Nothing incites panic in the ocean like an ominous shark fin cutting through the water in the direction of swimmers. Yet, sometimes the alleged shark is nothing more than a playful or curious dolphin. Although shark and dolphin fins share some characteristics, a closer look reveals which type of animal actually is swimming nearby.
The dolphin’s dorsal fin is curved backward toward the animal’s rear. It has a definite arc, with a sharply curved tip. A shark’s dorsal fin is broad and stands straight up from the animal’s back. The front edge of a shark fin is angled backward, while the rear edge is straight. This generally is the easiest way to identify the animal at a glance.
Depending on the species, sharks have as many as four unpaired fins along their bodies. The second dorsal fin usually is visible above the water’s surface. It is much smaller than the first dorsal fin, and further back, so identifying it often requires a closer look. Dolphins have pectoral fins that normally are underwater. If you see a second fin, the animal is a shark.
While dolphins and sharks have almost infinite control over their fins, their manners of moving through the water are different. Dolphins have wide horizontal tails, which they move up and down as they swim. Sharks have tall vertical tails, which move side to side. This difference in the way the animals move creates different effects in the visible fins. If the fin appears to be bouncing up and down, you are likely observing a dolphin. If the fin moves back and forth, the animal might be a shark.
Although it is tough to see unless you’re very close, dolphin and shark fins are attached differently. A dolphin’s dorsal fin curves smoothly from the animal’s back. A shark’s dorsal fin has a triangular notch at the base on the rear side, separating that part of the fin from the shark’s back.
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