Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 22

The eyes of fish vary greatly. There are species that possess a rudimentary eye or eyespot, e.g. hagfish (Myxinidae), and there are eyeless fish such as the cavefish (e.g. Astyanax mexicanus). Fish with particularly large eyes relative to body size, e.g. some squirrelfish (Holocentridae) and rockfish (Sebastes spp.), seem more prone to ocular issues such as gas bubble disease and inflammation.

Fish do not have apposable eyelids, although many have a membrane known as the epidermal conjunctiva that covers the cornea or folds of tissue around the eye (Gelatt 2014). Some bony fish do have static eyelids to protect the eyes, e.g. salmonids (Salmonidae), jacks (Carangidae) (Figure A1.6) (Jurk 2002). Corneas in freshwater fish species are thicker than in saltwater species and some fish have two‐layered corneas (Gelatt 2014). The cornea of green moray eels (Gymnothorax funebris) has a dermal layer (secondary spectacle) and a scleral layer (Trischitta et al. 2013) and it is common to see abnormal lipid deposition in the dermal layer. Relevant microanatomical features include the epidermal conjunctiva, a basement membrane (Bowman's membrane), and the endothelial layer (Descemet's membrane) (Roberts and Ellis 2012). In some species, there is a normal corneal iridescence or pigmentation that is likely associated with glare reduction, e.g. pufferfish (Tetraodontidae) (Gelatt 2014).

Most fish have a fixed pupil so there is no pupillary light reflex, but there are some exceptions e.g. true eels (Anguilla spp.), turbot (Rhombus spp.), flounder (Pleuronectidae), and African lungfish (Protopterus spp.) (Gelatt 2014). The iris can be round, pear‐shaped, elliptical, or slit‐like. Deep sea fish lack an iris (Stoskopf 1993). Amphibious fish such as mudskippers (e.g. Periophthalmus spp.) need to see above and below water and so have a flattened cornea and two pupils in each eye (Colicchia 2007). Suckermouth catfish (Loricariidae) have a modified iris called an “omega iris”, which has a loop at the top that can expand and contract to control light exposure (Douglas and Djamgoz 2012).

Figure A1.6 Eyelid on a crevalle jack (Caranx hippos).

Source: Image courtesy of Carlos Rodriguez, Disney’s Animals, Science and Environment.

Lenses are dense and spherical to compensate for the lack of refraction at the corneas and typically protrude slightly through the iris (Roberts and Ellis 2012; Gelatt 2014). There is no mechanical separation of vitreous and aqueous humor as in other vertebrates. Ciliary bodies are either absent or rudimentary and ciliary processes are absent; vitreal fluid production is not understood (Gelatt 2014).

The sclera is cartilaginous. The orbit is bony and enclosed. In some fish, a tenacular ligament anchors the globe to the orbit. Some species have scleral ossicles, e.g. sturgeon (Acipenseridae) (Gelatt 2014).

The retina varies significantly between species (Ali and Anctil 2012). Rods and cones are present, with more cones in diurnal species. A tapetum lucidum and fovea are present (Ollivier et al. 2004). The European eel (Anguilla anguilla) is the only teleost with intraretinal vascular circulation (Trischitta et al. 2013). In other teleosts, there is an organ with a vascular rete called the choroidal gland, which wraps around the optic nerve and communicates with the pseudobranch (discussed later) (Gelatt 2014). The choroidal gland is important in oxygen secretion and has been implicated in intraocular gas bubble formation (Roberts and Ellis 2012). It is also a potential source of blood loss during enucleation in teleosts.