The Ultrastructure of the Spectacle, Cornea and Conjunctiva of the Short-Nosed Sea Snake Aipysurus apraefrontalis (Elapidae, Squamata) With Particular Reference to the Sub-Spectacular Space and Its Contents.

The eyes of snakes possess an outer protective spectacle, continuous with the dermis, and an inner cornea, continuous with the scleral eyecup, that encloses a fluid-filled space. While the spectacle and cornea provide transparency and contribute to the refractive power of the eye, little is known of their ultrastructure and role in maintaining the contents of the sub-spectacular space. Using micro computed tomography, light microscopy and transmission electron microscopy, this study reveals that the conjunctival portions of the corneal epithelial and spectacular endothelial lining of the short-nosed sea snake Aipysurus apraefrontalis possess high densities of microvilli to increase the surface area and aid in intra- and extracellular movement of nutrients and waste products, and at least two types of vesicles that contain what appear to be mucin fibrils and actin filaments, respectively, that may play a role in secretion of their contents into the sub-spectacular space. The discovery of several ducts in the conjunctiva suggests that the Harderian gland may also deliver (non-lipid-bearing) fluid and granules to this space via several routes. The likely functions of the conjunctival secretory tissue are discussed in the context of maintaining the fluid within the sub-spectacular space and the need to assist in the control of smooth eye movements beneath the immobile spectacle.