The study of scales is called Squamatology. Scales help to protect a fish from scrapes, parasites and other external injuries. They vary from one evolutionary line of fish to another and can be catagorized into several groups. The type of scales a fish has, as well as whether a species has scales at all, can often be tied to the behaviors and lifestyles of that species. I’ll describe the five basic types of fish scales below.
Scales usually grow on a fish in overlapping plates, with the free end of the scale pointed towards the tail, sort of like the shingles on your roof. They are anchored to the body by the anterior end of the scale, either embedded into the flesh or actually attached internally by bone. Because they are all attached at only one point, they provide flexibility so that the scales can slide over each other as the fish’s body bends while swimming. This growth pattern also makes the fish more hydrodynamic, allowing water to glide smoothly over the animal’s body as it swims.
The vast majority of bony fish ("bony fish" are those species that do not include sharks, skates or rays) have cycloid, or round scales. These have two basic parts; the inner solid structure which is made out of bone, and an outer collagen layer. Scales are not shed as the fish ages, but grow with the animal, and the number of scales on these fish do not increase. The result of this, as you can see in the photo above, is that a fish can be aged by counting the growth rings, just like you can do with trees and clams. It’s not quite that straight forward however and it takes and expert squamatologist to determine a fish’s age by the scales.
For example, in many species the females stop growing during the reproductive periods, dedicating all of their energy to egg production. After spawning, growth continues again and this growth break will result in a gap in the scale’s ring production. These growth rings are known as annuli.
Herring, anchovies and other groups of bony fish have large cycloid scales that fall off very easily. Hold one of these fish in your hand and when you put it down your skin will be coated with dozens of little silver scales. Skin structures that are shed easily are known as deciduous scales, just like the term used for trees that drop their leaves in Autumn.
Ctenoid scales are closely linked to cycloid scales and the two types are often grouped together simply as "bony-ridged" scales. The structure of these scales is similar to the description above except that the posterior end (the exposed part of the scale) is lined with spines or comb-like ridges. Again, the vast majority of fish have one of these two types of scales. Some, like flounders, actually have both.
At first glance, a flounder appears to be flattened from top to bottom, with two eyes on the top like a stingray. Actually it is flattened from side to side, but lives lying down on one side. It has two eyes on one side of its body and no eyes on the other. The "eyed-side" has ctenoid scales, to better protect it from predators, while the "blind-side" has cycloid scales, making it easier to smoothly glide along the bottom.
Ganoid scales are rare among modern fish, found only on relics such as gars, sturgeonfish and the coelacanth. These scales act like armor to form a nearly impenetrable barrier to predators. What’s unusual about these scales is that they do not overlap but instead fit together like pieces of a jigsaw puzzle. These scales also grow with the fish, not only in length, but in width and thickness as well. The larger the fish the more daunting is the armored shield.
Another very rare scale type, but one that was no doubt common in extinct species, is the cosmoid scale. These are limited today to the primative lobefins and lungfish. You’ve all heard of lungfish, those strange Australian fish that can curl up in a ball of hardened mud during the dry season and come back to life when the rain returns. Like ganoid scales, these scales also grow along with the fish.
Also known as dermal denticles, placoids are tooth-like scales found on sharks, skates and rays. Unlike the previous types of scales, placoids do not grow with the fish. As a shark gets larger it grows new scales to fill in the gaps between the scales. Placoids have a sharp ridge down the center of the scale made out of an enamel-like substance called vitrodentine, a substance very similar to that covering our teeth. This enamel ridge is called an ectodermal cap, and this is important because without this enamel cap the sharks could not feed. Technically, sharks are toothless. The structures we call the "teeth" are simply modified dermal denticles. That poisonous spine on the tail of the stingray is the same thing. A greatly modified placoid scale.
Some groups of fish lack scales completely, including gobies and toadfish. Instead, these fish are covered in a coating of slime which will serve the same purpose of fending off injury and ectoparasites. The slime layer may serve the added function of making them especially hard for a predator to grab hold of.
Note: Apparently you can’t edit old diaries, so if you read that one on toadfish the movie at the end can now be found here.
Other diaries in this series can be found here.