The following taxon apparently lies within the circumscription of Blennothrix lyngbyacea but has not yet been transferred or reduced to synonymy:
Hydrocoleum lyngbyaceum Kützing var. rupestre Kützing, 1849: 259 (type locality: Calvados, France).
Hydrocoleum lyngbyaceum Kützing forma rupestre (Kützing) Jadin, 1894: CLXV.
INDIAN OCEAN DISTRIBUTION: Mauritius.
Note: Drouet (1968: 272), after examining the holotype (in L), assigned it to Microcoleus lyngbyaceus (Kützing) P. Crouan & H. Crouan.