Describing a tail fin that is asymmetrical, with the upper and lower lobes unequal in size.
From Greek aniso- (unequal) combined with -cercal (from Greek kerkos, tail). This ichthyological term describes the heterocercal tail structure found in sharks and some ancient fish.
A shark's unequal tail fins actually generate lift and help steer the shark upward as it swims—what looks like imperfection is actually an engineering feature that lets sharks be aggressive hunters in three dimensions.
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