Welcome to Fins 101. There are five kinds of fins found on a fish, although not all fish possess all five. The variety of fin adaptations among species could fill a book. These include the dorsal fin, found on the back of a fish; the ventral fin (sometimes called the anal fin), found on the belly; the caudal fin, which is on the end of the tail; the pectoral fins, which are paired and found on the sides just behind the gills; and the pelvic fins, also paired and found on the fish’s chest.
These five fins can be classified into two groups: Paired fins (the pelvic and pectorals) and median fins (dorsal, ventral and caudals). The placement, and in fact the presence or absence, of these fins depends on both the center of gravity and life style of each species. I’ll keep this diary very basic and expand on it later with specific examples.
Dorsal fins are probably the most well known of all the fins, made famous by movies like "Jaws" where the image of this fin cutting through the water surface portends the approach of the killer shark. Dolphins and whales, including the orca pictured above (despite the common name "killer whale" this species of porpoise is neither a killer nor a whale), also have prominent dorsals. We’ll be dealing mainly with fish fins in this diary.
The purpose of the dorsal fin is stability, although it also plays a role in helping the fish make sharp turns. Like the keel of a ship, the dorsal is vital to keep a moving fish from rolling over as it swims. Most fish have two of these fins, called the "first" and the "second" dorsals. In the vast majority of fish the first dorsal is larger than the second. This first dorsal is often referred to as the "spiny" dorsal and the second is the "soft" dorsal. The first dorsal is supported by thick fin rays that keep the fin rigid as the fish swims. The rays on the second dorsal are much thinner and add flexibility to this structure. Some species of fish have only a single dorsal (usually the "spiny") while others, such as members of the cod family, have three (one "spiny" and two "soft").
Also known in some texts as the "anal" fin, the ventral is a single fin found on the belly of a fish, usually close to the lower lobe of the tail fin. And to make things more confusing, the pelvic fins are sometimes referred to as the ventral fins in these texts, and some authorities call both bottom fins ventrals in general. We’ll be using my preferred terminology here. Just to explain, because the ventral is the opposite of the dorsal and used for the same purpose, I prefer to call them by their antonyms. Also, after teaching children about marine life for 20 years, I find avoiding the term "anal" eliminates the distracting giggle factor.
Like the dorsals, this fin also helps to stabalize the fish as it moves. The ventral works in concert with the dorsals to prevent a fish from rolling while swimming.
The pelvic fins are paired and found on the chest of a fish. These fins aren’t used for propulsion, but rather for steering. In some species of fish the pelvics are used like little hands, helping the fish to crawl along the bottom or among seaweeds. See this diary on Sargassumfish for an extreme example (although in this fish's case it is the pectoral fins that act like hands).
Pectorals are like a fish’s brakes. They help to balance and steer the fish as it swims, but are used mainly to slow the fish down, bring it to a stop or even move the animals backwards.
Some fish, like the sea robin above, have greatly enlarged pectorals. These are used as a protective device, spread out when the fish is threatened to make the animal look larger to a predator.
Here’s my personal favorite fin. The fin at the end of a fish’s tail is the most diverse by far. It can be rounded to give a fish bursts of speed over short distances or lunate (half moon-shaped) to give an open water fish power over long hauls. Most sharks have a heterocercal tail fin, meaning the top half is longer than the bottom lobe. Most bony fish have homocercal caudals, meaning both lobes are of equal length. As you can see above, thresher sharks take the heterocercal fin description to an extreme. In heterocercal fins the fish’s backbone enters the tail fin. The spine of most other fish ends at the base of the tail.
One adaptation of open water fish can be found at the base of the tail, known as the caudal peduncle. These fish have ridges known as scutes which strengthen and support the tail fin for extra fast open water movement.
Although I stated above that there are five fins on fish, some families have a sixth type, called an adipose fin. This is a fleshy fin without fin rays and is found between the second dorsal and the caudal fin. Salmon and their relatives have an adipose fin.
And to add one more to the mix, manta rays have a pair of fleshy appendages on each side of the head, used to funnel water into the mouth as the fish feeds. These aren’t technically fins, but are known as cephalic (head) fins. No other species of fish has these structures.
Other diaries in this series can be found here.