Vegetative and reproductive morphology of Nemastoma damaecorne (Gigartinales, Rhodophyta) from Western Australia

Abstract

The vegetative and reproductive morphology of the red alga Nemastoma damaecorne Harvey (Gigartinales) have been examined. Thalli are flattened and variously branched, either foliose with marginal proliferation or deeply incised. Structurally, the thallus is multiaxial, with a filamentous medulla and cortex, the latter subdichotomously branched, anticlinally directed, and incorporating large, intercalary gland cells. Plants are monoecious, with spermatangia formed on outer cortical cells. Carpogonial branches are three-celled and borne on an inner cortical cell. Following presumed fertilization, the carpogonium fuses with nearby subsidiary auxiliary cells—the latter producing several branched connecting filaments. Generative auxiliary cells are transformed intercalary cortical cells. Connecting filaments fuse with generative auxiliary cells and continue growth. The gonimoblast initial arises from the upper surface of the generative auxiliary cell and forms a carposporophyte composed entirely of carposporangia. Tetrasporophytes are unknown, but the germination pattern of carpospores in culture suggests a crustose habit. These results indicate that the species is incorrectly placed in Nemastoma and conforms more closely to the genus Platoma. Furthermore, a comparison between N. damaecorne and published descriptions of Platoma cyclocolpum (Montagne) Schmitz suggests that the two species are conspecific, and as such the former is regarded as an heterotypic synonym of the latter.