6. DIGESTIVE ANATOMY OF SELECTED FISHES
The gross anatomy of four cultured fishes, having diverse feeding habits are described in this section, emphasizing noteworthy structures which show relationships to feeding and digestion. The line drawings are intended to serve as a general guide for the examination and dissection of actual specimens and are not intended to show definitive details of visceral organs. The four species illustrated were selected primarily for variety of feeding habits and life styles among a dozen or more cultured species with no intent to indicate their importance or representativeness compared to other culture species.
6.1 Rainbow Trout (Salmo gairdneri)
Rainbow trout are largely carnivorous, but show few anatomical specializations for capturing and digesting prey. Teeth are simple and small with no other elaboration of structures to capture, hold, or swallow prey. Salmonids swallow their food whole via a wide oesophagus into a Y-shaped stomach. Many pyloric caecae branch near the pyloric end of the midgut, their numbers often being of taxonomic importance among the various salmonid species. The pancreas is diffusely scattered in the fat and connective tissue around the pyloric caecae and is not readily visible. The gall bladder extends from the middle lobe of the liver and the bile duct can usually be traced from there to the upper midgut in larger specimens. The midgut merges into the hindgut without any particular demarcation.
Other visceral organs include a thin-walled, nearly transparent swim bladder, the kidney just dorsal to that and running the full length of the visceral cavity. The kidney covers the dorsal aorta on the ventral surface of the vertebral column and encloses the posterior vena cava. The urinary ducts can usually be seen on the ventral surface of the kidney. They meet somewhat anterior to the posterior end of the kidney and descend as a single duct around one side of the swim bladder. An expansion of this descending portion of the urinary duct serves as a urinary bladder. The bladder is connected to the urogenital papilla as are testes in mature males. The gonads develop dorso-laterally in the anterior visceral cavity in both sexes, but the ovaries have no ducts connecting the urogenital papilla, eggs are simply shed into the visceral cavity. The only major organ remaining unmentioned is the spleen. In salmonids this lays ventrally, just above the pelvic fins, attached to the posterior side of the major visceral mass.
In general.) the rainbow trout is representative of most salmonids. It is a relatively primitive (unspecialized) fish, a typical carnivore with good swimming ability for capturing prey, a stomach which can easily extend posteriorly for ingesting relatively large prey, and a short intestine for handling food containing minimal amounts of indigestible material. The total length of the gut (oesophagus to anus) is 0.6 to 0.8 times the body length, about as short as any teleost (Figure 1a).
6.2 Channel Catfish (Ictalurus punctatus)
The channel Catfish is characteristic of most Ictaluridae. The mouth is wide and large, is fringed with gustatory barbels, and is generally adapted for foraging and sorting in mud to obtain the organic material - predominantly insects, but also snails, worms, plants, and general organic debris. The buccal cavity can be closed completely for squeezing mouthfuls of mud through the gill rakers and gill bars. The oesophagus is longer than in salmonids and leads to a round stomach which is located ventrally. The intestine originates at the anterior, ventral edge of the stomach, then turns dorsally to form several convoluted, half-circles around the stomach before proceeding posteriorly.
Arrangement of several non-digestive organs in the visceral cavity is noteworthy. The swim bladder has bulges on each side which raise the centre of bouyancy above the centre of gravity so that the fish do not turn belly-up when sick or unconscious, as in salmonids. These bulges also come very near to the body surface and probably enhance hearing. The relatively high position of the swim bladder is displaced anteriorly and dorsally so that it partly overlays the stomach. The gall bladder is in the mesenteries just posterior to the liver.
In general, ictalurids are versatile omnivores. They are relatively inactive, although marine catfish (family Arridae) appear to be more active than freshwater species and spend more time off the bottom. None of the species are particularly streamlined for efficient or rapid swimming, except that some species (channel cats, marine cats) have forked tails, which suggests some degree of swimming specialization. The maxillary and other barbels maximize their ability to find food at night or in turbid water where sight is largely useless. Other than the barbels, the digestive tract is of moderate size and length, showing little specialization (Figure 1b).
6.3 Common Carp (Cyprinus carpio)
Common carp are representative of many cyprinids, including goldfish, squawfish, minnows, dace, chubs, and tench in North America. Most of these fish, including common carp, are omnivores, similar in several respects to catfish, but also differing significantly in several respects. Carp have maxillary barbels (most cyprinids do not) and forage in mud like catfish. However, carp ingest a considerably greater amount of plants than catfish and then chew the plants using a set of interdigitating pharyngeal teeth placed just anterior to the oesophagus. Carp lack a stomach, but have a long intestine which winds extensively throughout the visceral cavity. The gall bladder rests on the dorsal surface of the anterior midgut and the bile duet opens into the intestine just anterior to the gall bladder. In addition, the liver has no specific shape, but seems to serve as packing material around the intestine. Food seems to be ingested in small particles in a relatively steady stream instead of intermittently in large units, so the storage function of a stomach probably is not missed. With the liver filling all the available visceral space, there would be no room for accommodating the stomach expansion of a large meal anyway. The remainder of the visceral organs are relatively unremarkable (Figure 1c).
Fig. 1. Diagrammatic representation of the digestive systems of four fish described in the text, arranged in order of increasing gut length.
a. Rainbow trout (carnivore);
b. Catfish (omnivore emphasizing animal sources food);
c. Carp (omnivore, emphasizing plant sources of food);
d. Milkfish (microphagous planktovore).
6.4 Milkfish (Chanos chanos)
Milkfish are specialized in several respects. The body shape, the streamlined cover over the eyes, and the widely forked tail all go with a fast-swimming life style. Fine (almost membranous) gill rakers suggest filter feeding, making a designation as carnivore, omnivore, or herbivore impossible since plankton is a mixture of many life forms, including some which are too simple to be clearly plant or animal. Milkfish are most frequently designated as microphagus planktovores. A specialized epibranchial organ above and behind the gills may help to concentrate microplankton, although no one has really demonstrated how it might do so. The stomach is a simple tube, somewhat convoluted, and of moderate size. The pyloric end of the stomach has thick, muscular walls and is usually described as a gizzard. The pyloric region of the long, narrow intestine has numerous pyloric caecae, also of small diameter. The swim bladder and the lining of the visceral cavity are membranous, similar to those in salmonids except for being jet black (Figure 1d).
In general, milkfish are cultured in enriched, saltwater lagoons in which they swim rapidly, straining their food from the typically turbid water.