Photoreception and vision are fundamental aspects of animal sensory biology and ecology, but important gaps remain in our understanding of these processes in many species. The colour-changing brittle star Ophiocoma wendtii is iconic in vision research, speculatively possessing a unique whole-body visual system that incorporates information from nerve bundles underlying thousands of crystalline ‘microlenses’. The hypothesis that these form a sophisticated compound eye-like system regulated by chromatophore movement has been extensively reiterated, with consequent investigations into biomimetic optics and similar ‘visual’ structures in living and fossil taxa. However, no photoreceptors or visual behaviours have ever been identified. We present the first evidence of photoreceptor networks in three Ophiocoma species, both with and without microlenses and colour-changing behaviour. High-resolution microscopy, immunohistochemistry and synchrotron tomography demonstrate that putative photoreceptors cover the animals’ oral, lateral, and aboral surfaces, but are absent at the hypothesised focal points of the microlenses. The structural optics of these crystal ‘lenses’ are an exaptation and do not fulfil any apparent visual role. This contradicts previous studies, yet the photoreceptor network in Ophiocoma appears even more widespread than previously anticipated, both taxonomically and anatomically.
Sumner-Rooney et al. S1
Arm segments from Ophiocoma wendtii. The 3D data included here were generated from x-ray tomography scans of specimens of brittle stars Ophiocoma wendtii, Ophiocoma echinata and Ophiocoma pumila. For full methodology, please refer to the Method Details text. All models are downsized for practicality in PDF form; STL files with full detail are available on request. 3D files require Adobe Acrobat or Acrobat Reader (v. 7 or higher) for full interactive functionality. Use left-click to rotate, Ctrl+click to pan, and shift+click to zoom. Default view is the dorsal surface.
Sumner-Rooney et al. S2
Arm segments from Ophiocoma echinata. The 3D data included here were generated from x-ray tomography scans of specimens of brittle stars Ophiocoma wendtii, Ophiocoma echinata and Ophiocoma pumila. For full methodology, please refer to the Method Details text. All models are downsized for practicality in PDF form; STL files with full detail are available on request. 3D files require Adobe Acrobat or Acrobat Reader (v. 7 or higher) for full interactive functionality. Use left-click to rotate, Ctrl+click to pan, and shift+click to zoom. Default view is the dorsal surface.
Sumner-Rooney et al. S3
Arm segments from Ophiocoma pumila. The 3D data included here were generated from x-ray tomography scans of specimens of brittle stars Ophiocoma wendtii, Ophiocoma echinata and Ophiocoma pumila. For full methodology, please refer to the Method Details text. All models are downsized for practicality in PDF form; STL files with full detail are available on request. 3D files require Adobe Acrobat or Acrobat Reader (v. 7 or higher) for full interactive functionality. Use left-click to rotate, Ctrl+click to pan, and shift+click to zoom. Default view is the dorsal surface.
Ophiocoma echinata 1
Image stack generated by synchrotron scan of Ophiocoma echinata arm segments. Cropped for import and reconstruction.
Ophiocoma echinata 2
Image stack generated by synchrotron scan of Ophiocoma echinata arm segments. Cropped for import and reconstruction.
Ophiocoma echinata 3
Image stack generated by synchrotron scan of Ophiocoma echinata arm spine. Cropped for import and reconstruction.
Ophiocoma pumila 1
Image stack generated by synchrotron scan of Ophiocoma pumila arm segments. Cropped for import and reconstruction.
Ophiocoma pumila 2
Image stack generated by synchrotron scan of Ophiocoma pumila arm segments. Cropped for import and reconstruction.
Ophiocoma pumila 3
Image stack generated by synchrotron scan of Ophiocoma pumila pair of arm spines. Cropped for import and reconstruction.
Ophiocoma wendtii 1
Image stack generated by synchrotron scan of Ophiocoma wendtii arm segments. Cropped for import and reconstruction.
Ophiocoma wendtii 2
Image stack generated by synchrotron scan of Ophiocoma wendtii arm segments. Cropped for import and reconstruction.
Ophiocoma wendtii 3
Image stack generated by synchrotron scan of Ophiocoma wendtii arm segments. Cropped for import and reconstruction.