Dive Inside The Weird World Of The Ocean Sunfish | HowStuffWorks

In Latin, the word "mola" means "millstone." Being flattened, roundish and often gray in color, these animals do bear a passing resemblance to their namesake rocks.

All known sunfish have an unusual pseudo-tail that contributes to their vaguely oval-shaped appearance. Most fish possess caudal (tail) fins at the end of their spinal columns. Bass, sharks, sturgeons, and the vast majority of other fish use these things to propel themselves forward.

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Yet, bizarrely, sunfish don't have caudal fins. Instead, the rear end of a sunfish is covered by portions of two other fins: the dorsal fin on top of the creature and the anal fin on its underbelly. Together, they form a "pseudo-tail" known as the clavus.

Short, stubby pseudo-tails make sunfish heads seem disproportionately large. In Germany, the sea beasts are popularly called "schwimmender kopf," or "swimming heads." We'll talk about those strange noggins in good time, but first, there's another fin-related issue that warrants discussion.

Remember the dorsal and anal fins that we mentioned earlier? Well, the frontal regions of both appendages are tall, triangular and wing-like. By flapping those structures from side to side, sunfish propel themselves forward. Meanwhile, the clavus functions as a rudder, steering them as they go.

"They are fine swimmers and divers capable of swimming against currents," Thys says. She adds that, in the span of 24 hours, a sunfish may cover 12.4 to 18.6 miles (20 to 30 kilometers) as it swims. Moreover, the fish can dive "up to 1000 meters" (or 3,280 feet) below the surface "multiple times a day into chilly waters" with temperatures as low as 35.6 degrees Fahrenheit (2 degrees Celsius).

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