Shifts In Habitat Have Changed This Shark’s Body Shape

Talk about growing pains – a recent publication in the Biological Journal of the Linnean Society has highlighted that when a shark shifts its ecology, there is allometric growth in certain regions of the body in response.

The only thing life can guarantee is that change is inevitable. The rain will turn to snow, the empty nubs on the end of tree branches will blossom into brightly colroed leaves that will eventually die, and animals will grow and change as they age. Major shifts in a critter’s habitat often occurs during their life, and it can have significant impacts on the morphology and function of an animal; however, little is known about how such ecological changes influence the locomotor system of large aquatic vertebrates.

Enter scientist Phillip Sternes, who currently is studying scalloped hammerheads (Sphyrna lewini), a large shark species found worldwide in warm temperate and tropical waters. Younger and smaller scalloped hammerheads are generally found in near-shore habitats, but as they grow larger, these individuals spend more and more time in deep-water, pelagic habitats. “Given the shift in both habitat and feeding requirements, changes to the locomotor system are likely to be important,” Sternes and his colleagues proposed. Considering the drastic differences in the ecology of shallow- and deep-water habitats, they believed it was likely that different morphological traits of scalloped hammerheads 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 gathered 13 morphometric linear measurements and five area measurements for each specimen. As the tape measure ran from one end of the shark to the other, questions swam in their heads: “Do the pectoral fins of scalloped hammerheads exhibit allometric growth, given the reliance on vertical migrations for foraging in deep-water habitats? Does this also lead to a more symmetrical caudal fin and/or changes to other parts of the body?” They hoped that from the collected data, they could address whether there are any morphological differences present in these sharks and the implications it would have in relation to their overall life history.

“We found [that] pectoral fin length and area, pectoral fin and caudal fin aspect ratio, dorsal fin height and the length of the caudal fin lower lobe exhibited positive allometry. These changes point towards an increase in swimming efficiency through reductions in drag, which are indicative of sustained swimming activity,” the authors explain. “This suggests that the changes in ecology have profound influences on the functional morphology of scalloped hammerheads.”

Each fin on a shark plays a specific role. The caudal fin, known also as the tail fin, is likened to a motor helping propel the shark forward through the water. “The caudal fins of scalloped hammerheads in our study transitions from a more asymmetric shape in smaller individuals to a more symmetrical shape in larger, more pelagic individuals,” explained Sternes. “This is likely to be directly associated with the changes in ecology, because smaller scalloped hammerheads live in shallow-water habitats, with limited home ranges. As individuals grow larger [and] they move into the pelagic realm to perform 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 for this species. Depending on the location along the body axis, the dorsal fin (the one commonly seen slicing through waves in movies) of sharks aids in either stability or thrust production; for this species it is thought to help in stability. Scalloped hammerheads swim at rolled angle of 90°, a pecular swimming behavior known as ‘side swimming’ that is thought to reduce drag or, more importantly, the cost of transport compared with normal, upright swimming. Yet, the researches pointed out that the individuals doing this ranged from subadult to adult. “[We believe a] larger dorsal fin might benefit larger pelagic scalloped hammerheads when they perform long-distance migrations,” explained the authors.

Could these changes in body shape be seen throughout multiple shark families? Perhaps. The team hopes for future studies to quantify the hydrodynamic changes associated with changes in fin and body shape through a shark’s life to better identify any functional consequences. They would be especially 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 hammerheads.