How fish use their fins – S.E.A. Aquarium, Dolphin Island blog

Fish fins are typically composed of bony spines or rays protruding from the body, with skin covering them and joining them together. They either end up in a webbed fashion as seen in most bony fish, or a flipper as seen in cartilaginous fish like sharks.

Then there’s the fleshy, lobed, paired fins joined to the body by a single bone, found in this class of bony fish called Sarcopterygii.

fish fins — The Sarcopterygii. Image credit

The key function of fins is to help the fish balance and turn as it swims. Here are the different types of fins typically found on a fish:

Fins located at different parts of the fish serve different purposes:

Dorsal fin
Protects the fish against rolling, and assists in sudden turns and stops. Most fish have one dorsal fin, but some have up to three.

Ventral (pelvic) fins
Comes in a pair, assists with moving up or down through the water, turning sharply, and stopping quickly.

Caudal (tail) fin
The main propelling fin in most fish

Anal fin
Helps to maintain stable equilibrium

Pectoral fins
A pair of fins used for balancing and braking.

Adipose fin
Scientists do not have a definitive answer for the purpose of the adipose fin. Some said it serves as a pre-caudal flow sensor.

Different fish, different fins

Not every fish has all the above fins. Depending on their body structure, nature has equipped them with different combinations of fins to help them swim efficiently.

With its upright body and lack of caudal fin, the seahorse propels itself using its small dorsal fin on its back, and uses its even smaller pectoral fins near the back of its head for steering.

The Featherback has no ventral fins. It has a small and narrow dorsal fin and a very small caudal fin that is fused with its anal fin, which runs most of the length of its body. It swims by holding its body rigid and rippling its anal fin to propel itself forward or backwards. Image credit

Other uses for fins

While most fish use fins when swimming, there are some that use their fins for rather different purposes. Such as the flying fish which use their pectoral fins for gliding, and the frogfish and epaulette shark that use theirs for ‘walking’. In species like gobies and lumpsuckers, their pelvic fins are fused into a sucker disk which they use to attach themselves to objects.

Balloon Lumpfish
The lumpsucker’s pelvic fins have evolved into adhesive discs on their undersides.

Epaulette sharks use their pectoral fins as prototype legs to ‘walk’ from one tide pool to another.

The next time you’re at S.E.A. Aquarium, don’t forget to take a closer look at their fins.