Down at about 23 meters while doing dive #2 of the PADI Deep Diver Specialty Course today, we had the pleasure of seeing one of Koh Tao’s strangest underwater residents: the razorfish.
Razorfish are so named because they resemble the straight razors of old. They are very thin, about 7 cm in length and they have a long dark band that goes down the length of their bodies. But the really strange thing about them is that they always swim totally vertically, upside-down in the water: Head facing the bottom, tail facing the surface.
Why such strange behavior? The answer is a wonderful example of evolutionary adaptation. The thin, vertical, dark band resembles the spines of a sea urchin. So, if the razorfish feels threatened, it simply darts into a nearby sea urchin and hides out. No predator is going to risk getting stuck with an urchin spine just for a snack. Even better, the razorfish feeds on small invertebrates that, in turn, feed on sea urchins. So the razorfish can hide from predators and grab lunch at the same time! Cool stuff.

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