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.  Like other marine mammals, phocids swallow their fish whole. The fish travel down the esophagus to the stomach. Phocids like otariids, have a single stomach (in contrast with cetaceans) and it is lined with thick ridges (rugae) which helps with the grinding function of the food.

 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. It is difficult to distinguish the jejunum an ileum are difficult to distinguish grossly. Most of the absorption of nutrients occurs in the first quarter of the small intestine. A vestigial cecum can be found at the junction between the small and large intestine and from the large intestine the waste finally reaches the colon and rectum and is excreted from the anus. All pinnipeds have very long small intestines, especially elephant seals which can have small intestines about 25 times their body length. The function of the long small intestine may be related to diet but is not fully understood.

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 more about digestive anatomy see Rommel, S. and Lowenstine, L. (2001).

 Detailed annotated images of the digestive system in two phocids, the harbor seal and elephant seal, are shown below.  CLICK on an image to see an enlarged view.