and digestive systems. Feeding mechanisms are highly varied. Functionally, the mouth is for prehension of food, not chewing or predigestion. The lining of the oral cavity is thick epithelium and dermis bound to the bone or muscle (Roberts and Ellis 2012). The jaws are comprised of several fused bones and can be complex in pattern. Some fish have ornate protruding maxillary rostrums (e.g. paddlefish, Polyodontidae) while others, like the slingjaw wrasse (Epibulus insidiator), have a telescopic mouth, which protrudes for prey capture (Burgess et al. 2011). A few species have pharyngeal jaws, e.g. moray eels (Muraenidae) (Figure A1.8). Fish tongues have limited mobility and are simply used to propel food into the esophagus. In some species, there are teeth on the tongue to help hold prey. Buccal glands produce mucus; there are no salivary glands (Stoskopf 1993). One of the most common causes of oral masses in fish is thyroid hyperplasia (goiter), which typically presents in bony fish as a mass along the gill arches.
Dentition varies depending on feeding ecology. Teeth are acrodont with ankyloses or a fibrous attachment to the jaw (Teaford et al. 2007). Basic tooth types are canine (Figure A1.9a), molariform, incisor, or plate‐like. Dental plates are seen in lungfish (Dipnoi) and gar (Lepisosteidae) (Fishbeck and Sebastiani 2012). Pikes (Esocidae) have hinged teeth that are pointed backward; they bend during swallowing of prey and are erected again by elastic ligaments (Berkovitz and Shellis 2016). In parrotfish (Scarinae) and pufferfish (Tetraodontidae), the front teeth grow continuously, necessitating trimming when not fed hard food items (Roberts‐Sweeney 2016). Some fish lack teeth altogether, e.g. filter feeders, seahorses and pipefish (Syngnathidae), sturgeon (Acipenseridae). Pharyngeal teeth are often present in bony fish for holding, masticating, or grinding foodstuffs (Figure A1.9b). Fish that rely heavily on pharyngeal teeth often do not have a muscular stomach (Gerking 2014).
Figure A1.8 Computed tomography of the skull of a moray eel (Muraenidae) showing the pharyngeal jaws.
The esophagus is a short, muscular tube. While straight in most species, it is important to be aware of differences in position and angularity when gavaging medications or food. The angle into the stomach can be dramatic (Figure A1.10) and a misplaced gavage tube can perforate the esophagus and damage the heart, liver, or swim bladder. In some fishes, the esophagus has blind diverticula (esophageal sacs) lined with calcified, esophageal teeth (Isokawa et al. 1965). In sturgeon (Acipenseridae), the esophagus has folds. The epithelium may have abundant mucus (esophageal) glands, particularly in carnivores, and these may be noted during gastrointestinal endoscopy (Roberts and Ellis 2012).
Figure A1.9 Teeth in a California sheephead (Semicossyphus pulcher) (a) and lateral radiograph of a rainbow parrotfish (Scarus guacamaia) showing pharyngeal teeth (b).
Source: Images courtesy of Catherine Hadfield, National Aquarium.
Gastrointestinal System
The stomach varies in size depending on food items. Some species are agastric, e.g. goldfish (Carassius auratus), common carp and koi (Cyprinus carpio), and zebrafish (Danio rerio). The stomach becomes a grinding organ in sturgeons (Acipenseridae), gizzard shads (Dorosoma spp.), and mullets (Mugilidae) (Helfman et al. 2009). Histologically, the cardia consists of striated muscle and transitions to smooth muscle in the pyloric stomach. The mucosa has numerous mucus glands (Roberts and Ellis 2012). Pyloric caeca are diverticula off the stomach that are present in salmonids and many other teleosts. They increase the absorptive surface area. These organs are dissimilar to the ceca of birds and mammals as they are not fermentative (Buddington and Diamond 2016).
The intestines are variable in length and shape but fairly simple. The colon is minimal or not distinguishable (Roberts and Ellis 2012). In the lungfish (Dipnoi) and sturgeon (Acipenseridae), the intestine is spiral‐shaped, similar to the elasmobranchs (Figure A1.11). A rectum is present (Roberts and Ellis 2012). Gastrointestinal emptying times are highly variable and both temperature‐ and volume‐dependent; more details can be found in Chapter A4.
Pufferfish are unique in being able to inflate their bodies with water or air. Mouthfuls are pumped into the stomach which then expands. The pectoral girdle and head have modifications that function as a pump and the skin is very distensible; ribs are absent to accommodate inflation (Wainwright and Turingan 1997). In some species the skin has spines which stand erect on inflation.
External intestinal and urogenital openings differ among species. In most, there is a separate anus and reproductive opening or urogenital pore (Yanong 2003). In some bony fish, there is a cloaca: a common area where the intestinal, urinary, and gonadal ducts empty, e.g. lungfish and coelacanths (Latimeria spp.).
Liver and Gallbladder
The liver is a fairly large organ (single or bilobed) and predominantly on the left side of bony fish. It is typically orange to brown. If the liver is tan or yellow, it can indicate fatty infiltration, which may be a normal seasonal change or related to a high‐fat diet. The liver is separated from the pericardial cavity by a septum (Roberts and Ellis 2012). A gallbladder is present in most species, with some exceptions, e.g. burbot (Lota lota) (Dutta and Datta‐Mushi 1996). The gallbladder is located within or between the liver lobes in most fish, but lies in the caudal coelom in some, e.g. rockfish (Sebastes spp.). The gallbladder can become distended with anorexia (Stoskopf 1993). The bile duct enters at the stomach or small intestine. Most bile is made of bile salts and taurine conjugates of bile acids, except in carp where the principal bile salt is in the form of alcohol sulfates (Stoskopf 1993).
Figure A1.10 Angling of the esophagus seen at necropsy of a lookdown (Selene vomer), shown by the dashed red line.
Source: Image courtesy of Carlos Rodriguez, Disney’s Animals, Science and Environment.
Respiratory System
Respiration does not involve inspiration and expiration but rather a continuous flow over the gill epithelium. Ventilatory rate reflects the muscular/opercular pumping of water over the epithelium. Gills are the primary organ for respiratory exchange in most fish; they are covered by an operculum or skin with gill slits. There are two sets of gills bilaterally which are made up of gill arches (holobranchs) with paired rows of primary gill filaments (hemibranchs) (Figure A1.12). Each primary filament has perpendicular secondary filaments. Most bony fish have four gill arches. Some gill arches also have gill rakers that function as a sieve to collect food from the water. Normal gill appearance is a uniform, dark red. In cases of anemia, the color fades to pink, light pink, or even white. This color change is also seen after death, along with autolysis. Methemoglobinemia may cause a brown discoloration. The epithelium of the gill is thin to allow gas