Studies of Marine algae in the lesser-known families of the Gigartinales (Rhodophyta). II. The Dicranemaceae

Abstract

The red algal family Dicranemaceae (Gigartinales) has been studied with regard to its vegetative and reproductive morphology, The group is composed of two Dicranema species (D. revolutum (C. Ag.) J. Ag. and D. cincinnalis sp, nov.), Peltasta australis J. Ag., Reptataxis rhizophora (Lucas) gen. et comb. nov., and Tylotus obtusatus (Sond.) J . Ag. All except Reptataxis, from Lord Howe I., are endemic to southern Australia. The last three genera are newly added to the family, which is redefined to embrace their early gonimoblast similarities to Dicranema. The species are all multiaxial, zonately tetrasporangiate and monoecious. Dicranema, Peltasta and Reptataxis have broad, cellular cortexes and filamentous medullas, while Tylotus is pseudoparenchymatous throughout. Tetrasporangia are nemathecial in Dieranem, Reptataxis and Tylotus, but scattered in Peltasta. Spermatangia in Dicranema are formed in deeply buried catenate clusters, and are similarly derived but non-catenate in the other genera. The species are all monocarpogonial, and only Tylotus is procarpic. In none of the genera are sterile cells associated with carpogonial branches. In Tylotus the supporting cell of the carpogonial branch fuses with the presumably fertilized carpogonium and becomes the diploidized auxiliary cell. In Dicranema, Peltasta and Reptataxis, 2- or 3-celled carpogonial branches are directed to the thallus surface and presumably fertilized carpogonia fuse with an auxiliary cell not necessarily on the same cortical branch system as the supporting cell. Diploidized auxiliary cells in all four genera form small, irregular fusion cells and emit multiple, filamentous gonimoblasts. Gonimoblast growth is mostly thallus-inward in Dicranema, initially radial in Peltasta and Reptataxis, and lateral and outward in Tylotus and advanced stages of Peltasta and Reptataxis. In Dicranema, Peltasta and Reptataxis an extensive placenta of mixed and connected gonimoblast and vegetative cells forms between the fusion cell and the peripheral carpospore-producing layers. In Tylotus vegetative/gonimoblast cell connections occur mainly at the floor of the cystocarp. Carposporangia develop singly on elongate penultimate gonimoblasts in Dicranema and Tylotus, but form chains in Peltasta and Reptataxis. Cystocarps in all the genera are surrounded by thick ostiolate pericarps. The genera of the Dicranemaceae are distinct from one another on frond and holdfast habits, vegetative structure, carpogonial branch shape, auxiliary cell position, gonimoblast orientation, carposporangial size, and mature cystocarp location and cross section. None of the four genera seem obviously derived from or particularly closely related to any others outside the family, although both Peltasta and Reptataxis show some features susesting possible links to the Sarcodiaceae.