Molecular data confirm the existence of attached crustose tetrasporangial thalli in Phymatolithon calcareum (Melobesioideae, Hapalidiaceae, Rhodophyta) from the Mediterranean Sea

• Molecular analyses are necessary to identify cryptic coralline algae.
• Tetrasporangial thalli that have the ability to form both rhodoliths and crusts.
• Morphological adaptations to colonize more habitats of the water column.

Phymatolithon calcareum is a species of non-geniculate coralline red algae that commonly forms maërl beds. Tetrasporangial thalli of this species are characterized by unattached growths (rhodoliths), which develop as a branched system without a central nodule, while the attached crustose forms are uncommon and are known only from Brittany (Atlantic coasts of France). In this study we confirm with molecular data the existence of attached crustose tetrasporangial thalli and describe for the first time epilithic, fruticose tetrasporangial specimens from the Mediterranean Sea (Vis Island, Croatia). Since the identification of coralline algal species is difficult due to the high morphological plasticity of the group, molecular analyses based on the nuclear nSSU and the plastidial psbA markers were necessary to correctly identify the specimens. Our study shows that P. calcareum tetrasporangial thalli have the ability to grow both unattached and attached, and we argue this to be in relation to the environmental conditions found at different depths. This ability permits the alga to colonize different habitats of the coastal environment.