Data from: Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars

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.