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).