Morphological diversity of the gastrointestinal tract in fishes

The anatomy of the gastrointestinal tract of fishes follows the same basic plan as in other vertebrates with a degree of variation reflected in phylogeny and ontogeny, diet, and environment. Morphological studies provide us with a context for understanding the spatial organization and relationship of physiological and biochemical data, and the molecular machinery that is rapidly being elucidated through molecular techniques directed at the genome, transcriptome, and proteome. Morphological data are also key to understanding fish nutrition in ecology and aquaculture, and during development as well as mechanisms for physiological adaptations to a changing environment. A number of the multifunctional roles of the fish gut that are discussed in the following chapters (e.g. respiration and ion regulation) of this volume incorporate distinctive morphological features that will be highlighted in this chapter. The stomach represents a significant vertebrate innovation and is also the most highly diversified region of the gut, yet it has undergone a number of independent secondary losses with stomachless fishes accounting for approximately 20% of species. The diversity of fishes makes the identification of the causes of stomach loss elusive. An updated survey is given.