WOURMS, J P; Clemson University, SC: The Abdominal Gut Appendage, An Unique Feature of Mesopelagic Fish Larvae

The abdominal gut appendage (AGA) occurs in some mesopelagic fish larvae within the Orders Stomiiformes. Ophidiiformes, Myctophiformes, and Pleuronectiformes. It is a long, thin, cylindrical extension of the ventral body wall, coelom, blood vascular system, and hindgut. It arises via differential growth and can attain 5X the body length. Within the body, the foregut is tubular and may have a typhlosole-like ridge. In contrast, the hindgut intestinal epithelium of the AGA is organized into longitudinally oriented, scroll-like sheets that extend from the gut wall into the lumen similar to a spiral valve. Thus, absorptive surface area is greatly increased. In Heterophotus, longitudinally oriented smooth muscle is associated with the exterior surface of the hindgut. A large blood vessel in contact with the gut traverses the AGA. Cells with pleated apical surfaces form a series of 5um wide bands that extend along the exterior of the AGA. Fine filaments fill their cytoplasm. In Heterophotus intestinal tissue protrudes from the terminal gut orifice at the distal tip of the AGA and forms prototypic trophotaeniae. TEM of the AGA hindgut in specially processed museum specimens reveals tall columnar cells with basal nuclei, a well-developed brush border of apical microvilli and sometimes cilia, evidence of endocytosis, an extensive and highly ordered endoplasmic reticulum, many large vacuoles with electron lucent content, and many �empty� vacuoles. These cells appear to function in absorption. (Supported by the NSF)