Countershading coloration in blue shark skin emerges from hierarchically organized and spatially tuned photonic architectures inside skin denticles

Abstract

The blue shark (Prionace glauca) exhibits a striking dorsoventral color gradient, transitioning from vibrant blue dorsally to silver and white ventrally, a pattern widely interpreted as pelagic countershading. Despite its ecological significance, the physical basis of this coloration remains unresolved. Here we show that this color system does not arise from dermal chromatophores, as in most vertebrates, but from a previously unrecognised photonic architecture housed within the pulp cavity of individual dermal denticles that cover the skin. Optical imaging reveals discrete color domains within denticle crowns, while external denticle morphology remains similar across color zones. Using spectroscopy, micro-computed tomography, histology, and correlative electron microscopy, we demonstrate that color variation is organized across coupled micro- and nanoscale architectures. In blue denticles, iridophores and melanophores form a densely packed tessellated reflector-absorber system within an expanded crown-restricted pulp cavity. Transition-zone denticles exhibit partial cellular layering, whereas white denticles lack melanophores and contain only reflective cells. At the nanoscale, ordered purine-crystal stacks generate narrowband blue reflection, whereas disordered assemblies produce broadband white scattering. Together, these results reveal denticles as mechanically protected optical "pixels" whose hierarchical cellular and nanocrystal organization generates the shark's countershaded coloration.

Figures (9)

• Figure 1: Structural and optical differences in denticles across blue shark skin color zones. (a) Photograph of a blue shark (image courtesy of Samuel Iglesias) showing the dorsal blue (BZ), lateral transition (TZ), and ventral white (WZ) regions used for sampling. (b) Multimodal characterisation of denticles (apical views). (Far left) Optical image of a fresh skin strip showing the pronounced color gradient across zones. (Left) $\mu$CT reconstructions (2.5µm resolution) revealing broadly similar external denticle morphologies across BZ, TZ, and WZ. (Right) Representative optical micrographs of denticles from each region showing distinct internal color domains. (Far right) High-magnification images of individual denticles confirm that color is directly linked to internal denticle architecture. (c) Reflectance spectra collected from the three regions (n = 9per zone), with mean curves (solid lines) and standard deviations (shaded areas), highlighting distinct spectral signatures associated with each colour zone.
• Figure 2: Origin of structural coloration: Nanoscale components within the blue shark denticle pulp cavity. (a) High-resolution optical micrograph of apically viewed denticles from the blue zone, oriented from right to left toward the shark’s tail, revealing distinct subsurface color domains within individual denticles. (b) Schematic representation of a single denticle illustrating the position of the internal pulp cavity relative to the mineralized denticle wall. Top: apical view. Bottom: lateral view showing the crown and neck regions and the location of the pulp cavity within the denticle crown. (c) Optical micrograph (z-stack, bright field and polarized light) of the basal side of a trimmed denticle crown (inverted orientation), exposing the pulp cavity; the dashed rectangle indicates the region shown at higher magnification in (d). (d) Optical micrograph of the cleaned denticle crown (apical view) after centrifugation and ultrasonic treatment, demonstrating the transparency of the denticle wall once internal components are removed. (e) Higher-magnification optical micrograph of the
• Figure 3: (cont'd.) boxed region in (b), revealing dark brown and bluish/shimmering internal domains within the pulp cavity. (f) Environmental scanning electron micrograph (ESEM) of the same region shown in (d), with pseudo-coloring highlighting nanoscale structures associated with these optically distinct domains. (g) Higher-magnification ESEM images showing two classes of nanoscale components within the pulp cavity: rod-like and spherical bodies, and flake-like platelets exhibiting variation in lateral dimensions and aspect ratios.
• Figure 4: Micro- and nanoscale organization of color-producing components within blue-zone denticles.(a) High-resolution optical micrograph of denticles from the blue zone viewed apically, showing continuous blue coloration across the crown surface. (b) Optical micrograph of a blue-zone denticle sectioned along
• Figure 5: (cont'd.) the sagittal plane, revealing a crown-restricted pulp cavity occupying 25% of the total denticle volume. The cavity is densely packed with iridophores and melanophores arranged in a mixed, non-layered (“chessboard-like”) microscale organisation, as illustrated by the schematic overlay. (c) Histological section of a blue-zone denticle showing the pulp cavity enclosed by the mineralized denticle wall. (d) Higher-magnification optical micrograph resolving cellular and subcellular optical components. (e) FIB–SEM cross-section beneath the denticle crown wall showing purine crystals within iridophores and melanosomes within melanophores. (f) Three-dimensional FIB–SEM segmentation illustrating micro- and nanoscale organisation. (g) ESEM of melanosomes revealing ($\sim$1060nm) protruding granules. (h) TEM showing electron-dense internal inclusions. (i) Higher magnification showing parallel-aligned purine crystal platelets. Optical micrographs in (b, d) were generated from merged z-stacks using combined bright-field and polarized light microscopy.