Abstract

Iridophores in the mantle of the giant clams Tridacna gigas and T. crocea comprise an aggregation of cells (iridocytes) containing stacks of regularly arranged platelet, (iridosomal platelets) of uniform thickness (c. 70 nm). The platelets are separated by cytoplasm of roughly the same thickness permeated by a single flattened cisterna, the bounding membrane of which is continuous with the plasmalemma of the iridocyte. Iridosome formation starts with the laying down of the cisternae and subsequent deposition of proteinaceous platelet material between them in a regular crystalline arrangement. The iridosomal platelets form a space lattice giving maximum light interference at a wavelength of around 400 nm or just above. Interference may extend into the blue and green portions of the visible spectrum, which would be consistent with the observed interference colours displayed by the giant clams in situ. The iridosomes may also function as diffraction gratings or they may redirect light to the zooxanthellae.