Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 41

Eye anatomy in elasmobranchs is diverse. Eyelids are usually fixed, but are mobile in some species, e.g. nurse sharks (Ginglymostoma spp.) and catsharks (Cephaloscyllium), and there is a blink reflex. There is a third eyelid or nictitating membrane in some, e.g. requiem sharks (Carcharhinidae). Pupil type and shape are characterizing features of some species. In rays, the upper iris is modified into an operculum pupillare which covers the iris during light adaptation (Figure A1.18). The pupillary light response is highly variable: diurnal sharks exhibit rapid constriction, nocturnal sharks have an intermediate response, and batoids show the slowest response. Dilation can be produced using topical acetylcholine (Kuchnow 1971). The sclera is thick with a cartilaginous layer. The cornea has the same layers as other vertebrates. Many sharks have a partially or totally occlusible tapetum, meaning that melanophores can migrate over the retina and block photophores to adapt to light. Some species have a fixed tapetum, e.g. catsharks and deep‐sea sharks (Gruber 1977). The retina is avascular and there is no choroid gland (Gelatt 2014). Many elasmobranchs are able to pull their globes back into their eye sockets using extraocular muscles (Jurk 2002; Hart and Collin 2015). They also possess an optic pedicle which is a cartilaginous structure connecting the globe to the cranium (Gelatt 2014). The scleral cartilage, optic pedicle, and the size of the optic nerve, vessels, and muscles make enucleation much more challenging than in teleosts.

Figure A1.18 Modified iris of a clearnose skate (Raja eglanteria); the spiracle is visible caudal to the eye.

Source: Image courtesy of Catherine Hadfield, National Aquarium.

In addition to the eyes, the pineal organ/eye (epiphysis) is well‐developed in most elasmobranchs, although absent in some of the electric rays (Torpediniformes). The photoreceptors are located superficially on the dorsal aspect of the chondrocranium (Gruber 1977).