Talk about growing pains – a recent publication in the Biological Journal of the Linnean Society pointed out that when a shark changes ecology, there is allometric growth in certain regions of the body in response.
The only thing life can guarantee is that change is inevitable. Rain will turn to snow, empty bumps at the end of tree branches will bloom into brightly colored leaves that will eventually die, and animals will grow and change as they age. Major changes to a creature’s habitat often occur during its lifetime, and this can have significant impacts on an animal’s morphology and function; however, little is known about how these ecological changes influence the locomotor system of large aquatic vertebrates.
Enter scientist Phillip Sternes, who is currently studying scalloped hammerhead sharks (Sphyrna lewini), a large shark species found worldwide in warm temperate and tropical waters. Younger and smaller scalloped hammerhead sharks are usually found in nearshore habitats, but as they grow these individuals spend more and more time in deep-water pelagic habitats. “Given changing habitat and food requirements, changes to the locomotor system are likely to be significant,” Sternes and colleagues proposed. Given the drastic differences in the ecology of shallow and deep water habitats, they thought it was likely that different morphological traits of scalloped hammerhead sharks might be favored in each habitat.
By measuring 50 different museum specimens of scalloped hammerhead sharks (with body sizes ranging from 32 to 130 cm TL), they collected 13 morphometric linear measurements and five area measurements for each specimen. As the tape measure passed from one end of the shark to the other, questions arose in their heads: “Do the pectoral fins of scalloped hammerhead sharks show allometric growth, given the dependence on vertical migrations for feed in deep-sea habitats? Does this also lead to a more symmetrical tail fin and/or changes in other body parts? They hoped that from the data collected, they could determine if there are any morphological differences present in these sharks and the implications this would have for their overall life history.
“We have found [that] pectoral fin length and area, pectoral fin to caudal fin aspect ratio, dorsal fin height, and caudal fin lower lobe length showed positive allometry. These changes indicate an increase in swimming efficiency through reductions in drag, indicating sustained swimming activity,” the authors explain. “This suggests that ecological changes have profound influences on the functional morphology of scalloped hammerhead sharks.”
Each fin of a shark plays a specific role. The caudal fin, also known as the tail fin, is likened to an engine helping to propel the shark forward through the water. “The caudal fins of scalloped hammerhead sharks in our study change from a more asymmetrical shape in smaller individuals to a more symmetrical shape in larger, more pelagic individuals,” Sternes explained. “This is likely directly associated with ecological changes, as smaller scalloped hammerhead sharks live in shallow water habitats, with limited home ranges. As individuals grow [and] they move in the pelagic domain to carry out long-distance migrations, […] a more symmetrical caudal fin with a high aspect ratio would greatly improve the cost of transport for such distances.
Another interesting find was the dorsal fin of this species. Depending on location along the body axis, the dorsal fin (the one commonly seen slicing through the waves in movies) of sharks helps with stability or thrust production; for this species, it is thought to help with stability. Scalloped hammerhead sharks swim at a 90° roll angle, a particular swimming behavior known as “side swimming” which is believed to reduce drag or, more importantly, the cost of transportation compared to normal swimming. in a vertical position. Yet research has pointed out that individuals who do this range from subadult to adult. “[We believe a] a larger dorsal fin could benefit pelagic great hammerhead sharks when performing long-distance migrations,” the authors explained.
Could these changes in body shape be observed in several families of sharks? Maybe. The team hopes that future studies will quantify the hydrodynamic changes associated with fin and body shape changes throughout a shark’s life to better identify functional consequences. They would be particularly keen to explore if possible sexual dimorphism (the differences in appearance between males and females of the same species) would be useful for comparisons with patterns seen in other hammerhead sharks.