Just a bit more:
And here's a quote from the site, studying different swimming actions in fish, think you'll like the bit about the Fineness Ratio:
Quote:
o Thunniform: Lateral movement of the torso is limited to the caudal peduncle (tail) and fin. The caudal fin is semilunate - i.e. tall and half-moon shaped.
In thunniform swimmers, water displacement is achieved by lift, generated by the caudal fin, rather than drag. The caudal fin has a high aspect ratio (span of fin/chord). Because lift is more efficient at higher speeds and Reynolds numbers, the thunniform morph is optimized for high absolute speed. The body must be highly streamlined. Indeed, even the caudal peduncle, the source of drag displacement in slower swimmers, is streamlined to reduce drag in its side-to-side motion. The strong selection for streamlining results in a typical maximum fineness ratio (diameter/length) of .25 at about 1/3 of the body's length. Example tuna.
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Biology of Extinct Animals