Structural colour in red seaweeds is more common and diverse than has been presumed.

The brightest colorations observed in nature are the result of structural colour, a physical phenomenon relying not on pigments but on the interactions of light with nanostructured materials. Research on structural colour in seaweeds has been growing and hints that the phenomenon is considerably more widespread in these organisms than previously understood. In this review, we combine information from published literature, herbarium specimens and our own observations to clearly outline and reframe the current state of knowledge on the phenomenon in red seaweeds (Rhodophyta). We describe structural colour and the structures responsible for it in rhodophytes, identifying clear categories and their variations. Through an overview of the phylogenetic, geographic and ecological distribution of the phenomenon, we confirm that it is more widespread and diverse than had been indicated by casual recording. We finally discuss hypotheses on the biological significance of structural colour for red seaweeds. Our investigation emphasizes the need for more extensive research in order to fully assess the evolutionary mechanisms at play, the development of the nanostructures and their relation to environmental conditions. This review provides a framework for understanding and classifying structural colour in red algae to encourage a more comprehensive reporting of the phenomenon.