Meeting Abstract
We study the Pacific Hagfish in an evo-devo framework, with special attention to the first appearance of the vertebrate eye over evolutionary time. Other cyclostomes (lampreys) and all jawed vertebrates share the familiar camera-style eye. The last common ancestor of lamprey and vertebrates also had a photoreceptive pineal gland. Thus the Hagfish, oft-reported to lack a pineal, eyes and vision, are positioned perfectly to understand the appearance of the vertebrate eye over evolutionary time. The eyes of adult hagfish are quite small and buried under a layer of epidermis, leading credible sources to suggest hagfish lack vision altogether and their eyes are pineal-like in several characters. Here we revisit the rarely-examined Pacific Hagfish (Eptatretus stoutii), with special attention to the smallest (≈youngest) individuals we can acquire, and applying contemporary histology methods to reveal novel compelling arguments that hagfish possess almost all features of a vertebrate eye; this codifies hagfish eyes as functional, though degenerating during ontogeny. Three independent opsin antibodies all reveal robust photoreceptors at the outer nuclear layer and indistinguishable from vertebrates. Importantly, young hagfish retinas have three recognizable nuclear layers separated by synapses as detected by multiple stains and antibodies. Thus, several data sets all independently demonstrate that young hagfish have eyes that are not pineal-like, but have organization at the tissue- and molecular-level shared with lampreys and all jawed vertebrates. The data form a new baseline for interpreting evolution of rods from cones. In sum, the retina of Pacific Hagfish degenerates with ontogeny and adult tissues have been mis-interpreted to implicate that the eyes of hagfish never develop and never evolved; our data dramatically revise our understanding of the early vertebrate eye evolution.
