Monterey Bay Aquarium Research Institute
Marine Botany

Chondracanthus exasperatus iridescence

The iridescence of some of the red algae, including C. exasperatus, is one of the most aesthetically beautiful characteristics associated with any alga.  If you have not been fortunate enough to see it for yourself scuba diving or at an aquarium, iridescence on the thalli of the reds is similar to the dancing blues, green, and reds of light reflecting off soap bubbles.  It is most brilliant underwater, and truly one of the more amazing sites in the phycology world.

    Iridescence is one of the most easily recognizable characteristics of algae, and hence commonly used for taxonomic classification.  However, the mechanisms behind iridescence for most algae is not well understood.  Two different mechanisms have already been discovered to be behind the phenomena of iridescence.  Iridescent bodies have been identified as the source of iridescence in Chondria and Gastroclonium, while in Iridaea it has been identified as a multi-layered cuticle (Lee, 1980).

    Iridescence in C. exasperatus is caused by this second mechanism.  A study by Gerwick and Lang (1977) identified a multi-layered cuticle in Iridaea, and found that mechanical removal of this cuticle also removed the iridescent potential of the alga, at least until the cortical regenerated.  Iridescence is found to be a purely physical interference of light by overlapping layers of cuticle with opposing refractive indexes.  The cuticle is formed of very thin electron opaque layers separated by very thin electron translucent layers.  These layers are uniform and produced by periodic secretion and deposition, and they do regenerate.   As light passes through these layers, it interacts with these different layers and their contrasting refractive indexes, producing the phenomenon called iridescence.  Under a microscope, this phenomenon can be broken down into individual tear drop-shaped spots of iridescence, each its own color, which then combine to form the soap bubble appearance we recognize (Gerwick and Lang, 1977).

©1999 Devon Lake