Digestive System

The digestive system of marine mammals consists of the mouth, esophagus, stomach, small and large intestine, colon, and rectum. Accessory organs are also critical for digestive function including the liver and pancreas.

Most marine mammals (except sea otters) don’t chew their food. Rather they pluck it out of the water and swallow it whole. The fish travel down the esophagus to the stomach. In cetaceans, the stomach has multiple compartments. Stomach anatomy in cetaceans is diverse with the number of chambers varying across species. There is also significant diversity in the anatomical names that have been given to these various compartments. In harbor porpoises, the compartment into which the esophagus terminates is called the forestomach or esophageal stomach. It is lined with thick ridges (rugae) and is mainly a storage compartment for food though some protein digestion does occur here. The next chamber is called the fundic or mainstomach and is a dark red in color. This is the chamber where HCl is secreted along with digestive enzymes. The next chamber is the pyloric stomach which is lined with mucus cells and is usually yellow to brown in color. After moving through the pyloric sphincter, the food, now called chyme, moves into the duodenal ampulla which is usually smooth in texture and is often full of bile (brownish fluid) which is secreted into this region from the liver via the hepatopancreatic duct. The chyme next travels to the duodenum which is the most anterior part of the small intestine where it gets mixed with the products of the pancreas including bicarbonate to increase the pH. The chyme then makes its way to the rest of the small intestine which consists of the jejunum and ileum. Most of the absorption of nutrients occurs in the first quarter of the small intestine. The large intestine follows the small intestine and then finally reaches the colon and rectum and waste is excreted from the anus.

During necropsy, it is important to examine each part of the digestive system. Often clues as to the cause of death can be found by looking at whether there is food in the stomach or in the small or large intestine. For example, an apparently healthy animal with a large stomach full of partially digested food might indicate a toxin, such as domoic acid, could be responsible for the animal’s death. You should also examine the digestive system for parasites and note whether there are extensive worms in the stomach for example. High parasite loads can sometimes indicate the animal was sick for weeks or months prior to death.

For a full review of stomach cetacean anatomy see Mead, 2007.

Detailed annotated images of the digestive system of a harbor porpoise are shown below.  CLICK on an image to see an enlarged view.