Comparative Morphology and Taxonomy of Sphacelaria Species with Tribuliform Propagules (Sphacelariales, Phaeophyceae)

Abstract

Four species of Sphacelaria producing tribuliform propagules in the section Tribuloides were distinguished from each other on the basis of the branching pattern and propagule morphology including the division type of the subapical cells and the shape of the lateral apical cells. Sphacelaria plumula shows complanate tufts composed of erect filaments bearing pinnate determinate laterals arranged in one plane, while the other species, S. tribuloides, S. brachygonia and S. novae-caledoniae, form erect tufts with irregular indeterminate branches. Sphacelaria tribuloides is distinguished by typical tribuliform propagules with acute conical lateral apical cells. Sphacelaria plumula has slightly larger propagules with obtuse conical lateral apical cells and a rather vague constriction at the waist. The distinguishing features of S. brachygonia are its slightly tribuliform, more often transversely ellipsoid propagules with round conical lateral apical cells and small lower parts under the waist. In S. novae-caledoniae, the filaments are slender and its mature propagules have the rectangular lateral apical cells produced by 2–3 longitudinal and transverse divisions of the subapical cells. The morphometric features of segments, propagules and zoidangia in the species of the section Tribuloides, including S. californica and S. novae-hollandiae, were found to be variable, but in some species they can be used to distinguish taxa. In addition, the holotypes of S. cornuta and S. nipponica, previously placed in the section Tribuloides, were examined. The results suggest that S. cornuta can be considered as a synonym of S. tribuloides, and S. nipponica should be placed in the section Furcigerae, based on the presence of diagonal divisions in the subapical cells as well as long stalks and prominent arms in the propagules.