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.
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
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 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.
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.
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.
Modified dorsal fins for a Triggerfish, Anglerfish, Remora, and Lionfish.
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.
Pectoral fin -
Pelvic fin -
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