It’s Derby time and all across the Island, anglers will be spinning fish tales. Some will have only stories, while others with more skill (or perhaps greater luck) will be down at the weigh station. All eyes at Derby time are on the scales, the ones used for official weighing, of course, but also the ones that adorn the fish themselves.
For protection, coloration and even sensory awareness, most fish have scales. Scales can vary in size, shape and structure, and can even be tiny or absent in some species. Eels have microscopic scales that cannot be seen, while catfish, lamprey, hagfish and sunfish lack scales completely.
Simple is the least likely way to describe the diversity of these complex coverings. There are many different types of scales to consider.
Placoid scales, also called dermal denticles, are the variety most analogous to vertebrate teeth. Like teeth, placoid scales have a soft central pulpy inside core with a middle layer of dentine and an outer layer of enamel. Cartilaginous fish, which include sharks and rays, have these types of scales.
While placoid scales protrude from the fish’s epidermis, they do not cover the skin entirely. Nor do these types of
scales grow individually; instead more scales are added as the fish increases in size. By creating vortices as they travel through water, these scales can reduce drag, making for more efficient and quieter swimmers. Sounds about right when you are talking about sharks!
Cosmoid scales are hardly worth a mention since they really only exist in extinct species of fish. The ganoid variety is a hard, shiny, diamond-shaped scale that is found on sturgeon, paddlefish and gars, among a few others.
The scales that will be seen at the Derby over the next month are elasmoid scales, the most common variety, which are found on lobe-finned or bony fishes. These scales can be further divided into two types; cycloid and ctenoid.
Cycloid scales are found on herrings, minnows, trout and many other varieties of fish. Each individual scale has smooth margins and they collectively overlap from the fish’s head to tail, thus reducing drag.
Ctenoid scales have tiny teeth called cteni on the posterior edge. They also overlap and grow in size by adding to the scale’s margins. This growth creates uneven bands called annuli which can be used to determine the age of the fish.
Other fish have modified scales that don’t easily fit any of the above categories. For example, seahorse and pipefish scales appear as bony plates, and porcupine fish scales become spines.
Clearly there is more than meets the eye when it comes down to the dermis. Those angling for a Derby win care little for those petit pieces on the fish’s surface, since they will likely never be enough to tip the scales in their favor. But scientists as well as nature lovers
— among whom I count fishermen and women — surely refer to the iridescent wonders of scales, not the proverbial pounds of flesh, when they say admiringly, “Now that’s a beautiful fish!” Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown.