Scale up the scale
By Yuan Xue Materials Science Program  


home salmon bass red snapper sea bass








As seen on most fish in supermarket, such as salmon, bass, red snapper and so on, fish scales are very common. The outer body of fish is covered with scales, which are part of the fish's integumentary system. However, have you ever noticed that the shape and color of scale differ from one fish to another? Actually, the use of scale morphology for fish classification, also known as lepidology, can be tracked back to the middle of nineteen century. Agassiz, a Swiss palaeontologist, was the first one to use fish scales for taxonomy. In his work, he divided fishes into four groups according to the structures of their scales: Placoidei, Ganoidi, Ctenoidei, and Cyloidei.

The left figure is showing the descriptive terminology of a typical fish scale. Each category of fish possesses different characteristics, for instance, ctenii, radii, and inter radial pattern. Here, I pick up some scales from four kinds of fish, as seen above, salmon, striped bass, red snapper, and sea bass, to look into the morphology and materials. Thanks to the electron microscope, a fantastic microworld is awaiting. Ready? Let's scale it up!



Seven methods are involved in this project:

1. Light Microscopy: traditional light microscopy equipped with laser 3-D scanning.

2. Scanning Electron Microscopy (SEM): obtaining the morphology of sample.

3. Sputter Coating: making the biology sample conductive for SEM analysis.

4. Atomic Force Microscopy (AFM): measuring the height of different scale patterns.

5. Energy Dispeprsive Spectroscopy (EDS): carrying out the accurate elemental analysis for scale samples.

6. Backscattered Electron (BSE) imaging: giving general elemental information of sample in a contrast picture.

7. Colorization: using photoshop to colorize SEM greyscale images to make them visually appealing by photoshop.

8. Panorama image assembling: more than 30 micrographs are taken to assemble to a panorama image for the scale sample.



Salmon Striped bass Red snapper Sea bass
(top: light microscopy images; bottom: SEM images)

Four types of fish scale sample are observed mainly by SEM, and also by light microscope and AFM. As a powerful tool in material study, SEM can only give greyscale micrographs, lacking the real color information of the sample. Therefore, I colorized the micrographs with pseudo colors to reflect their natural looks (see images in the second row above). Based on the results, the salmon scale demonstrates a distinct characteristic from the other three scales. The salmon scale has a smooth edge without any "tooth", which makes the salmon scale come into the form of cycloid. The striped bass, red snapper, and sea bass scales all have a toothed outer, so called Ctenii that gives them a rough texture. This feature makes these three scales are categorized as ctenoid.



I would like to express my gratitude to Brian L. McIntyre for giving such a wonderful course of Opt307. I would also like to thank our TA, Rohit for the great help with this project. I am grateful to Suju Gurung, Maria Abreu-Sepulveda, and Xiang Li for their inspiring duscussion and advice.


[1]Laith A. Jaward Comparative scale morphology and squamation patterns in triplefins (Pisces: Teleostei: Perciformes: Tripterygiidae). Tubinga (2005)16: 137-167.




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