Changes in the Digestive Tract and Feeding Behavior of Anuran Amphibians during Metamorphosis

Abstract

During metamorphic climax, anuran amphibians undergo dramatic changes as they pass from aquatic to terrestrial life. In particular, the digestive tract is completely modified. The pharyngeal filtering apparatus disappears. Striking transformations take place in the gastric region. The tadpole's glandular sheath regresses and ceases protruding from the gut wall. At the same time, a large permanent stomach is formed to replace the sheath, gastric glands develop, and the muscle layer thickens. The intestine also changes greatly. An overall shortening and rearrangement of its coils occurs. The histological changes that affect the intestine are largely confined to the epithelium. Degeneration occurs in the larval epithelium, while intense proliferation of stem cells results in the development of a new epithelium that is folded. During metamorphosis, the endocrine cells of the digestive tract are also redistributed. Such changes are associated with modifications in feeding behavior. Most tadpoles are microphagous and feed on particles suspended in the water, which they retain on the surface of the gill filters and other buccopharyngeal structures. Anuran tadpoles usually stop feeding at climax because the filtering apparatus degenerates. Moreover, at climax the larval intestinal epithelium is too altered to absorb anything, and the new intestinal epithelium has not yet developed. Juveniles and adults are predators (carnivores or insect eaters). The acquisition of binocular vision permits efficient perception of prey in a terrestrial environment. The new feeding behavior is also associated with the appearance of new enzymes. The secretion of proteolytic enzymes at the pancreas (trypsin) and stomach (pepsin) levels prepares the animals to digest meat. Chitinases are secreted by the stomach as soon as it differentiates and allow the digestion of chitinous insects. Moreover, the new intestinal mucosa is well equipped for the absorption of protein hydrolysis residues. Control of the digestive secretions and feeding behavior in developing anurans is also discussed in the present article.