A LOOK AT INTERNAL AND EXTERNAL ANATOMY

Every fin on a fish has an important function. The dorsal and anal fin aids in keeping the fish upright and stable, the pectoral and pelvic fins aid in direction and stopping, and the caudal fins help to move forward.

Characins and some other types of fish (6,000+ noted) possess a small, ray-less, “adipose” fin. In hatchery raised fish such as salmon, it usually is not present. This is made up solely of fatty material. It is found behind the dorsal fin and before the caudal fin. While there is no known use for it, a study performed by T.E. Reimchen and N.F. Temple has pointed towards the idea that it is used as a sensory organ to detect water flow changes before they reach the caudal fin. This would allow the fish to make precise changes to keep with the flow. During the study, fish without the adipose fin had an 8% increase in caudal fin movement.

(See #97 in REFERENCES for full citation)

The majority of the fish’s body is composed of muscle, the exception being the bones and the small cavity which houses the organs towards the front of the body. You can tell where a fish’s abdominal cavity ends by where the anal fin starts, since the anal fin marks the end of the digestive tract.