Southern Australian Species of Ceramium Roth (Rhodophyta)

Abstract

Fifteen species of Ceramium are recognized from the coast of southern Australia, and their distri- bution and ecology are outlined. Two species (C. rubrum and C. flaccidum) are of widespread distribution in other parts of the world, but the remaining species appear to be largely confined to southern Australia.

The species are separated mainly on the development of the nodal cortication from the ring of periaxial cells at the upper end of each axial cell; other useful taxonomic features are the dimensions of the filaments, the branching pattern, and the arrangement of the tetrasporangia.

The pattern of cortical development at the nodes is usually readily followed after aniline blue staining and mounting in corn syrup, since slight contraction of the cells in this process renders the pit-connections visible. Each periaxial cell typically cuts off two cells both acropetally and basipetally and in most species these branch further in a similar pattern; in some species three cells may occasionally be cut off, and in one species (C. flaccidum) only one cell is cut off basipetally. In a few species no cells (or only odd ones) are cut off basipetally, and in many species cortical development is greater acropetally than basipetally. The margin of the nodal cortication (whether straight through synchronous cell development or irregular) may be characteristic of certain species.

A variation giving a distinctive pattern in a few species is where the periaxial cells cut off one or two pseudoperiaxial cells which interpose in the periaxial ring (e.g. C. shepherdii, C. australe and C. macilentum).

In many species the cortical development is limited and soon reaches a stage which is then consistent throughout the thallus. In some species (e.g. C. tasmanicum) an extending cortex later develops which may largely or completely close the internodal space; in fully corticated species (C. rubrum and C. pusillum) the cortex extends from an early stage and maintains complete cover of the axial cell. Heavily corticated species usually have an outer cortex of small cells lying outside the larger inner cortical cells. Gland cells may be characteristic of certain species but are usually variable in frequency and not always present.

The nature of the branch apices, while characteristic in some species, depends on the activity of the apical growth. Carposporophytes and spermatangial plants provide little help in taxonomic distinctions. V

The European species C. gracillimum sensu Harvey is now recognized as a very widely distributed species distinct from C. gracillimum Kuetzing. The oldest name is apparently C. flaccidum (Kuetzing) Ardissone, for which there are many synonyms. It is characterized by the formation of only a single cell basipetally from each of the periaxial cells and also by having unicellular rhizoids; the single basipetal cell may produce one or two further cells and commonly itself divides into 2-4 cells.