Fish scales
A fish scale is a fish scale, right? Think again. There are a number of types of fish scales and they come in all sorts of shapes and sizes.
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Do all fishes have scales?
No. Many species of fishes lack scales. All the clingfishes (family Gobiesocidae) for example, are scaleless. Their bodies are protected by a thick layer of mucous.
Why do fish have scales?
The primary purpose of scales is to give the fish external protection.
How many types of scales are there?
There are four main kinds of scales and numerous variations of each kind.
- Placoid (sharks and rays)
- Cosmoid (lungfishes and some fossil fishes)
- Ganoid (bichirs , Bowfin, paddlefishes, gars, sturgeons)
- Cycloid and Ctenoid (most bony fishes)
Different fishes, different scalation
It is interesting to think about the lifestyle and habitat of a fish, then look at its scales. It is not always the case, but faster swimming sharks usually have overlapping denticles. Slower swimming sharks often have courser non-overlapping scales.
Are all scales the same size?
No. Scale sizes vary greatly between species. Some fishes, such as the freshwater eels have tiny embedded scales.
Fishes such as the tunas have tiny scales often found in discrete areas of the body. Many fishes such as the Coral Snappers have medium sized scales whereas the scales of others such as the Tarpon, Megalops cyprinoides, are large enough to be used in jewellery. The scales of the Indian Mahseer, Tor tor, are known to reach over 10 cm in length.
How old is a fish scale?
As cycloid and ctenoid scales increase in size, growth rings called circuli become visible. These rings look a little like the growth rings in the trunk of a tree. During the cooler months of the year the scale (and otoliths) grows more slowly and the circuli are closer together leaving a band called an annulus. By counting the annuli it is possible estimate the age of the fish. This technique is extensively used by fisheries biologists.
Can a fish have more than one type of scale?
Yes. Some species of flatfishes (flounders, soles, etc) have ctenoid scales on the eyed side of the body and cycloid scales on the blind side.
Can scale type vary with sex?
Yes. In some species of flatfishes, males have ctenoid scales and females have cycloid scales.
References
- Briggs, J.C. in Paxton, J.R. & W.N. Eschmeyer (Eds). 1994. Encyclopedia of Fishes. Sydney: New South Wales University Press; San Diego: Academic Press [1995]. Pp. 240.
- Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
- Michael, S.W. 1998. Reef Fishes. Volume 1. A Guide to Their Identification, Behaviour, and Captive Care. Microcosm. Pp. 624.
- Roberts, C.D. 1993. Comparative morphology of spined scales and their phylogenetic significance in the Teleostei. Bull. Mar. Sci. 52(1):60-113
