Sitting on the beach, you might start to panic if you see a gray fin at the ocean’s surface. However, what you’re seeing could be either a shark or dolphin as the dorsal fins (fins on the back/top of the body) are quite similar. So what are the similarities and differences between sharks and dolphins? 

Type of Animal

The first difference between sharks and dolphins is the type of animal they are — sharks are fish while dolphins are mammals (like us). Dolphins, and other mammals, are warm blooded, give birth to live young, nurse their young, are born with hair, and breathe air. Sharks, like other fish, have gills to remove oxygen from the environment, are cold blooded, and have scales. Another fact about sharks is that they are what are known as cartilaginous fish, meaning their skeletons are made of cartilage (like our ears and noses) rather than dense bone. Sharks are part of the elasmobranch family, which includes sharks, rays, skates, and sawfish, while dolphins are part of the cetacean family, which includes toothed and baleen whales, as well as porpoises.  

Appearance

Both sharks and dolphins generally have what is called countershading where their bellies are lighter than their backs, which is a helpful way to blend in to the marine environment. Dolphins and sharks both have dorsal fins on their backs, two pectoral fins on their sides, and a tail. Sharks’ tail fins are vertical and they use them in a side-to-side motion to swim, while dolphins’ tails are horizontal and they use them in an up-and-down motion to swim. Sharks have gills on the side of their body while dolphins have a blowhole on the top of their head. 

Social structure

Dolphins are known to be vocal and quite socials. They often travel in groups called pods, and sometimes even megapods, or superpods, form where hundreds or thousands individuals come together to travel or mate. Sharks on the other hand are more solitary and cannot vocalize, however research on shark social behavior is not as well studied as with dolphins. 

Diet and Habitat

Both sharks and dolphins are opportunistic feeders, meaning they eat whatever is available and easiest to catch, and carnivores. Their diets can include fish, crustaceans, squid, and shrimp. Where sharks’ diets differ is that they will sometimes eat other sharks, marine mammals, or plankton depending on the species of shark. 

Dolphins need to live where they can surface to breathe air while sharks spend their whole lives underwater, calling the open ocean, shallow coastal waters, coral reefs, and in some cases even rivers home.