Study of epiphytic non-geniculate coralline algae Reveals an Evolutionarily significant Genus, Pseudoderma gen. nov. (Lithophylloideae, Corallinophycidae)

Epiphytic non-geniculate coralline algae (ENCA) are distributed broadly, while limited research on their diversity has constrained our understanding of their ecological roles in marine environments, and impeded a comprehensive understanding of coralline algae. In this study, the diversity and ecological characteristics of ENCA epiphytic on 113 red macroalgal specimens collected from coastal China were examined. Three species delimitation algorithms revealed 24 primary species hypotheses (PSH), of which 22 were corroborated through phylogenetic analysis based on the psbA gene. Further multi-gene concatenated phylogenetic analyses and morpho-anatomical assessments revealed a new genus within Lithophylloideae, Pseudoderma gen. nov., which is closely related to the genus Titanoderma and exhibits morphological similarities. A hypothesis concerning the growth patterns was proposed to elucidate the morphological differences among Titanoderma, Lithophyllum, and Pseudoderma: the presence or absence of hypothallial palisade cells indicated distinct thallus thickening patterns, either dominated by elongation of the hypothallial cells or by division of the perithallial cells. These processes resulted in fast-growing thin-crust thalli or longer-lasting thick-crust thalli, adapted to epiphytic or epilithic lifestyles, respectively. Pseudoderma included at least six new taxa, and Pseudoderma sinicarum sp. nov., was designated as the holotype species of this genus. In conclusion, this study has underscored the unexpected biodiversity of ENCA, and the identification of the novel genus Pseudoderma from ENCA groups carried significant evolutionary implications for enhancing our understanding of coralline algae systematics.