In a general sense, form and function go hand in hand; an understanding of anatomy enhances the understanding of physiology. Fish can be classified broadly by their feeding habits into the well-known classes of detritivores, herbivores, omnivores, and carnivores. Within each category, organisms can be thought of as either euryphagous (eating a great variety of foods), stenophagous (eating a limited variety of foods), or monophagous (eating only one type of food) (Moyle and Cech 1982). The majority of fish targeted for aquaculture are either euryphagous carnivores (such as salmon, basses, breams, halibut, turbot, flounders, and groupers), euryphagous omnivores (such as channel catfish and tilapia), or euryphagous herbivores (such as some carp and milkfish). While exceptions occur, the gross anatomy is often somewhat similar within each class but different between classes (Fig. 7.1; see also Section 7.7). Figures 7.2–7.8 illustrate the differences and similarities among the classes.
At first glance, carnivorous “flatfish” (Fig. 7.2) appear to be somewhat dif- ferent from carnivorous “round” fish (Figs. 7.3–7.5); however, these distinc- tions are due largely to the different shape of the body cavities in each form.
In reality, the digestive tracts of halibut (Hippoglossus stenolepis;Fig. 7.2) and sablefish (Anoplopoma fimbria; Fig. 7.3) are more similar than is the digestive tract of sablefish to that of either Atlantic salmon (Salmo salar; Fig. 7.4) or lingcod (Ophiodon elongatus; Fig. 7.5). Both sablefish and halibut have sim- ilar feeding habits and occur in the same areas deep in the north Pacific Ocean.
Pacific halibut (Fig. 7.2), sablefish (Fig. 7.3), Atlantic salmon (Fig. 7.4), and lingcod (Fig. 7.5) are all carnivores. Displayed (in Figs. 7.2–7.5) are the dentition (teeth and gill rakers), mouth, eyes, nasal pits, gills, tongue, esophagus, esophageal (also called cardiac) sphincter, stomach, pylorus, pyloric ceca, intestine (upper and lower), gall bladder, spleen, kidney, and liver. Figure 7.6 is of a channel catfish (Ictalurus punctatus), and Fig. 7.7 is a Nile tilapia (Sarotherodon niloticus), both omnivores. Catfish prefer animal sources of food, while tilapia typically eat plant material and detritus in the
FIG. 7.1
Diagrammatic representation of typical digestive configurations. (a) Euryphagous carnivore with a y-shaped stomach (salmon, trout, lingcod, sablefish, and halibut).
(b) Euryphagous omnivore emphasizing animal sources of food; pouched stomach or intestinal sac (catfish and tilapia). (c) Euryphagous omnivore emphasizing plant sources of food; stomach absent (carp and goldfish). (d) Stenophagous planktivore;
tabular stomach with muscular gizzard (milkfish). From Smith (1989).
wild (though it will aggressively feed on pellets made with animal products, and its morbid tankmates in culture). Both have a pouch-shaped stomach, no pyloric ceca, and a long intestine; otherwise the gross anatomy is similar to that of carnivores. Tilapia consume a great deal of algae, which can be resistant to digestion unless an extremely low stomach pH (a pH of about 2–3) is available to rupture the cell walls (Smith 1989). Tilapia also have a very long intestine, which may compensate for the lack of ceca. Figure 7.8 shows the common carp, an herbivore. The carp (Cyprinus carpio) lacks a stomach and pyloric ceca; however, the length of its intestine is very long compared to that of carnivores.
In the following sections, each area of the digestive tract is examined in more detail. Refer to Figs. 7.1–7.8 as well as to the figures associated with each organ system. Many of the figures in the following sections contain histology
(A) Dissection of a halibut showing the digestive tract. (a) Oral cavity, (b) gill arches with teeth-like rakers, (c) liver, (d) gallbladder, (e) pyloric ceca, (f) upper or small intestine, (g) lower or large intestine, (h) anus, and (i) kidney. (B) Digestive tract removed, showing (a) oral cavity, (b) liver, (c) gallbladder, (d) spleen, (e) esophagus, (f) stomach, (g) pylorus, (h) pyloric ceca, (i) upper or small intestine, (j) lower or large intestine, (k) anus, and (l) kidney. Photographs by Michael Rust.
(A) Dissection of a sablefish showing the digestive tract. (a) Gill arches, (b) liver, (c) gallbladder, (d) pyloric ceca, (e) upper or small intestine, (f, g) lower or large intestine, (h) anus, (i) ovaries, and (j) kidney. (B) Digestive tract removed.
(a) Oral/pharyngeal cavity, (b) pharyngeal tooth plate, (c) esophagus, (d) stomach, (e) pylorus, (f) pyloric ceca, (g) gallbladder, (h) spleen (hard to see), (i) liver, (j) upper or small intestine, (k) lower or large intestine, (l) anus, (m) ovaries, and (n) kidney. Photographs by Michael Rust.
372
(A) Dissection of an Atlantic salmon. (a) Oral cavity, (b) pharynx, (c) liver,
(d) gallbladder, (e) pyloric ceca, (f) upper or small intestine, (g) spleen, (h) lower or large intestine, and (i) anus. (B) Digestive tract removed. (a) Esophagus, (b) stomach near the esophagal sphincter, (c) pyloric ceca, (d) liver, (e) gallbladder; (f) upper or small intestine, (g) spleen, (h) lower or large intestine, (i) anus, (j) kidney, and (k) ovaries. Photographs by Michael Rust.
373
FIG. 7.5
(A) Dissection of the anterior portion of a juvenile lingcod. (a) Jaw teeth, (b) gill arches, (c) liver, (d) stomach, and (e) pyloric ceca (somewhat behind liver). (B) Same as A, with the liver removed. (a) Oral cavity, (b) gill arches, (c) cardiac stomach, (d) fundic stomach, (e) pyloric ceca, (f) upper or small intestine, (g) gallbladder, (h) spleen, (i) lower or large intestine, and (j) anus. Photographs by Michael Rust.
FIG. 7.6
(A) Dissection of a channel catfish showing the digestive tract. (a) Gills, (b) liver, (c) gallbladder, (d) swimbladder, (e) stomach, (f) esophagus, (g) upper or small intestine, (h) lower or large intestine, (i) adipose (fat) tissue, (j) ovary, and (k) kidney.
(B) Channel catfish with digestive tract removed. (a) Gills, (b) liver, (c) gallbladder, (d) spleen, (e) stomach, (f) esophagus, (g) upper or small intestine, (h) lower or large intestine, (i) swimbladder, (j) ovary, and (k) kidney. Photographs by Michael Rust.
FIG. 7.7
(A) Dissection of a Nile tilapia with the intestinal tract in place. (a) Liver and (b) coiled gut. (B) Dissection of Nile tilapia with the intestinal tract removed.
(a) Kidney, (b) ovary, (c) esophagus, (d) stomach, (e) liver, (f) gallbladder, (g) spleen, and (h) intestine. Photographs by Michael Rust.
FIG. 7.8
(A) Dissection of the common carp. (a) Swimbladder, (b) liver, (c) spleen, (d) ovary, and (e) coiled gut. (B) Digestive tract removed. (a) Swimbladder, (b) ovary, (c) liver (under), (d) esophagus, (e) spleen, (f) intestine, and (g) pancreatic tissue (around the gut in several locations). Photographs by Michael Rust.
photographs chosen to show common structures in fine scale. The histol- ogy sections were produced by fixing the tissue or organ, then embedding it in a paraffin-based medium, and cutting thin (about 4- to 5-μm) sections (slices) of tissue once the medium hardened. These sections were mounted on a microscope slide, the paraffin-based medium was removed, and the section stained so that the cells and tissues could be seen. The stains used in the histological photographs in this chapter were hematoxylin and eosin, the most common stains used in animal histology. Hematoxylin stains acidic structures a purplish blue. Structures that contain acids (e.g., RNA or DNA), such as the endoplasmic reticulum and nuclei, turn blue, as do gastric cells and glands. Eosin stains basic structures various shades of red to pink. Most cytoplasmic proteins are basic and stain red or pink. Cells that contain huge amounts of lipid, such as adipose tissue, do not stain very well and show up as clear or white because the embedding process extracts the lipids.
Many of the tissues that are important in nutritional physiology con- tain an epithelium of columnar-shaped cells supported by a lamina propria and/or other subsurface cell layers. The epithelium cells interact with the environment or the lumen of an organ and include those that sense the environment (chemoreceptive cells), those that take up nutrients (entero- cytes), and those that secrete mucus, enzymes, and/or other compounds.
For more detailed information on general histology and cellular physiol- ogy of fish, the reader is referred to Hibiya (1982). In addition, several species-specific histological atlases cover fish histology extensively. Examples include those by Anderson and Mitchum (1974a) for trout, Grizzle and Rogers (1976) for catfish, Gorman (1982) for striped bass, Yasutake and Wales (1983) for salmon, Bell (1986) for sablefish, and Morrison (1993) for larval cod. All are excellent sources of histological/anatomical information on fish.
7.3