The morphological distinction of Ralfsia expansa and R-hancockii (Ralfsiaceae, Phaeophyta) from Mexico

Abstract

The brown alga Ralfsia expansa described by Borgesen (1912) and Tanaka & Chihara (1980a) may actually be two different species. To evaluate this possibility, a comparison of R. expansa with the related species R. hancockii was made. The following specimens were examined: the holotype of R. hancockii and several other specimens of this species from 11 localities along the Mexican tropical Pacific coast, and three specimens of R. expansa from the type locality (Veracruz, Mexico). The observations confirm the diagnostic value of: (1) the number of cells per stalk of the unilocular reproductive structures

and (2) the presence of a cortical layer of several cells differentiated from the medulla

they also confirm the lack of diagnostic value of certain other features such as the symmetry of the thallus in longitudinal radial section. We consider R. expansa sensu Borgesen to be a distinct species, typically found on the tropical Atlantic coast. Specimens of R. expansa sensu Tanaka & Chihara, described in previous reports from the Pacific coast of Mexico, are actually R. hancockii, which appears to be restricted to the Gulf of California and the Pacific coast of Mexico.