This is a followup to my diary entitled "Fins 101". In that essay I just touched briefly on the caudal fin, which is the fin found on a fish’s tail. What’s really interesting about this particular fin is that once you’ve learned how the different shapes this fin comes in work, you are then able to take any unknown species and guess pretty accurately how it lives, its habitat and even what it feeds on.
So I’m going to give you these tools below so you can amaze your friends and stun your enemies with your uncanny ability to analyze any dead fish placed in your hands.
Fins are used for swimming, and the combination of the five main types of fins help the fish to move forward, turn, balance and maneuver. The tail fin, or caudal, is normally the one fin responsible for actually propelling the animal forward through the water. The shape of this fin varies with species depending mainly on its habitat.
A few groups of fish, such as rays and seahorses, lack a caudal fin altogether and rely on the other fins for movement. These are rare examples, however. Most species of fish move forward through the water with a sideways sweeping motion of the caudal. This is known as carangiform locomotion. Nearly as important as the fin itself is the base of the tail, called the caudal peduncle, and we’ll see some adaptations on this area that can be used to affect movement in some species below.
First, we’ll start with the very basic division of caudal fins by comparing the tails of sharks and bony fish. Normally a fish’s tail has two lobes, an upper and a lower. Most bony fish have a homocercal tail, meaning the two lobes are about equal in size and shape. Fish with homocercal tails have their vertebrae ending at the base of the tail. A shark’s vertebrae extend into the upper lobe of the tail which results in a larger upper caudal lobe than the lower. This asymmetrical fin is called a heterocercal tail.
Homocercal Caudal Heterocercal Caudal
Epicercal heterocercality, such as that found in sharks, is by far most common. That is, the upper lobe is larger than the lower. I can only think of one family where the lower lobe is larger (hypocercal heterocercality), and that would be in the group of fish known as flying fish. These species use the longer lower lobe as sort of a propeller to skim across the surface and launch the fish into the air. You can read about flying fish here.
While the difference in the size of the two lobes is usually not all that great, one notable exception is the thresher shark. This fish has an upper caudal lobe that is half the length of its entire body. The extended lobe of the tail is used to corral fish into tight schools that can then be easily attacked.
Homocercal tails come in a variety of shapes. The tail of fast moving oceanic fish are usually lunate, or half-moon shaped. This shape is the most effective in giving a fish speed, and is often supported by other adaptations designed solely for maximizing speed. In many cases the other fins retract into grooves in the body to reduce drag. And the caudal peduncle can be studded with bony ridges called scutes, which strengthen the peduncle and gives the side to side tail movement more power. Below is a crevalle jack, the young of which I find in Rhode Island waters every fall (they don’t survive the winter). Notice the scutes at the tail’s base.
Open water fish that don’t rely on speed for protection and feeding, but tend to swim constantly and over long distances, have a forked tail. This can be deeply forked, like the tail of a pilotfish. This is that little black and white striped fish you’ve all seen on TV following sharks and manta rays around. Keeping up with their much larger hosts requires non-stop, intensive swimming, and the forked shape is the best option for the tail.
Open water schooling fish, which may swim constantly but rely on the schooling behavior rather than speed for protection, tend to have a shallow forked tail fin. An example is the scup family.
Notice the deeply forked tail fin of these pilotfish.
When we get into shallow water fish, maneuverability and quickness becomes more important than speed and stamina. These fish tend to have either rounded or truncated tail fins. A round shape, like those of most coastal minnows, allows a fish to make sharp turns and quick starts. Although the round shape is energy inefficient, the increased surface area means that more power goes into each stroke, enabling the fish to move in short bursts very fast from a standstill. This is very important when your enemies are shorebirds and other shallow water predators.
Round caudal fin of a shallow water minnow.
Finally, among the conventional tail shapes, we have the truncated tail. Think of this as a sort of triangular shape where there really isn’t a pair of discernible lobes. These tend to occur on bottom dwelling fish such as flounders and sculpins. Like the round-lobed fish, these fish don’t need speed except over short distances. Fish with a truncated tail are either camouflaged animals that stay still for long periods of time, but when threatened may need a quick burst of speed, or they tend to slowly and constantly swim along the bottom searching for food. The flounders are an especially interesting group because they are actually lying on their sides. The tail fin is therefore horizontal, like the flukes of whales and dolphins, rather than vertical like all other fish.
The variety of fish species throughout the world is stunning, so there are of course many examples of unusual caudal fin shapes. The coelacanth, that ancient fish thought to have been long extinct until discovered in the 1930’s, has a diphycercal shaped tail with three lobes and a vertebrae that extends all the way to the end of the tail.
Some species have a slightly indented caudal known as an emarginate fin. The great barracuda is unusual in that its tail is double emarginate. You can see an image of that here. Then there are the fish most famous for not having any lobes at all. The eel family. True eels have what is called a continuous vertical fin. That is, the dorsal, caudal and ventral fins are all connected to form one fin that starts on the back and wraps around the length of the body to the belly. I did a diary way back on the american eel that describes this fin.
Now let’s see what you’ve learned. Without googling the fish below, tell me a little bit about how it lives. To stay objective, I’ve purposely chosen a fish I know nothing about, not even its name.
Other diaries in this series can be found here.