By Blaise Jones
Sharks are amazingly adapted animals, and nowhere is that more apparent than in their body structure. Though sharks have evolved to possess vast differences in shape and size, there are a few universal traits that all shark species share, such as their fins.
All fish are classified into one of two families, Osteichthyes (bony skeleton) or Chondrichthyes (cartilage skeleton), and because of their different skeletal construction, shark fins are slightly different from their fins of their prey. Most bony fish (Osteichthyes) have fins that are branched, jointed, and segmented, allowing the fins to move independently from their bodies, giving them increased maneuverability to avoid becoming a shark’s next meal.
With skeletons made of cartilage, sharks (Chondrichthyes) have fins that are rigid and stiff, without any joints. Shark fins are unable to move independently from the body, instead the fins are only able to flex slightly to the sides and up and down, providing lift and steering for the sharks as they propel themselves through the water.
Sharks have five different kinds of fin: dorsal, pectoral, pelvic, anal, and caudal fins. The caudal fin is the shark’s main method of propulsion. It sits on the back of their tail in two lobes, forming a scythe-like shape. The top lobe is always longer than the bottom lobe, though in some species, such as the great white shark, the two lobes are almost identical in size. The spinal column of the shark extends into the top of the upper lobe.
The pectoral fins are the two fins that extend horizontally from either side of the shark. They function like airplane wings and provide lift. Pectoral fins are used for changing the shark’s position in the water column, whether that is moving up, down or turning.
The three remaining fins – dorsal, anal, and pelvic – all share in a single purpose: to keep the shark stabilized while swimming. If it weren’t for these fins, sharks would be rolling over and flipping upside down every time they attempted to make a turn.
Finning = Slow Shark Death
Because each of a shark’s fins plays an important role to its survival, shark finning is lethal to sharks even though fishermen return the shark to the water alive. Without its fins, the shark is doomed to slowly drown and bleed to death if not eaten by other predators first.
If even one fin is damaged or missing on a shark, the animal is incapable of swimming properly. They’ll be unable to turn, or prevent themselves from spinning. In the end, a shark with damaged fins will either drown, starve or become prey for other predators.
The dorsal fin has an added feature besides helping with stability. Not only does the dorsal fin (or fins, in the species that have two) serve as an iconic image for sharks, it also aids in propulsion.
The dorsal fin of a shark cannot be folded like those of bony fish, and thus produces a lot of drag as the shark moves through the water. While normally this would be a hindrance for an animal focused on staying streamlined, sharks have managed to use this drag to their advantage. Sharks use this drag to create pivot points, which the shark is able to push off of to create more forward thrust.
The dorsal and caudal fins work in tandem to create a swirling vortex that travels down the shark’s body. As the vortex swirls down the length of the shark’s body, the shark’s second dorsal fin (if it has one) and caudal fin pushes against the vortex, creating additional thrust.
A Walk in the Park
However, some sharks have decided to use their fins for something completely different. The epaulette shark has large, rounded pectoral and pelvic fins which it uses to walk over the rocky terrain of its home environment. When startled, this shark actually tries to run away instead of swim. The epaulette shark looks like “a salamander with ping-pong paddles for legs, ” according to R. Aiden Martin, director of the ReefQuest Centre for Shark Research.
Nurse sharks have also been observed using their pectoral fins to lay traps for their prey. Young nurse sharks will actually curl their pectoral fins underneath their bodies, raising them up and creating artificial caves and hiding places for their favorite prey, crabs. The crab, thinking it has found a safe cavern, walks directly beneath the waiting mouth of the hungry nurse shark.
“The Encyclopedia of Sharks” by Steve Parker
“Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess