Once food is sensed and located, a fish must capture it so it can be in- gested. The oral or buccal cavity (less specifically, the mouth) is the area where food is first consumed by the fish. It extends from the jaw to the esophageal sphincter. The posterior portion of the mouth, near the esopha- gus and gills, is called the pharynx. The structures which aid in capture and retention of food are the taste buds (previously discussed), teeth, gill rakers, tongue, and esophagus (Fig. 7.13). They all work in concert to capture, taste, retain (or reject), and then swallow (or expel) food items.
Mouth structure is highly variable among species and invariably related to feeding habits. Because these structures are easy to observe, and relate to how and what organisms are consumed in the wild, they are a logical place to start when developing a diet for a new species or predicting the appro- priateness of a pellet type (hard or soft, floating or sinking). For example, does an organism possess the appropriate structures to feed on shellfish (then it can crush hard pellets)? Does it normally feed off the bottom (use sinking pellets) or on the surface (use floating or slowly sinking pellets)?
and so on. By looking at the mouth it is easy to tell if the fish is a carnivore or
FIG. 7.13A
FIG. 7.13B (continued)
(A) Mouth of an adult sablefish. (a) Jaw teeth, (b) tooth-like gill rakers, (c) pharyngeal pad with multiple small pharyngeal teeth, and (d) esophageal sphincter. (B) Mouth of a juvenile lingcod, with the opercle removed. (a) Jaw teeth, (b) base of gill arches containing teeth-like rakers, (c) middle gill arch, (d) individual tooth-like gill rakers, (e) gill filaments, (f) olfactory pit, (g) eye, and (h) tongue. Photographs by Michael Rust.
omnivore, which may suggest what types of feedstuffs will be palatable, what potential feeding attractants might work, and what the general proximate composition of the diet is. The size of the mouth and the esophagus can also be used as a general guide for pellet size. While these traits can be used as a general guide, there are also many exceptions, and fish are often adaptable in a culture situation to foods that they would never eat in the wild.
7.4.1. Teeth
Teeth are located in several areas of the oral cavity: on the jaw (maxil- lary and premaxillary bones above and dentaries below), in the mouth (on the bones of the roof and sides of the oral cavity and, sometimes, on the tongue), and on plates in the pharynx (Figs. 7.3B and 7.13A). Tooth-like structures (gill rakers) also occur on the gill arches, especially in preda- tory fish.
The function of the teeth relates to their form (Lagleret al. 1977; Gosline 1985; Motto 1985; Hermanet al. 1992; Peterson and McIntyre 1998). Sharp needle-like teeth (canine or cardiform) aid in grasping, puncturing, and holding prey, sharp-edged cutting teeth (incisors) help to slice prey into smaller bits, and broad, flat teeth (molariform) are used for crushing or grinding. The number and type of teeth relate to the natural feeding habits of the species. The dentition and oral anatomy of fish can also change over ontogeny, as larval, juvenile, and adult fish occupy different feeding niches during different life stages (Nakajima 1979, 1984; Luczkovichet al. 1995).
7.4.2. Gill Structures
The structure and function of gill rakers (Figs. 7.2, 7.5, 7.10, and 7.13) parallel or complement the structure and function of teeth in most respects.
Like teeth, gill raker structure relates to feeding habit (Singh 1976; Wright et al. 1983; Livingston 1987; Guinea and Fernandez 1992; Chaudhuri and Datta 1994). Gill rakers in some species are fine and comb-like (or feather- like) and are used to strain small particles from the water in much the same way that whales use baleen to filter krill (Hamplet al. 1983). The gill rakers on predatory fish are sharp and pointed to hold and puncture prey. Other species have gill arches with molar-like teeth (rakers) used for grinding.
While the gill rakers play a major role in feeding, they also serve to protect the delicate gill filaments located on the other side of the gill arches (see Figs. 7.2–7.8 and 7.13).
7.4.3. Esophagus
The esophagus serves as the transition from the mouth to the rest of the digestive system. Figure 7.13A (and Fig. 7.10) shows the anterior portion of the esophagus and the closed cardiac (or esophageal) sphincter of a sablefish. The portion of the esophagus (visible in Figs. 7.2–7.8), posterior to the sphincter, is continuous with the stomach in gastric fish or the intestine in stomachless fish and is similar histologically and functionally to those
organs (Hibiya 1982). In fact, some authors consider the cardiac sphincter to be the anterior of the stomach, and for all practical purposes this is the case (Smith 1989). The esophagus is highly elastic in predatory fish, expanding to hold the prey in the mouth while it is fed at a more controlled rate into the stomach. Mucus-secreting esophageal glands are located in the anterior portion of the esophagus to help lubricate the alimentary tract, while digestive juice-secreting glands (similar to chief cells or gastric glands in fish with stomachs) are located posterior of the sphincter. Evidence is lacking in fish whether cells anterior to the sphincter secrete any digestive enzymes or substances other than mucus (Smith 1989).
7.5