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Revealing the Inner Fish


The fins of the scalyhead sculpin are related to our arms.

The scalyhead sculpin is a flathead fish that lives in the intertidal zone of the Eastern Pacific, along the coasts from Alaska to Southern California. It spends much of its time on the ocean floor, or hiding under rocks, and can often be found by flipping rocks at low tide. This particular fish was stained with dyes that color bone red and cartilage blue, and treated with chemicals that make the body transparent. This process reveals the intricate details of the sculpin’s bony skeleton, including bony scales on the head and flanks and “rays” of bony spines inside the fins. Fish and modern humans share a common ancestor, and the bones in the sculpin’s fins are equivalent to our arms and hands.

To find out more about our fishy past, find related educational resources at the Your Inner Fish page on BioInteractive.

Technical Note:
The fish was stained with an Alcian Blue dye to highlight cartilage and Alizarin Red S to turn bone bright red. The surrounding tissue was dissolved using trypsin, an enzyme that breaks down proteins. The fish was then submerged in a solution of glycerin, a sugar alcohol, to preserve the fish and make the remaining tissue transparent. 

Image courtesy of Adam Summers, PhD, Friday Harbor Laboratories, University of Washington, USA



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