Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 25

The oral cavity is shared by the respiratory, endocrine, 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.