Marine Biology

Anatomy of Rainbow Trout

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"Anatomy of Rainbow Trout"
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External Anatomy:

Rainbow trout (Oncorhynchus mykiss) have a silver, streamlined body with a pale to dark pink stripe that starts just behind the rose colored gill plate, or operculum. The stripe extends the length of the side of the body, ending just before the tail, or caudal fin.  The belly area is white to pale yellow. Coloration of these fish varies with diet, spawning and from male to female. The fins, body and tail are speckled with black spots that contrast against a silver to dark red cover of scales, creating a camouflage effect in the water. The scales are covered with a protective slime to keep the fish healthy.

Rainbow trout live in streams, rivers, and lakes; can grow to a length of thirty-two inches, and reach a weight of up to sixteen pounds. Trout that live in streams tend to stay smaller than the fish that inhabit lakes and large rivers.  Their bodies are well adapted for quick maneuvering while hunting.

On the top of their bodies, rainbow trout have one dorsal fin and an adipose fin, both of which keep the fish steady and flat in the water.  The caudal fin, which is the tail, is large and used to propel and steer the fish as it swims.  Low on the sides, rainbow trout have a pectoral fin just below the gill plate. Although small, the fish uses this fin to slow down. On the bottom, these fish have larger pelvic fins that are used to move up and down in the water.  Low on the bottom are anal fins that also help the fish keep steady in the water.

Internal Anatomy:

The internal anatomy of a rainbow trout is fairly unremarkable from other trout.  They have a brain for processing, a neural spine to send signals to the body, gills for pulling oxygen from the water, a heart to pump blood, and just under the boney spinal column they have a swim bladder, or gas bladder, that helps them float.

Rainbow Trout have earstones, or otoliths, located behind their eyes.  The earstones are used for hearing and also to help the fish keep their balance.  Nares, or nostrils, are located on the snout.

Encased in a boney ribcage, these fish have the typical list of digestive organs such as a stomach, gall bladder, kidney, liver, intestines, and anus. Their digestive organs are developed for their diets of plankton, insects, other fish, crayfish and crustaceans, so their digestive tracts have an organ called a the pyloric ceca to help break down hard foods such as the exoskeletal material from the crayfish and bony pieces of small fish they eat. They have an upper jaw that is made up of two bones to allow for flexibility when feeding and a single-boned lower jaw that is lined with small, but sharp teeth for snagging their prey. Rainbow Trout are known for having a voracious appetite, and as this article illustrates, their bodies are designed for hunting.

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