The role of dermal photoreceptors during the sea lamprey (Petromyzon marinus) spawning migration.

Light avoidance in larval lampreys is mediated by dermal photoreceptors located in the tail. These photoreceptors continue to function in adults, but they seem redundant because post-metamorphic lampreys possess well-developed eyes. This study examined the role of dermal photoreceptors in adult sea lampreys by testing whether temperature-induced changes in refuge-seeking behavior are mediated by a reduction in dermal photosensitivity. In a lighted arena containing a single shaded refuge platform, lampreys at 22 degrees C displayed five times less search activity and were less likely to attach beneath the refuge platform than lampreys at either 7 or 15 degrees C. A behavioral assay for tail photosensitivity (locomotor response to tail illumination) revealed a corresponding reduction in dermal photosensitivity at 22 degrees C. Moreover, the responses to head illumination (eyes and pineal) did not correspond with the observed light avoidance behaviors. The head was less responsive to light than the tail and was not influenced by temperature. These results provide strong evidence that the dermal photoreceptors continue to mediate light avoidance in adult lampreys, even though adults possess fully functional eyes. The fact that the eyes apparently do not take on this role suggests that there is functional specialization between these two light sensing systems.