Studies of marine algae in the lesser-known families of the Gigartinales (Rhodophyta). I. The Acrotylaceae

Abstract

The Acrotylaceae is composed of Acrotylus australis J. Ag., Hennedya crispa Harv., Amphiplexia hymenocladioides J. Ag. and A. racemosa (J. Ag.) comb, nov. (all endemic to southern Australia), Reinboldia polyearpa Schmitz from South Africa and Ranavalona duckerae gen. et sp. nov. from southern Madagascar. The species are multiaxial, zonately tetrasporangiate and monoecious. In Acrotylus and Ranavalona the tetrasporangia develop in raised nemathecia, whereas they are scattered in the other species. In Amphiplexia hymenocladioides, tetrasporangia are intercalary in contrast to the other species with known tetrasporophytes. Acrotylus and Ranavalona are polycarpogonial, with the carpogonial branches of Ranavalona being additionally nemathecial. The remaining genera are monocarpogonial, with scattered carpogonial branches. The carpogonia in all species are directed laterally or thallus-inwardly and provided with reflexed trichogynes. The species are all procarpic (except possibly Reinboldia), with supporting cells of carpogonial branches functioning as auxiliary cells. Fusion cells do not form, and several gonimoblast initials arise directly from diploidized auxiliary cells and enter an adjacent region of 'nutritive' cells. Gonimoblast development is thallus-outward in Amphiplexia and thallus-inward in Acrotylus, Ranavalona, Hennedya and possibly Reinboldia, the last genus being known only from a surviving fragment of the type specimen. In all but Amphiplexia, breakdown of the auxiliary cell, some adjacent cortical cells and some older gonimoblasts results in a cavity which is lined by gonimoblast derivatives connected to surrounding nutritive and other vegetative cells. With expansion of the cavity, branched gonimoblast filaments grow centripetally and produce terminal or short chains of carposporangia that are released through distinct ostioles. In Amphiplexia, nutritive cells do not completely surround the auxiliary cell, and most gonimoblast filaments initially cover the interior surface of the pericarp rather than a cavity lined by specially produced nutritive cells.

The genera are separated on wide differences in habit, vegetative cross section, carpogonial branches, gonimoblast orientation, carpospore morphology and mature cystocarp cross sections. The two African genera Ranavalona and Reinboldia appear on vegetative and certain reproductive grounds to be relatively primitive in the group, with possible links to the less reproductively complex family Solieriaceae. The most closely related genera within the Acrotylaceae, Acrotylus and Ranavalona, are also about the most geographically separated. The present-day distinctiveness, uncommonness and distribution of the genera of the Acrotylaceae perhaps suggest that it is a relict and isolated group.