Synonymy of Pachyarthron compressum (Corallinaceae, Rhodophyta) with Corallina officinalis and proposal for Corallina officinalis subsp. cretacea

Pachyrthron compressum is morphologically and anatomically very similar to Pachyarthron cretaceum, with the two species overlapping in most characteristics. After P. cretaceum was placed in synonymy under Corallina officinalis, P. compressum became orphaned. To determine whether P. compressum is a distinct species and the sole representative of a distinct genus, or whether it is conspecific with P. cretaceum and should be considered a synonym of C. officinalis, comparative morphological and anatomical, and molecular phylogenetic analyses using 5.8S-ITS2, COI, and rbcL were performed. The morphological and anatomical analysis did not support P. compressum as distinct from P. cretaceum (=C. officinalis from the Pacific). No genetic differences were found between P. compressum and C. officinalis, or the isolectotype of Corallina cretacea (=P. cretaceum), indicating that P. compressum is also a synonym of C. officinalis. Fixed differences were found in all molecular loci between the Pacific and Atlantic populations of C. officinalis. Together with the clear morphological and anatomical differences between the populations, this fact suggests their separation and allows us to propose C. officinalis subsp. cretacea for the Pacific population of the species.