Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Fishes Fins

The physiology or anatomy of a fish enables it to do all the things which are needed in order to thrive in its natural environment. Fish have evolved over millennia and are still evolving in order to meet the demands of their dynamic environment. Their fins help them with stability and locomotion within their environment.  Many different species of fish have specialized fins which enable them to live in environments that other species of fish would not be able to inhabit.

 Fins of a fish 

The ray counts in the fins are one of the first identifying factors when classifying a fish.  Learning to spot the differences between the spines and soft rays and being able to count them accurately will help narrow down the possibilities for identifying different species.  When used for classification, the number of spines are indicated using Roman numerals, while the soft rays are indicated using Arabic numbers.

For example the Rock Goby, Gobius paganellus, has the following ray count:

D1 VI; D2 I + 13-14 (12-15); A I + 11-12 (10-13); P 21-22 (18-23).

D1 - The first dorsal fin consists of 6 spines
D2 - The second dorsal fin normally consists of  1 spine and 13-14 rays, but can range between 12-15
A - The anal fin normally consists of 1 spine and 11-12 rays, but can range between 10-13
P (P1)- The pectoral fins normally consists of 21-22 rays, but can range between 18-23 rays
V (P2)- Ventral (Pelvic) fins have not been stated in this count
C - Caudal fins have not been stated in this count 

Fin Composition for Ray Finned Fish

Fish fins are composed of hard spines and/or soft rays covered with a translucent membrane.  If both spines and rays are present in the fin, the spines are always at the anterior of the fin followed by the softer, more flexible rays.

 Rigid and soft rays of a fishes fin
Rigid and soft rays of a Perciformes dorsal fin

Spines, sometimes called hard rays, are composed of a single shaft and are generally sharp and ridged but can be flexible in some species.  Some species of fish have modifications that allow their spines to preform secondary functions such as members of the Triggerfish family can lock their spines up making it very difficult to remove them from any crevices they use for protection.

Soft rays are generally flexible and segmented and may even be branched.

Fin Composition for Lobe Finned Fish

The fins in Lobe-finned fish are structured very differently than the fins found in other fish.  They have bones that extend into the fins as well as rays which are covered in a fleshy material rather than a translucent membrane.

Single fins

 Soft rayed dorsal fin
Dorsal fin

Dorsal fin - There can be up to three separate dorsal fins composing of only hard rays, only soft rays or a combination of both hard and soft rays. The dorsal fins are generally used to help keep the fish balanced upright, similar to the keel of a ship, and to assist in sudden stops or turns.  Many fish have modified dorsal fins for more specialized uses.

Three modified dorsal fins
Modified dorsal fins for a Triggerfish, Anglerfish, Remora, and Lionfish.

  •  Members of the marine sunfish family, Molidae, use their dorsal fin for swimming. 
  • The large dorsal fin in sailfish, Istiophorus spp., have large blood vessels that may be used for heating and cooling allowing the fish to swim faster and longer. 
  • Trigerfish, Balistidae, have a small but very strong first dorsal fin that can lock upright, securing it into tight crevasses for protection.
  • Anglerfish, from the order Lophiiformes, have a highly specialized first dorsal that they use as a lure called an esca or illicium.
  • The dorsal fins on the remora, family Echeneidae, have modified dorsal fins that look and act  like suckers so they can hitch a ride on other fish.
  • Lionfish and other scorpionfish have hollow venomous spines used for self defence.
  • Some fresh water catfish also have venomous spines.
  • Some fish do not even have any dorsal fins such as many members of the Gymnotiformes, Knifefishes, family.
 Adipose fin
Adipose fin

Adipose fin - Not present on all fish, is a soft fin located behind the dorsal fins and in front of the caudal fin. It is mainly present on Salmonidae, Characins and catfishes. Currently the function of this fin is not completely known or understood. Some places cut of this fin to differentiate captive reared fish from wild stock, however this practice is currently under revision as more information is coming to light. Recent studies have noticed that fish with the adipose fin removed beat their tails approximately 8% more than fish with the fin intact. It is also thought that this fin contains a network of nervous tissue, cells and fibers, possibly allowing it to navigate in rough waters.

Anal fin -

Gonopodium (modified anal fin).

Caudal fin - Located at the end of the caudal peduncle and is used mainly for locomotion.

  • Heterocercal - found on some species of sharks, where the vertebrae extend into the caudal fin making the top part of the tail longer. (can also be reversed making the bottom of the caudal fin longer)
  • Protocercal - where the vertebrae extend to the end of the tail and the caudal fin is symetrical on either side of the vertebrae
  • Homocercal - most common type of caudal fin in modern fish. The fin appears symetrical, however the vertebrae extends for a very short distance into the fin.
    • Rounded
    • truncated
    • forked
    • lunate
    • emarginate
  • Diphycercal - The vertebrae extends to the tip of the tail and the caudal fin appears symetrical and extended.

Finlets -

Paired Fins

Pectoral fin -

  • The Pectoral fins of the Sea Robins can open into sails used to glide around in the currents.
  • Fresh water butterfly fish, Pantodon buchholzi, use their pelvic fins for gliding.
  • Flying fish, from the family Exocoetidae, propel themselves out of water, open up their pelvic fins and glide through the air on average of about 50m to escape from predators.  Some fish have been recorded to use updrafts from the leading edges of waves to glide more than 400m and have been recorded as high as 6m above the waters surface.

Pelvic fin -

  • Gouramis can taste with their modified pelvic fins.
  • Gobies have fused pelvic fins
  • Sea Robin have pelvic fins that can be used for "walking" along the substrate.


Anterior - The part closest to the nose of the fish.

Posterior - Closest to the tail end of the fish.

Soft Ray - A segmented, generally flexible and sometimes branched structure to provide support for a fin.

Spine - A single single-shafted structure to provide support for a fin 


Wikipedia - Fish fins

Wikipedia - Molidae

Wikipedia - Flying Fish

Marine Species Identification Portal - Rock Goby (Gobius paganellus)

Fin Fish by Dr. V. Ramaiyan* and Dr. M. Kalaiselvam

Clipping Adipose fins on salmon might hurt fish's ability to swim in rough waters, study finds. by  Mihir Zaveri, The Oregonian