The glow of the night: The tapetum lucidum as a co‐adaptation for the inverted retina

• Published in: BioEssays 2022-10, Vol.44 (10), p.e2200003-n/a
• Main Authors: Vee, Samantha, Barclay, Gerald, Lents, Nathan H.
• Format: Article
• Language: English
• Subjects: Adaptation; Arthropods; cephalopods; Chemical composition; evolution; Photons; Photoreception; Photoreceptors; Photosensitivity; Retina; tapetum lucidum; Vertebrates
• ISSN: 0265-9247; 1521-1878
• DOI: 10.1002/bies.202200003

The vertebrate retina is said to be inverted because the photoreceptors are oriented in the posterior direction and are thus unable to maximize photodetection under conditions of low illumination. The tapetum lucidum is a photoreflective structure located posterior to the photoreceptors in the eyes of some fish and terrestrial animals. The tapetum reflects light forward, giving incident photons a “second chance” to collide with a photoreceptor, substantially enhancing retinal photosensitivity in dim light. Across vertebrates (and arthropods), there are a wide variety of tapeta that vary in structure, chemical composition, and even tissue architecture, indicating repeated convergent evolution. To date, the tapetum has not been observed in any cephalopod, however, which also possess a camera‐like eye, but with the retinal photoreceptors oriented in the anterior direction. We therefore hypothesize that the tapetum lucidum is a compensatory adaptation for the suboptimal design of the inverted retina of vertebrates. The light‐reflecting tapetum lucidum exhibits striking structural, chemical, and histological homoplasy in vertebrates, having evolved separately in numerous lineages. Because the tapetum has not evolved in any cephalopods, we hypothesize that this structure is a compensatory co‐adaptation for the sub‐optimal inverted orientation of the photoreceptors in the vertebrate retina.