The anal fin is located on the bottom of a fish, in between the tail (caudal fin) and the anus. Along with the dorsal fin, it stabilizes the fish against sideways rolling movement, much like the keel of a ship. The anal fin is not typically used for propulsion, except in rare cases like the Knifefish.

In 1998, while warming up before a contest at Niijima beach in Japan, the legendary surfer Tom Carroll had the fin of his surfboard lodged up his anus in what is generally considered one of the most cringe-inducing wipeouts in surfing history.

The injury resulted in 13 stitches, 5 of which were internal. After misunderstanding his doctors, Tom (who spoke no Japanese) accidentally applied an antiseptic without diluting it, irritating the area around the injury greatly.

The net result of this was a short Australian man running around a Japanese hotel with no pants on and a bandaged arse, screaming incomprehensibly at people and pointing at his blistered balls. True story.

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