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Convergent evolution in the mechanical design of lamnid sharks

Submitted by kheredia on Thu, 10/31/2019 - 11:00

The driver of evolutionary change is natural selection, a process where communities thrive or perish depending on the environmental conditions. Despite the unique traits that darwinism has brought, these evolved bodily functions are not exclusive to one species. Lamnid sharks and tunas are an example of two species thought to have independently evolved similar traits. However, there is little data regarding the mechanisms behind their convergent evolution. Under these circumstances, researchers have investigated the evolutionary relationship between mako sharks, Isurus oxyrinchus, and tunas’ swift, continual movement and morphological design. They also compared swimming kinematics, and muscular function to determine this. When exposed to a controlled swim tunnel, scientists observed how both species concentrate movement in their posterior, more specifically, the caudal region of the tail.

Results showed that mechanistically, allowing the mid-body to become virtually stiff and focusing muscle activation in the rear has evolutionarily allowed these fish to become more energy efficient. Because the tuna and shark resemble one another in their structural design, both are able to swim for longer periods of time compared to other fish, without the cost of travel. The important feature in tunas which allow this specialized movement is the physical uncoupling of the red and white muscle when in motion. In other species, the muscles act synchronously. This thunniform-like mobility in tuna was tested for similarity in mako shark via sonomicrometry, by shortening the muscle during passive and active swimming in the hopes to detect uncoupling. Throughout active swimming, they recorded asymmetrical muscle activity. There was a delay in red muscle strain compared to white muscle. This confirmed that red muscle was in fact uncoupled, supporting the claim that tunas and mako’s are evolutionarily similar. This is also indicative of strong posterior movement. The analogous relationship between the two species was supported morphologically as well. Elongated tendons were measured in both fish. Its association with the red muscle creates a system which allows the transfer of great power from the anterior of the animals’ to the posterior.

However, the driving force behind the system was found in the hypaxial lateral tendons in sharks, whereas the tuna’s primary source of transmission is located in the horizontal septum of the tendon. Despite this regional difference, the study overall was representative of a phenomenal evolutionary relationship between two separate species; though it does have its complications. Studying animals such as sharks without sedation can serve to be quite difficult. These predators pose great danger to the handlers, so precautions must be taken and some methods may be carried out quickly; resulting in fewer studies and limiting the amount of information available to others. To avoid potential risks in the future, studying the similar, less harmful tuna in the place of lamnidae can be useful.

A future experiment may include a common ancestor of the tuna to study the mechanisms in which they diverged. This will help map out the history of how tunas were able to develop different characteristics over time, and eventually become similar to the mako shark.