It’s all in the fins: Pectoral fin rotation in bonnethead sharks

We think of sharks as quick and maneuverable animals able to leap out of the water and turn on a dime to capture prey. But, like most of us, sharks don’t spend all of their time cruising through the water at top speed. Most of the time, they are moving at a more moderate pace. 

Unlike extreme movements, moderate kinematics are understudied in fish. Dr. Sarah Hoffmann and colleagues at Florida Atlantic University decided to investigate the steady swimming in sharks focusing on the turning behavior of the bonnethead (Sphyrna tiburo). She used a technique called Video Reconstruction of Moving Morphology (VROMM) to measure the motion of the shark’s pectoral fins in 3D.

Using VROMM on freely swimming sharks is no easy task. Dr. Hoffmann had to figure out a way to attach markers onto the parts of the animal she was interested in. For this experiment, black beads were super glued on to an anaesthetised bonnethead before releasing it into a large circular filming tank. Two GoPros captured the 3D motion of the shark turning casually through the water as well as the specific degrees of rotation of its pectoral fins.

(Hoffmann et al Figure 1a – Example of the bead layout on a bonnethead shark)

It turns out that bonnethead sharks rotate their pectoral fins in a very complex way depending on where they are in the water column when they are turning. They are even able to adjust their behavior to use the ocean floor turing a turn. Dr. Hoffmann found that this complicated pectoral fin rotation plays a significant role in how the animals move! Previously, researchers assumed that it was only the bending of the body that caused a shark to turn. But pectoral fin rotation directly relates to how fast and sharp the animal turns around in the water.

Dr. Hoffmann also described the muscles that control pectoral fin movement and again used VROMM to measure how each muscle contributes to the way a bonnethead turns. She did this by using the same black bead markers on an anesthetized fish, and shocking the muscles individually with an electric current to see how they cause the fin to move. Each of the three muscles contributes in a very specific way to the rotation of the fin. The shark needs to activate different muscles at different times throughout the turn to achieve the specific rotations we see in a freely swimming fish.

(Hoffmann et al Figure 7. Illustrations of Bonnethead Shark pectoral fin muscles and their individual roles in fin rotation)

Why is it important for us to look at the way sharks move while they are casually meandering through the water? Dr. Hoffmann says that researchers spend so much time focusing on extreme movements, that we forget that animals spend most of their time doing other things. Looking at day to day casual kinematics can help us to better compare and understand the relationship between morphology and kinematics in a larger variety of species.

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