red muscle stores more lipids than white (Sheridan 1988).
Fat deposits fluctuate seasonally in wild animals but are less likely to change across the year in fish under human care. Over‐conditioning of fish is a common problem under human care, particularly in large, mixed species habitats. Appropriate diets, targeted feeding, and suitable fasting periods can help in those situations.
Ocular Anatomy
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.
Auditory Anatomy
The acoustic organs provide information on acoustical stimuli, gravitational forces, and linear and angular accelerations of the head. Fish make use of a labyrinth that includes semicircular canals, ampullae of the inner ear, and otoliths or otoconia (discussed in more detail under auditory anatomy of elasmobranchs) (Hoar et al. 1983). Otoliths are calcified stones that overlay sensory epithelium and are surrounded by endolymph, which facilitates their movement for sound perception and equilibrium (Roberts and Ellis 2012). In most ray‐finned fish, there is a single otolith in each otic chamber, but there may be several. Otoliths can be used to age and identify bony fish. Pathology of the inner ear can lead to loss of equilibrium.
The swim bladder and other gas cavities can conduct sound using bones known as the Weberian apparatus (or ossicles) in several species, e.g. carp (Cyprinus carpio), bowfin (Amia calva), and tetras (Characidae) (Schulz‐Mirbach et al. 2012). Fish that show a direct or indirect connection from the swim bladder to the perilymphatic system of the inner ear can perceive higher frequency sounds (e.g. up to 4000 Hz in carp and catfish, Cyprinidae and Ictaluridae) than those without (up to 520 Hz in cod, Gadiformes) (Roberts and Ellis 2012).
There are three methods of sound production: stridulatory (teeth, fins, spines, and bones), hydrodynamic (swimming movements), and muscle vibrations around the swim bladder (Hoar et al. 1983).
From a clinical perspective, it is difficult to evaluate these structures. Swim bladders and otoliths can be identified on radiography, computed tomography, or MRI (Figure A1.7). Animals with swim bladder disease may show reduced functional hearing. In catfish, swim bladder damage decreased the hearing frequency range (Kleerekoper and Roggenkamp 1959).
Olfactory and Gustatory Anatomy
Water‐soluble chemical compounds are detected by olfaction (smell) or gustation (taste). For olfaction, teleosts have paired nares on the rostrum lined with olfactory epithelium. Hagfish and lampreys (Agnatha) are unique with only a single nare (Evans et al. 2004). Water passing through the nares stimulates receptors in the olfactory tracts, which send signals to the olfactory bulbs within the forebrain (telencephalon) (Roberts and Ellis 2012). Some teleosts have nasal sacs and accessory nasal sacs that actively pump water over the epithelium (coinciding with opercular movement) (Hara 1975). Some rely heavily on olfaction and have large olfactory pits extending from the rostrum to the eye, e.g. moray eels and true eels (Anguilliformes). Others rely on visual cues and lack nasal sacs, e.g. pufferfish (Tetraodontidae) (Hara 1975). In some species, males have a larger olfactory capacity (Hara 1975).
Figure A1.7 Otoliths visible on lateral radiograph of a red drum (Sciaenops ocellatus).
Source: Image courtesy of Shane Boylan, South Carolina Aquarium.
Taste buds are epidermal and can be found in the oral cavity, lips, head, barbels, body wall, fins, and esophagus (Evans et al. 2004; Roberts and Ellis 2012). In some fish, the external taste buds outnumber intraoral taste buds by as much as 90% (Hara 1975). Fish have up to three anatomically different taste buds (Reutter et al. 1974). The taste cells are receptive to amino, nucleic, and organic acids (Oike et al. 2007). Fish do have aversive and preferential responses to some chemical stimuli but extensive research on gustatory preferences has not been done (Oike et al. 2007).
Oral/Pharyngeal Cavity
The oral cavity is shared by the