The morphology of Ralfsia expansa (J. Agardh) J. Agardh (Ralfsiaceae, Phaeophyta) from Veracruz, Mexico

The holotype specimen of Ralfsia expansa is examined, and a description including the characteristics that are presently used to distinguish the species of the genus is provided. Considering that the type specimen is sterile and that different interpretations of the species exist, its reproductive structures are described based on specimens obtained from the type locality, in Veracruz, Mexico. The observations show that the unangia of the species are characteristically borne on a one-celled. This feature distinguishes the species from the very near R. hancockii which has multicelled stalks. Ralfsia expansa can be distinguished from R. fungiformis, R. verrucosa, R. hancockii, R. hesperia, R. integra, R. longicellularis and R. huanghaiensis by a combination of reproductive features, the presence of a clearly defined cortical layer, and its growth form. Ralfsia pacifica does not have a cortical layer and a unangial stalk and is not distinct from R. verrucosa. R. expansa has been reported from the tropical and subtropical Atlantic, the Mediterranean Sea, and the coasts of India.