Fish scales

I previously mentioned that the scale count along the lateral lines of some fish is often diagnostic of the species. But I then read something about the diagnostic value of the shape of the scales themselves - they are often similar within a family or genus, but vary between families and genera.

Gudgeon, Perch and Pike scales

The scales also show growth rings or 'annulae'; and rather like the rings inside a tree, they can be used to determine the age of the fish. The Delaware Division of Fish and Wildlife have a little document about this.  The obvious rings are formed during the winter when the fish (and its scales) grow much more slowly.

The scales above are from bony fish, but cartilaginous fish like sharks have scales that evolved separately. The latter are structurally like our teeth with a pulp cavity and a layer of dentine. And they completely appeal to my love of repeating patterns:

Lemon shark CC image by Pascal Deynat

Shark scales feel rough but apparently the vortices the shapes produce in the water actually cut hydrodynamic drag (ahem, or maybe I've got the wrong end of the stick and they're a bit like winglets on a plane - whichever way, the point is the fish moves more efficiently).

It strikes me that this subject could be something to include in passing in your monograph, especially if you have specimens that represent the different groups (and of course some fish have no scales at all).

Wikipedia  does have a very good overview and there's some more information on a page from Earthlife too.

If you wanted to look at your scales under a microscope that would certainly be possible. And if you got really interested then pursuading my colleague with his elecron microscope to get involved might not be out of the question. And to think that until yesterday I had barely given fish scales a second thought.