Cystoseira sensu lato C.Agardh (Ochrophyta) forests are important habitat formers in the Mediterranean Sea, but they have mostly been studied in the western basin where many species are under decline due to human pressures. In the eastern basin, where fewer species occur, Cystoseira rayssiae Ramon was described in the year 2000 as an endemic species, based on morphological characteristics from herbaria samples collected on the Israeli coast. No further investigations have been conducted on this peculiar species since, but recently it has been recorded in contiguous Lebanon and outside the Mediterranean. Our work was aimed at confirming the taxonomic validity and endemic nature of this species, including its position among recently split Cystoseira sensu stricto, Carpodesmia Greville and Treptacantha Kützing genera, by sequencing the mitochondrial COI gene and by examining morphological characterics using samples from three different sites in northern Israel. Nothwithstanding considerable morphological plasticity, molecular analyses revealed a single unique COI sequence. Phylogenetic analyses show that Cystoseira rayssiae belongs to the resurrected genus Treptacantha and hence, the new combination Treptacantha rayssiae (Ramon) M.Mulas, J.Neiva & A.Israel, comb. nov. is proposed. Unique sequences and a restricted range support its Levantine-endemic status. Intriguing extra-Mediterranean reports from the Red Sea and the Persian Gulf are probably misidentifications rather than reflecting a disjunct distribution or recent invasion. 

*MM and JN contributed equally to this work.