Anatomy and Histology of the Spiral Valve Intestine in Juvenile Australian Lungfish, Neoceratodus forsteri

The Australian lungfish, Neoceratodus forsteri, is the only vertebrate that possesses a complete spiral valve intestine with pre-pyloric coiling. This study describes the anatomy and histology of the spiral valve intestine in juvenile N. forsteri and compares it to a previous study of adult N. forsteri, thus providing a broader picture and better understanding of the intestine of the Australian lungfish. Not surprisingly, most features of the spiral valve intestine in juvenile and adult N. forsteri are similar. However, our study goes further to show that, unlike most other vertebrates, the stomach (pre-pyloris) is non-distensible (lacks rugae). Rugae are confined to the post-pyloric duodenum. The epithelium of the pyloric fold, between foregut and midgut, is ciliated and the presence of lymphoid tissue in the pyloric fold implies the involvement of this region in the immune system. Lymphoid tissue is also present around the posterior spleen in the medial axis, which indicates a broader gut-associated lymphoid tissue (GALT) in juvenile Neoceratodus than has been previously recognized in adult Neoceratodus. This study also found some node-like structures in the epithelium of the mucosal tissue, which resemble the Peyer's patches of other more advanced vertebrates. Furthermore, a previously unreported parasite was found in the spleen encased in fibrous tissue, indicating an immune response had been mounted by the host against it. These latter observations suggest that a thorough investigation of GALT in Neoceratodus is warranted.