Pure, Unadulterated Speed
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Image Source: National Geographic
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Bluefin Tuna
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Few fish embody the quintessence of speed like the tuna does. The sleek body, the narrow caudal peduncle, its mesothermic metabolism, the fin layout, and a host of specialized adaptations combine to make one of the fastest fish in the ocean. Oh, and let's not forget that beautiful lunate, bony caudal fin.
Drag increases with the square of velocity, so any adaptation that mitigates drag is going to be of significant benefit to fast swimmers. Tuna, dolphin, and mackerel have been observed to experience cavitation behind their caudal fins. This saps efficiency, reduces thrust, and depending on when and where the cavitation occurs, it can be damaging. For dolphins and other cetaceans this cavitation damage is painful for their flukes.
But for tuna, the caudal fin is not innervated, but rather a solid bony structure. Because tuna do not feel this damage, they ignore it as evidenced by cavitation damage on their caudal fins. [K. Schmidt & Nielsen][New Scientist][Iosilevskii & Weihs][Kishinouye]
The caudal fin is lunate in shape, and has a large aspect ratio (Long and skinny caudal lobes). The lunate design has a minimum of surface area on which water can impose drag. At high speeds, the tailbeat amplitude is small to reduce drag and cavitation, and the tailbeat frequency is high. [JEB #4][Nature #5]
But for tuna, the caudal fin is not innervated, but rather a solid bony structure. Because tuna do not feel this damage, they ignore it as evidenced by cavitation damage on their caudal fins. [K. Schmidt & Nielsen][New Scientist][Iosilevskii & Weihs][Kishinouye]
The caudal fin is lunate in shape, and has a large aspect ratio (Long and skinny caudal lobes). The lunate design has a minimum of surface area on which water can impose drag. At high speeds, the tailbeat amplitude is small to reduce drag and cavitation, and the tailbeat frequency is high. [JEB #4][Nature #5]