Introduction

Structural coloration is common in dynamic visual displays by terrestrial and marine animals [1–5]. The spectral environment in which they live influences communication methods that use coloration [1,6–11]. In the ocean's euphotic zone, the ‘disco’ or ‘electric’ file clam Ctenoides ales (family Limidae) is found inside small crevices at depths of approximately 3–50 m. At these depths, the majority of wavelengths available for visual displays are in the blue-green range (400–500 nm) [12]. This is true even at shallow depths where long wavelengths have not yet attenuated (less than 15 m), as the crevices in which C. ales are found are dominated by horizontal light composed of short wavelengths (electronic supplementary material, file S1).

The flashing display on the mantle lip of C. ales has been mischaracterized as bioluminescence [13,14], although it is actually mediated by light scattered from photonic nanostructures [15]. Structures of this sort typically use materials that have high refractive indices relative to the substrate, such as collagen, chitin, keratin and guanine [16]. Silica also has a high refractive index (n = 1.43 at 589 nm) [16] but has only rarely been used as a biophotonic structure, such as in diatoms [17–19] and the weevil Pachyrhynchus argus [20]. Photonic nanostructures of any substance however can enhance reflectance, such as the ultrathin, aperiodic filaments in scales of Cyphochilus spp. beetles [21] and the bead-studded scales in the wings of certain pierid butterflies [22].

Within bivalves, studies of light manipulation are limited to the bioluminescence of the marine clams Pholas dactylus and Gastrochaena sp. [23], and the iridophores of the giant clam Tridacna, which are thought to scatter light towards symbiotic zooxanthallae [24]. Ctenoides ales, however, is the only known bivalve with a behaviourally mediated photic display. The fundamental characteristics of this display are described here to determine its potential as a signal. Ongoing studies of the function of the display are being conducted in the context of habitat-specific sensory ecology. In order to provide a preliminary comparative framework, we collected data from the morphologically and ecologically similar congener Ctenoides scaber, which does not flash.