Structural colour in Asparagopsis taxiformis (Bonnemaisoniales, Rhodophyta) and its possible role in communicative functions

Abstract

Many organisms, especially animals and land plants, are known to use structural colouration, i.e. colour produced by an object’s microstructure rather than by pigments, for communicative functions such as mating, camouflage and aposematic (warning off predators) purposes combined with toxic or distasteful substances. Some macroalgae show structural colours, typically produced by three mechanisms: (1) multilayered structure of the thallus surface, (2) arrays of microfibrils or (3) iridescent/refractile bodies within the cell. Suggested functions include the optimization of photosynthesis by reducing stresses of strong ambient light, especially in species that grow in the intertidal zone, although some subtidal species also show structural colour. Regarding other possible functions of structural colour, some macroalgae accumulate toxic materials such as secondary metabolites and highly acidic substances as defensive systems against herbivorous animals, yet few studies have linked herbivore defence with structural colouration as a possible communicative function. Here we report that refractile bodies in the red alga Asparagopsis taxiformis, which contain anti-herbivory substances such as bromides, also have optical effects causing a whitish appearance of the thallus. This masks the more typical reddish colour of the species, thereby functioning as camouflage from herbivorous animals that seek food by visual cues. Furthermore, the opal-like structure of the refractile bodies creates bluish structural colour in the meristematic region and so may provide aposematic functions. Similarly, in the brown alga Sporochnus dotyi, the iridescent bodies within cells of the assimilatory filaments create greenish structural colour. These refractile bodies incorporate some unknown highly reactive substances that apparently have anti-herbivory functions. Therefore, the iridescent bodies of S. dotyi may also have an aposematic function. Other tropical to warm temperate macroalgae, such as Martensia, Callophyllis and Dictyopteris, have structural colour that we consider to have communicative functions such as camouflage and aposematism. Based on these observations, we consider that structural colouration is probably widely used for communicative functions in macroalgae, as in animals and land plants.