Digestion in Teleost Fishes ( OVERVIEW)

Definition of the Gut and its Subdivisions

The gut is a tubular structure beginning at the mouth and ending at the anus. It is commonly divided into four parts. The most anterior part, the head gut, is most often considered in terms of its two components, the oral (buccal) and gill (branchial, pharyngeal) cavities. The foregut begins at the posterior edge of the gills and includes the oesophagus, the stomach, and the pylorus. In fish, such as the cyprinus, which lack both a stomach and pylorus, the foregut consists of the oesophagus and an intestine anterior to the opening of the bile duct. This posterior demarcation is arbitrary and primarily for convenience during gross dissection and may have little relation to the functional aspects. The midgut includes the intestine posterior to the pylorus, often with no distinct demarcation posteriorly between it and the hindgut. The midgut often includes a variable number of pyloric caecae (pyloric appendages) near the pylorus, although pyloric caecae are always absent in fishes which lack stomachs. The midgut is always the longest portion of the gut and ma be coiled into complicated loops (often characteristic for each species) when longer than the visceral cavity. In some fish, the beginning of the hindgut is marked by an increase in diameter of the gut. The posterior end of the hindgut is the anus. Only rarely is there a hindgut caecum in fish comparable to that found in mammals. A cloaca (a chamber common to anal and urogenital openings and formed from infolded body wall) never occurs in teleost fish, except the Dipnoi, although it is universal in sharks and rays.

Evolution and Ontogeny of the Digestive Tract

The gut of protochordates consists of a simple, straight tube through which food is propelled by ciliary action. An early elaboration of the gut is seen in lampreys where an infolding (typhlosole) of the gut wall presumably increases the absorptive area of the gut. A similar, but spiral, infolding of the hindgut occurs in sharks, rays, and the coelocanth (Latimeria) in the form of the spiral valve (spiral intestine). The gut wall in lampreys also contains diagonal muscle fibres, although true peristalsis (travelling wave of contraction) is thought not to occur. Teleost fish have a gut which is typical of the higher vertebrates in many respects, although the midgut villi (absorptive papilli) of mammals are absent in fish.
The gut forms very early during embryological development (ontogeny) and shows some of the same stages of development as in the evolution of the vertebrate gut, some larval fish having portions of their gut which are ciliated, for example. The general character and even the length of the gut may change during development. The gut appears to shorten, for example, in fish in which the larval stage is herbivorous and the adult stage is carnivorous. In other fish the gut length remains relatively constant in proportion to body size throughout life.

Generalizations

A number of generalizations about the gut of fishes have been attempted, many of them extrapolated from terrestrial vertebrates. The commonest of these, the observation that herbivores have longer guts than carnivores, appears only partially true in fish. While this may be true in limited groups of fish, it is not universal in teleosts as a whole. Gut lengths have been listed as 0.2-2.5, 0.6-8.0 and 0.8-15.0 times body length in carnivores, omnivores, and herbivores, respectively. Thus, the longest guts are found in herbivores, but not all herbivores have long guts; i.e., the gut lengths of some herbivores are shorter than those of some carnivores. Part of the explanation lies in the fact that many fish eat a variety of food, sometimes ingested with considerable indigestible material (e.g. mud) which often influences gut length. The size of the food particles - from submicroscopic plankton to whole fish - may also influence gut configuration.
One generalization so far appears to have no exception. In fishes having no stomachs, no acid phase of digestion occurs, even when the midgut develops stomach-like pouches anteriorly. Although gut tissues exhibit great versatility, the midgut appears unable (or does not need) to duplicate the stomach functions.
In general, most studies relating food habits to gut morphology show considerable relationship between the two. However, the gut also retains considerable reserve ability to respond to new foods, new environments, and new opportunities. This versatility has been demonstrated in a number of cases in which a single genus has adapted to new niches and evolved whole new modes of feeding and digestion to utilize otherwise unexploited food resources and done so over rather short evolutionary periods of time.
At the same time, there are usually severe constraints on adaptations to new food. As long as swimming continues to be important to a fish's lifestyle, any major change in body shape, such as a bulging visceral mass resulting from enlarging the stomach or lengthening the midgut, must extract a penalty in terms of increased effort needed for swimming. Feeding mechanisms must not interfere with the respiratory functions of the gills and vice versa. All in all, "packaged" so that any major change in the digestive system would call for major compromises in many other systems. Perhaps the best generalization is that teleost fish maintain an intimate relationship between the form and function of their gut and their food resource. In the final analysis, all of the other life processes continue to function only when sufficient materials and energy are obtained and assimilated via the gut.