A Pentaradial and Bilateral Combined Photoreception System in the Sea Cucumber is Mediated by a Blue Light-Sensitive R-Opsin.

Despite lacking a definitive visual organ, sea cucumbers clearly demonstrate an ability to perceive light. This study identifies the anterior oral tentacles, dorsal papillae, and ventral tube feet as the light-sensitive areas of the tropical sea cucumber Holothuria leucospilota. In H. leucospilota adults, only one active-site-containing opsin, specifically a rhabdomeric opsin (r-opsin), is expressed in the light-sensitive areas, thus providing an opportunity to characterize its photoreceptive structures through localization of this single gene. This r-opsin is functionally confirmed to be light-sensitive and capable of activating the Gq pathway. It exhibits a maximum absorption wavelength at 459 nm, which overlaps with the spectrum of blue light that triggers negative phototaxis responses in H. leucospilota, and it also mediates the responses to adjacent wavelengths. The r-opsin-expressing cells are distributed in the oral tentacles, papillae, and tube feet, forming distinct light-sensing structures with specialized morphologies. These structures are interconnected by nerve cords, establishing a comprehensive photoreception system throughout the body, which integrates the diffuse photoreception of pentaradial animals and the anterior photoreception seen in most bilateral animals. This study elucidates a unique sea cucumber photoreception system, shedding light on the last previously unexplored class within Echinodermata and offers evolutionary insights into deuterostome vision.