Digestive functions in sirenians

Sirenians, the only aquatic herbivorous mammals, include 2 families; Dugongidae as dugong (Dugong dugon) and Trichechidae as West Indian manatee (Trichechus manatus), Amazonian manatee (T. inunguis) and West African manatee (T. senegalensis). The previous works have concentrated on the digestive functions in sirenians that have been supposed to be specific and different from marine carnivorous mammals and terrestrial herbivores. This paper reviews previous works and gives a perspective for further studies on digestive functions in sirenians by focusing on the following three topics: (1) The anatomy of the digestive tract, (2) Species and their nutritive values of aquatic plants on which the sirenians feed (3) Researches regarding to digestion. (1) The sirenians are non-ruminant herbivores. Their stomach is simple sac-like and is characterized by a unique ""cardiac gland"" which contains the digestive enzymes and acid secreting cells. No one cannot explain the difference that the cardiac gland forms like a ball in dugongs and a finger in manatees. The intestine reaches to 9 to 16 times as long as the body length when sirenians becomes adult. The length of large intestine in adult dugong becomes twice as long as small intestine, whereas that of large intestine in adult manatees becomes to be same length as small intestine. There is a large cecum with a single diverticulum in the dugongs and paired diverticula in manatees. Based on anatomy and morphology of the digestive tract, sirenians belong to the hindgut digesters such as Perissodactyla, Proboscidean, Hyracoidea, Rodentia and the green sea turtle in Cryptodira. (2) The dugong feeds exclusively on marine phanerogams of 18 species, belonging to 4 families: Cymodoceae, Hydrocharitaceae, Potamogetonaceae and Zosteraceae. Except for the Amazonian manatee which only lives in fresh water, manatees feed on many aquatic plants in marine to freshwater environments. It has been reported that the West Indian manatees feed on over 60 species of aquatic plants. The seagrasses on which the dugong preferably feeds have relatively lower crude protein, higher crude fiber and lower calorific content than those of the major plants on which terrestrial herbivores primarily feed. On the other hand, the aquatic plants on which the manatees feed have similar crude protein, higher crude fiber and lower calorific content as the above