There are many different ways to study the locomotion of animals and humans alike; for birds, there are wind tunnels, for humans, there are treadmills, and for fish, there are swim tunnels. Jeanine M. Donley, Chugey A. Sepulveda, Peter Konstantinidis, Sven Gemballa & Robert E. Shadwick wanted to look further into the locomotion of sea animals, specifically tunas and lamnid sharks (examples would be both mako and white). The reason for their study was to determine if the morphological similarities between these two sea animals meant that their mechanical and functional design had also converged. To do this, they studied the musculature and structure of the mako shark. The scientists examined Isurus oxyrinchus, a species of shortfin mako shark. The lamnid sharks used in this study ranged from 80 to 112 cm in size. While observing the shortfin mako in a swim tunnel, the scientists were able to determine where the dorsal midline was located using digital images. The position on the midline is important because swimming modes in fishes are explained based on the proportion of the body that is used during movement of the tail, which is also known as thrust-producing movements. These modes are distinguished by different patterns of displacement. Tunas fall under the least undulatory mode, meaning the body is very rigid and the tail is the source of most movement. When studying the mako shark, it was found that they have a similar mode to the tuna due to the degree of lateral motion along the shark’s form, which was from 0.4 L to 0.8 L (L meaning total body length). Past 0.8 L, the amount of movement in the tail increases substantially. After noting the similarity between the two species, the scientists wondered if the shortening the red muscle found in the mako shark would result in a functional property found in tunas. This functional property is when the tuna uses its red muscle fiber found in the midsection/upper region of the body to propel and create thunniform kinematics in the posterior region of the body, mostly the tail. There are long tendons that link to their tail so that even the red muscle is build more interior, they can use the tendons and the tail to create the movement of the tail while keeping the rest of their body very rigid. If the shortening of the red muscles happens at the same time as the white muscle, the mako shark would swim the way most fish do; if they are not synchronized, it means the mako shark swims and propels itself in the same way a tuna fish does. Sonomicrometry and electromyography used to measure muscles lengths instantaneously during swimming sessions, both active and passive. The results showed that the shortening of red muscles was uncoupled from the other tissues, similar to tunas. The results from their study also show that mako sharks possess the same tendon structure which aids in producing thrust. The findings of this study confirm that lamnid sharks and tunas have converged on both mechanical design, as well as morphological, concluding in selection for fast and continuous locomotion. The study itself reveals more data than previously known about lamnid sharks, but because these types of sharks are large and aggressive predators, they are difficult to handle and leaves dynamic properties about their individual locomotor system unknown. Future studies could look into the reasons behind this morphological and functional conversion; the usage of certain muscles, the placement of fins, the size of fish, etc. all deserve further research.
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