Riccia lamellosa, previously thought to be straightforward in Australia, shows much more variation than expected and has proven to be problematic.
A number of complexes have been re-evaluated for the Flora of Australia. They have been well recognised for their taxonomic difficulties and past studies have left them unresolved. Thus care and time is being taken during this study to attempt to present a more coherent picture of the Ricciaceae.
The monospecific subgenus Triseriata, based on a single collection of R. singularis, is currently being re-evaluated. Plants attributed to R. albida exhibit a third cell layer, developed to a greater or lesser extent, similar to that found in R. singularis. A number of other morphotypes, probably one or two new species, have been found in collections that fit into this group.
It is proposed that the subgenus Triseriata be expanded to encompass Riccia species that have only smooth rhizoids (not pegged as in most Marchantiales), smooth spores (not all have been fruiting but the various morphotypes with spores have all been smooth) and a third tissue layer of airiferous tissue capped by cells covered in calcium deposits. Additional work is required by way of thin sectioning and histology to elucidate the details of structure in this cell layer as it is extremely fragile and difficult to section.
The revised subgenus will include: Riccia singularis, R. albida, R. crustata and R. corrugata.
Australian plants currently identified as Riccia albida are attributable to at least one or two new species, as well as R. crustata. None of the Australian plants in this group match the type of R. albida.
The emerging complexity of Riccia in Australia means there is still much to be done describing new species, further defining the relationships between them, and evaluating the subgenera.
Jovet-Ast, S. (2000), Documents pour la connaissance des Riccia australiens (Hepatiques, Marchantiales)— Nouvelles récoltes. Taxons nouveaux. Commentaires morphologiques et ecologiques, Cryptogamie Bryologie 21(4): 289–343.
McCarthy, P. (2003), Catalogue of Australian Liverworts and Hornworts. Flora of Australia Supplementary Series Number 21, pp 93–97. ABRS , Canberra.
Na-Thalang, O. (1980), A revision of the genus Riccia (Hepaticae) in Australia, Brunonia 3: 61–140.
Perold, S.M. (1999), Hepatophyta. Part 1: Marchantiopsida. Fascicle 1: Marchantiidae. In O.A.Leistner, (ed.), Flora of Southern Africa. National Botanical Institute, Pretoria.
Seppelt, R.D. (1998), The genus Riccia (Marchantiales: Ricciaceae), in South Australia, Hikobia 12: 317–341.