Buoyancy, Fins and Swimming

Movement through water is the least metabolically costly method of travel, followed by flight, walking, and burrowing (the most costly). The reason for this is the potential for an organism to reach neutral buoyancy and the subsequent reduction in the amount of energy needed to place the body in motion.

Several physiological and behavioral processes can help to achieve neutral buoyancy.

Passive: Reduce body structures, tissues, and organs with a specific gravity (s.g.) greater than the surrounding water.

HOH=1 (by definition)

Saltwater = 1.026

  1. Bone consists of calcium and some of the heavy metals( s.g. approximately 3). To reduce the negative buoyancy of bone the ratio of bone to body volume in fish is greatly reduced compared to terrestrial vertebrates. Additionally fish bones may contain lipids( s.g. of 0.9-0.93) that will reduce the negative buoyancy per volume of bone.

  2. Cartilage( s.g.= 2) in place of bone in the Chimaera (ratfish) and Elasmobranchii (sharks, skates, sawfish, and rays) can produce a significant reduction in negative buoyancy.

  3. Reduction of specific gravity of bodily fluids in elasmobranchs with urea and TMAO (trimethylamine oxide ), both of these have a specific gravity less than seawater.

  4. Concentration of oils, lipids, and squalene (s.g. =0.86 ) in tissue and organs such as the liver. This is especially true in sharks where the liver can comprise as much as 25% of the body weight.


  1. Control buoyancy through regulation of the swim bladder. However; not all fish have a swim bladder.

  2. Provide dynamic lift with a combination of forward motion and fin deployment.

Types of Fins

The source of propulsion for virtually all fish comes from:
  1. Undulation of the body
  2. Paired Fins:
    • Pectoral

    • Pelvic

  3. Unpaired Fins:
    • Caudal

    • Dorsal

    • Anal

  4. A combination of the above

Which of these methods or combination of methods is used for propulsion can vary depending on physiological abilities and urgency of spatial change.


Undulation of the body is one of the most efficient forms of propulsion and when used in combination with the caudal fin provides for maximum acceleration. This combination of body and caudal fin propulsion can be found in virtually all fish to some degree. One of the best examples of the use of body undulation is the eel (Anguillids).

Paired Fins

Paired fins can be used as the main means of propulsion or in combination with other swimming or crawling forms. The pectoral and pelvic fins are also commonly used as a means of quickly stopping the fish, controlling pitching (change of position from the horizontal) and to attract mates or startle other fish. Two well known examples of the use of the pectoral fins for primary propulsion are:

  • Labrids - the rowing type movement of the wrasse.
  • Rajiform - the bird like swimming of the skates and rays.

When the pectoral fins are used in conjunction with other swimming methods they are often employed to provide an upward lift much like the wings of an airplane (e.g. flyingfish and hatchetfish). This is a form of active control of buoyancy and provides dynamic lift to the fish. Fish, such as the sharks (Elasmobranchs) and tunas (Thunnids) are dependent on continual forward motion and their pectoral fins to regulate their place in the water column.

Pelvic fins are rarely used as a means of primary propulsion. Instead, pelvic fins are commonly used to provide stability and steering while swimming. In some species the pelvic fins are capable of permitting crawling either/or/both on land and underwater. Another modification of the pelvic fins results in a suction cup type structure that is used to maintain position in fast moving current.

In many fish the pelvic fins have become reduced to threadlike features or lost entirely.


Unpaired Fins

Unpaired fins in some fish provide the major source of propulsion both for steady and burst swimming.

Caudal Fin
The caudal fin provides the most familiar form of unpaired fin movement. The efficiency of each type of caudal fin varies and presents the fish with advantages and disadvantages depending on the situation encountered.


Dorsal Fin

The dorsal fin(s) lack the speed that can be obtained when using body undulation and/or the caudal fin for motion. However; the use of the dorsal fin(s) by fish such as the bowfin (Amia calva), Mormyridae, seahorse, and pipefish provides both precise movement and the ability to move forward or backwards with equal ease.

Starting at the anterior end of the fish, dorsal fin(s) are named; first dorsal, second dorsal, and third dorsal.


Anal Fin

The anal fin(s) is also not as powerfully as undulation and/or the caudal fin. The use of the anal fin(s) for primary propulsion produces the same maneuverability advantages as the dorsal fin(s). The knifefish are probable the most recognized fish to use the anal fin for motion.