Anatomy of finfish and shellfish (2+1)

Unit 4 - Gastro Intestinal Tract OR Digestive system and associated digestive glands

4.1.2. Stomach
Frequently in literature the fishes are classified as stomach or stomach less fishes. 85% telecast have stomach 15% had no stomach.
The stomach too shows various adaptations one of which is shape. In carnivore the stomach typically is straight, elongate for example in gars (lepisosteus), bowfins (Amia), Pikes (Esox) and barracudas (sphyraena).
In omnivorous species, the stomach is most often sac-shaped. A very special adaptation is the modification of the stomach into a grinding organ as in the sturgeons (Acipenser) gizzard shads (Dorosoma) and mullets (Mugil). Here the stomach is reduced in overall size but its wall greatly thickened and muscularized. The lining too is heavily strengthened with connective tissue and the lumen is very small. The organ is not for storage, mixing and primary digestion but rather a food grinder.
Great dispensability is the adaptation of the stomach in the predatory deep sea swallowers (saccopharyngidae) and gulpers (Eurypharyngidae) enabling these fishes to take relatively huge prey.
A remarkable modification of the stomach exists in the puffers (Tetradontidae) which can inflate themselves with water or air to assume often on almost globular shape. The adaptive value of this modification of the digestive tract is probably mainly one of defense, for many puffers and porcupine fishes have spines all over the body which can thus be created.
Not all fishes have a stomach that is a portion of digestive tube with a typically acid secretion and a distinctive epithelial lining different from that of the intestine.

In most of the herbivores and phytoplankton feeders the epithelial tissue of the esophagus grades directly into that of the intestine, thus termed stomach less. Though the primary criterian for being without stomach does not seem to be whether a fish is an herbivore or a carnivore but whether accessory adaptations for trituration and fine grinding of food exist either in the form of teeth or a grinding apparatus such as a gizzard. Where stomach exist most pronouncedly (strongly) in carnivores. They are characterized by a low pH and the prominent presence of pepsin (enzyme) among other digestive juices.