The Eyes of Hagfishes

Summary

Though probably functional light receptors, hagfish eyes are small, that of Myxine glutinosa only 500 μm diameter, and degenerate. Demonstrated extraocular photoreception may be more important for hagfish behaviour. Eptatretus species eyes are beneath an unpigmented skin patch, but Myxine glutinosa eyes are buried beneath muscle. All hagfishes have only an undifferentiated corneoscleral layer, and extraocular muscles are absent. We found no lens in any hagfish examined. Eptatretus species have a vitreous cavity, with scattered collagen fibrils, some forming dense aggregates. Choroidal capillaries, but not pigment, occur in all species examined. Eptatretus retain a hollow optic cup, but at the margin epithelium and neuroretina are continuous, without extension to ciliary body or iris, both of which are absent. Developmental anomalies are common in peripheral retina in all. The Myxine optic cup has no lumen, the margins meeting at a fibrous plug. Eptatretus species retinas contain photoreceptors, with clear outer segments in the periphery, but few or none in the fundus. Myxine has few, degenerate outer segments, indenting the opposing epithelium. Receptor synapses are sessile. Synaptic bodies, like vertebrate ribbons, occur in Eptatretus, but only simple synapses in Myxine. Myxine optic nerve contains a few hundred thin axons only.