There’s (arguably) nothing more iconic than the sight of a shark’s dorsal fin slicing through the water. But what’s their actual purpose, other than to strike terror into movie-goers?
What's the point of a dorsal fin?
One reason for a dorsal fin is for stability: having a dorsal fin prevents animals from flopping from side to side like an unruly log bobbing in the water. This also helps prevent them from rocking or rolling when making tight turns or changing direction.
The triangular fin of a great white shark might be the most famous but this species is not the only one with an impressive dorsal fin. Great hammerheads are known for their extraordinarily large dorsal fins – reaching up to around one metre tall – while basking shark fins can be up to two metres tall.
Most sharks have two dorsal fins: a larger one on their back and a smaller one closer to their tail. But some species living today have just one. These are from a primitive order called hexanchiformes and include bluntnose sixgill, broadnose sevengill and frilled sharks.
Sharks aren’t the only animals with dorsal fins, of course. Fish, dolphins, and most whales have them too.
If you ever trod on a weever fish at the beach as a kid (or an adult, for that matter), you’ll have experienced their self-defence mechanism firsthand. The searing pain pulsing through your foot was caused by venom injected through a defensive spine on their dorsal fin (not all fish have defensive dorsal spines, but some do).
In many cetaceans, dorsal fins are used for heat regulation because they don’t sweat to cool down like humans do. Their dorsal fins are full of blood vessels and, unlike the rest of their bodies, don’t have much blubber to insulate them. This exposes the fin to the water, meaning controlling the blood flow through the fin can help the animal cool down or warm up. More blood pumping through the dorsal fin allows more heat to escape into the water. Conversely, if they want to retain heat, they can constrict their blood flow to keep the heat inside their bodies.
Bowhead and beluga whales are noticeably lacking a dorsal fin. A fin on their back would make it difficult for them to swim under the ice sheets in their Arctic habitat. It might get stuck or injured. Not having a fin means they can use the ice as shelter when they need to get away from predatory orcas (which, incidentally, at up to 1.8 metres are the tallest dorsal fins of any cetacean)
Sadly, some dorsal fins put animals at risk. Many species of shark are targeted by fishers for their impressive fins, which often end up in shark fin soup. This is just one of many threats, including overfishing and bycatch, that imperils these magnificent animals. More than 100 million sharks are killed by humans every year.
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