A photosensory structure in the brain of the systellomatophoran gastropod <i>Peronia verruculata</i>

Most animals detect ambient light using their cephalic eyes as photosensory organs. However, some animals have different types of photosensors in other parts of their body. The marine gastropod Peronia verruculata possesses several types of extraocular photosensors such as dorsal eyes, dermal photoreceptors and brain photosensory neurons. In the present study, we identified a pair of follicle-shaped structures expressing Gq-rhodopsin in the lateral lobe of the brain in P. verruculata. This structure had numerous microvilli and a few cilia in its interior, which is reminiscent of the follicle gland in the lateral lobe of the brain of the pond snail Lymnaea. Retinal binding protein and retinochrome were localized to the cell bodies of the neurons that constitute this structure. Photoresponses were recorded in an isolated brain by extracellular recording, and the spike frequency increased in a light intensity-dependent manner. We thus named this structure the follicle photoreceptive organ (FPO). We also found that the FPO was positioned close to the optic nerve projecting from the stalk eye and had nerve connections with the optic nerve. We discuss our findings in the context of the epistellar body of octopus and the parolfactory vesicles of squid, as well as the follicle gland of Lymnaea.