The following taxon apparently lies within the circumscription of Rhodymenia sonderi but has not yet been transferred or reduced to synonymy:
Sphaerococcus australis (Sonder) Kützing var. constrictus Kützing, 1849: 784 (type locality: Western Australia).
Misapplied name (fide Kützing, l.c.):
Rhodymenia australis var. constricta.—Sonder, 1845: 56.— Sonder, 1848 [1846–1848]: 191.
INDIAN OCEAN DISTRIBUTION: Australia.
Note: When transferring Rhodymenia australis Sonder var. constricta (Turner) Sonder to Sphaerococcus, Kützing excluded Fucus constrictus Turner as a synonym but, confusingly, retained the varietal epithet.