Sharks, dolphins, porpoises, whales and certain types of fish depend on fins for myriad purposes. We took a look at the seven most common fins found in the ocean and provided a fin-tastic guide to what they do.
The dorsal fin is the most visible of all fishy appendages and fulfills the first of the two basic fin functions: stability.
Located on the back (the word comes from dorsum, the Latin for "back"), this is the main stabilizing fin, preventing the fish from rolling over as it moves through strong currents and helping it turn by pushing against the water. It basically works like an upside-down keel on a boat, keeping things upright and on the right course.
Some species have found added uses for the dorsal fin, though. The ocean sunfish (Mola mola) has a dorsal fin that has fused with its cloacal fin and that it uses for propulsion.
Lionfish (Pterois) and some catfish have developed poisonous dorsal spines that they can use for defense, while the anglerfish uses a modification of its prominent dorsal fin as a lure for prey.
More stability is maintained by a small cloacal — or anal — fin which is located, as the name suggests, at the rear and underside of the fish near the tail.
Though most cloacal fins are relatively small some (like those of the Japanese Tamasaba) can be long, elegant and flowing.
Also known as the ventral fin, the pair of pelvic fins also lends the fish stability as it moves quickly through the water. It helps the fish ascend, turn and stop.
Taking their alternative name from the word venter, Latin for "belly," these are usually located beneath the fish, opposite the dorsal fin — though sometimes they can be found positioned further forward (as in the haddock, or Melanogrammus aeglefinus) or toward the back (as found with the South Asian Rohu or Labeo rohita).
Photo Credit: Wikimedia, Jean-Lou Justine
The caudal is the fish's tail fin (from the Latin cauda, literally meaning "tail"). It represents the second fundamental purpose of fins: propulsion.
The fish uses body-caudal locomotion to propel itself. Basically, this means the fish creates a wave along its vertebrae from the top of its body to the tip of the tail, undulating through the water back and pushing itself forward.
Fish tails are usually two-pronged, either forked or lunate — that is, shaped like a moon. Some, however, like that of the salmon, have a straight edge and are described as truncated.
Though the caudal fin is usually pretty symmetrical (or homocercal), sometimes the vertebrae extend to make the upper half longer (heterocercal) — like the tail of most sharks — or the lower half longer (reversed heterocercal) like that of the angel shark (Squatina).
5. Pectoral and Cephalic
Pectoral fins are, as the name suggests, found on the breast area of the fish. They are used primarily for propulsion and steering and are usually located just behind the gills. However, the usefulness of pectoral fins doesn't stop there.
Fish without swim bladders — like sharks — use their pectoral fins to create "dynamic lift" to prevent themselves from sinking. The fish uses its fins like the wings of an aircraft, forcing a faster flow of water over the top of them, creating lift. The one drawback is that the creature must keep moving forward to maintain the right depth.
Flying fish (Exocoetidae) use their pectoral fins to take to the air. Like the dynamic lift generated by sharks, flying fish tilt their "wings" to create enough buoyancy to carry them distances of around 50 feet at speeds of up to 43 miles per hour!
Meanwhile, fish like the mudskipper (Periophthalmus barbarous) have reinvented their pectoral (and pelvic) fins and use them to walk on dry land. These creatures can take in oxygen through the skin, allowing them to pull themselves across land in a series of "skips" to find new pools or wait for the tide to return.
The manta ray (Cephalopterus manta) has two forward-placed cephalic fins, which are modifications of the pectoral fin. These horn-like projections are located by the mouth and mantas flatten them to channel food when they eat.
Finally, the adipose fin, which — ahem — poses something of a mystery. This fin is a fleshy appendage located on the fish's back between the dorsal and caudal fins. Although it probably offers a little extra stability, presently, no one is completely sure what it is for.
Though not every fish has one, it can be seen on salmon, trout and catfishes, among others. Some research suggests that there is a neural connection to the fin, and therefore it may be a primarily sensory organ, able to pick up changes in current, sound and pressure. Until the adipose can be fully explained, it's a secret the fish is keeping to himself.
From a few basic premises, the multitudinous seas have produced an infinite variety of fish appendages. Each creature has developed its fins to suit its own particular lifestyle — and if evolution has taught us anything, it's that fins can only get better.