Ribbontail Stingray Skin Employs a Core–Shell Photonic Glass Ultrastructure to Make Blue Structural Color

Abstract

Structural blue colors are common in animals, with the tissue nanostructures and material systems that produce them—especially bright blues—typically based on highly ordered nano‐architectures. In this study, we describe an unusually bright and angle‐independent structural blue from the skin of ribbontail stingray, arising from a more disordered array of scattering elements with a previously undescribed core–shell ultrastructure, involving nano‐vesicles enclosing guanine nano‐platelets. We show that this skin architecture functions as an intracellular photonic glass, coherently scattering blue, while broadband absorption from closely associated melanophores obviates the low color saturation typical for photonic glasses. Our characterization of skin ultrastructure and color in a stingray demonstrates how disordered systems can be harnessed to produce brilliant hues while illustrating that the capacity for guanine‐based colors likely arose extremely early in vertebrate evolution. Moreover, the material‐structure‐function associations underlying ribbontail stingray coloration, employing two distinct photonic phenomena, illustrate how the evolution of nanoscale architectures can have profound effects at much larger size scales (e.g., in visual ecology and communication), and provide fundamental guidelines for color‐saturated manmade photonic glasses.

Intermediate filaments spatially organize intracellular nanostructures to produce the bright structural blue of ribbontail stingrays across ontogeny

“…In contrast, we have recently shown that the blue color (reflectance peaks ∼447–452 nm) of the ribbontail stingray ( T. lymma ; Figure 1A ) is due to a novel type of iridophore that occurs in the epidermis and in exceptionally high density in the dermis, equipped with a stable colloidal system of crystal-containing vesicles (referred to as iridosomes in the present study) that coherently scatter incident light, while associated melanophores absorb the longer incident wavelengths ( Surapaneni et al, 2024 ). Surapaneni et al (2024) is the first study to describe structural color formation in elasmobranchs, using a combination of optical, histological and modeling methods to explain the basis for the non-iridescent blue of the ribbontail ray.…”

“…Specimens of the blue-spotted ribbontail ray T. lymma were from the same individuals used in Surapaneni et al (2024) , collected opportunistically from commercial fisheries operating off Singapore (Pulan Ubin–Changi area), Indonesia (Jakarta), and Kenya (Eastern Indian Ocean) between May 2017 and June 2022, following the sampling protocol by Mollen (2019) . Specimens in this study included two females, one adult of ∼26.0 cm DW (disc width) and 72.5 cm TL (total length) and one juvenile of ∼14.9 cm DW and 35.7 cm TL.…”

“…This can reveal hidden correlations among variables as starting points for more formal downstream hypothesis testing (e.g., pairwise correlation, chi-squared test). Variables included in our FAMD analysis related to the structural and optical properties of scatterers (summarized in Supplementary Table S1 ) from diverse eukaryotic groups (vertebrates, invertebrates and algae; Prum et al, 1999 ; Prum and Torres, 2004 ; Prum et al, 2004 ; Noh et al, 2010 ; Saenko et al, 2013 ; Teyssier et al, 2015 ; Gur et al, 2015a ; Gur et al, 2015b ; Gur et al, 2020 ; Chandler et al, 2015 ; Hsiung et al, 2015 ; Lopez-Garcia et al, 2018 ; Jeon et al, 2023 ; Surapaneni et al, 2024 ). FAMD analysis and plotting were performed using FactoMineR and Factoextra packages in R software version 4.2.2 ( Lê et al, 2008 ; Kassambara and Mundt, 2020 ; R Development Core Team, 2023 ).…”

“…Among fishes, elasmobranchs (sharks and rays) are generally not conspicuously colored and therefore have been of little concern to scientists interested in how nature makes color. Blue coloration, as far as we know, has only been explored in two species, an electric ray ( Torpedo ocellata, a junior synonym of T. torpedo; Linnaeus, 1758) and a ribbontail stingray ( Taeniura lymma ; Fabricius, ex Forsskål, in Niebuhr, 1775), both of which bear prominent blue spots on their backs ( Taylor and Bagnara, 1972 ; Bagnara et al, 2007 ; Surapaneni et al, 2024 ). In the electric ray , ultrastructural data on the cellular architecture of the skin have never been published, but in a literature review by Bagnara et al (2007) , an artist’s reconstruction shows histological characteristics of the spot and adjacent region, however with no cellular and intracellular details.…”

“…In addition, we demonstrate the presence of mixed pigment organelles (containing both pigments and crystals) in the blue regions and highlight the differences in cell composition and nanoscale architecture that distinguish blue from non-blue regions, thereby framing tissue alterations that decide body pattern and supported the evolution within elasmobranchs of nature’s rarest color. By contrasting the cellular anatomies and ultrastructures in blue and non-blue regions of skin, we provide a natural test of the optical morphology hypothesis posed by Surapaneni et al (2024) that cellular components and their nanoscale arrangements differ in characteristic, hierarchical and predictable ways between the two tissue regions.…”