Fish scale

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Cycloid scales cover these teleost fish (rohu)

The skin of most fishes is covered with scales, which, in many cases, are animal reflectors or produce animal coloration. Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as shrimpfishes and boxfishes, to microscopic or absent in fishes such as eels and anglerfishes. The morphology of a scale can be used to identify the species of fish it came from.



Placoid scales[edit]




The skin of sharks is entirely covered by placoid scales. The scales are supported by spines, which feel rough when stroked in a backward direction, but when flattened by the forward movement of water, create tiny vortices that reduce hydrodynamic drag, making swimming both more efficient and quieter compared to that of bony fishes. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was in making hand-grips for swords. The rough texture of the skin is also used in Japanese cuisine to make graters called oroshiki, by attaching pieces of shark skin to wooden boards. The small size of the scales grates the food very finely.


Leptoid scales[edit]


Cycloid scales[edit]

Cycloid (circular) scales
The cycloid scale of a carp has a smooth outer edge



Ctenoid scales[edit]

Ctenoid (toothed) scales
Three ctenoid scales from various locations of a perch were stained. Significant variation can be observed between the medial (middle of the fish), dorsal (top), and caudal (tail end) scales. The ctentii of each of the scales is labeled.



Ctenoid scales, similar to other epidermal structures, originate from placodes and distinctive cellular differentiation makes them exclusive from other structures that arise from the integument.[6] Development starts near the caudal fin, along the lateral line of the fish.[7] The development process begins with an accumulation of fibroblasts between the epidermis and dermis.[6] Collagen fibrils begin to organize themselves in the dermal layer, which leads to the initiation of mineralization.[6] The circumference of the scales grows first, followed by thickness when overlapping layers mineralize together.[6]

Ctenoid scales can be further subdivided into three types:



Ganoid scales[edit]

The longnose gar has diamond-shape ganoid scales

Ganoid scales are found in the sturgeons, paddlefishes, gars, bowfin, and bichirs. They are derived from cosmoid scales, with a layer of dentine in the place of cosmine, and a layer of inorganic bone salt called ganoine in place of vitrodentine. Most are diamond-shaped and connected by peg-and-socket joints. They are usually thick and have a minimal amount of overlap as compared to other scales.[8] In sturgeons, the scales are greatly enlarged into armour plates along the sides and back, while in the bowfin the scales are greatly reduced in thickness to resemble cycloid scales (see above).


Elasmoid scales[edit]



Cosmoid scales[edit]




  • an external shield-like bony plate, or


Thelodont scales[edit]



Bone, a tissue that is both resistant to mechanical damage and relatively prone to fossilization, often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. The scales comprise a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine base.[11] Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish.[12] Beyond that, there appear to be five types of bone-growth, which may represent five natural groupings within the thelodonts—or a spectrum ranging between the end members meta- (or ortho-) dentine and mesodentine tissues.[13] Interestingly, each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish.[12]

However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area,[14] with intermediate forms appearing between different areas—and to make matters worse, scale morphology may not even be constant within one area. To confuse things further, scale morphologies are not unique to taxa, and may be indistinguishable on the same area of two different species.[15]

The morphology and histology of thelodonts provides the main tool for quantifying their diversity and distinguishing between species, although ultimately using such convergent traits is prone to errors. Nonetheless, a framework comprising three groups has been proposed based upon scale morphology and histology.[13] Comparisons to modern shark species have shown that thelodont scales were functionally similar to those of modern cartilaginous fish, and likewise has allowed an extensive comparison between ecological niches.[16]





Cetonurus crassiceps scales.jpg Cetonurus crassiceps2.jpg
PSM V35 D070 Scale of eel.jpg Anguilla japonica 1856.jpg

See also[edit]


  2. ^ Perkins, Sid (16 October 2013). "The First False Teeth". Science. Retrieved 2 March 2018. 
  6. ^ a b c d Kawasaki, Kenta C., "A Genetic Analysis of Cichlid Scale Morphology" (2016). Masters Theses May 2014 - current. 425.
  7. ^ Helfman, Gene (2009). The Diversity of Fishes Biology, Evolution, and Ecology. Wiley-Blackwell. 
  8. ^ Sherman, Vincent R.; Yaraghi, Nicholas A.; Kisailus, David; Meyers, Marc A. (2016-12-01). "Microstructural and geometric influences in the protective scales of Atractosteus spatula". Journal of The Royal Society Interface. 13 (125): 20160595. doi:10.1098/rsif.2016.0595. ISSN 1742-5689. PMC 5221522Freely accessible. PMID 27974575. 
  10. ^ Turner, S.; Tarling, D. H. (1982). "Thelodont and other agnathan distributions as tests of Lower Paleozoic continental reconstructions". Palaeogeography, Palaeoclimatology, Palaeoecology. 39 (3–4): 295–311. doi:10.1016/0031-0182(82)90027-X. 
  11. ^ Märss, T. (2006). "Exoskeletal ultrasculpture of early vertebrates". Journal of Vertebrate Paleontology. 26 (2): 235–252. doi:10.1671/0272-4634(2006)26[235:EUOEV]2.0.CO;2. 
  14. ^ Märss, T. (1986). "Squamation of the thelodont agnathan Phlebolepis". Journal of Vertebrate Paleontology. 6 (1): 1–11. doi:10.1080/02724634.1986.10011593. 
  15. ^ Botella, H.; J. I. Valenzuela-Rios; P. Carls (2006). "A New Early Devonian thelodont from Celtiberia (Spain), with a revision of Spanish thelodonts". Palaeontology. 49 (1): 141–154. doi:10.1111/j.1475-4983.2005.00534.x. 
  16. ^ Ferrón, Humberto G.; Botella, Héctor (2017). "Squamation and ecology of thelodonts". PLoS ONE. 12 (2): e0172781. doi:10.1371/journal.pone.0172781. 

Further reading[edit]
