Jaw shapes of various shark species provide insight into evolutionary biology

Sharks occur in great diversity in the oceans. The great white shark with its characteristic back fin, large conical mouth and sharp, pointed teeth is just one of more than 500 species of shark living today. Depending on the lifestyle, the species differ in their anatomical characteristics. How the shape of the jaw correlates with habitat, size, nutrition and other factors has been investigated in an international study with the participation of Professor Christine Böhmer from the Zoological Institute of the Christian-Albrechts-Universität zu Kiel (CAU). The results were recently published in the journal Communications Biology. They show: In the most widespread shark species, the jaws show relatively little variation in shape over millions of years; the most variable jaws were found in deep-sea sharks.

The study was led by Dr. Faviel A. López-Romero of the University of Vienna. Besides the CAU, the Imperial College London, UK, the Muséum national d'histoire naturelle, Paris, France, and the Naturalis Museums, Leiden, The Netherlands, were also involved. The results illustrate the importance of prey, level in the marine webs and habitat in relation to jaw shape diversity among shark species. This also helps to uncover the evolutionary causes of the differences in jaw morphology related to habitats.

All marine habitats are populated by sharks

Today's sharks have a long evolutionary history with some taxa that can be traced back to as far as 180 million years ago. During all this time, they have been a key component in the fauna of the marine realm and its food webs, mainly occupying higher trophic positions as meso- and top predators. At the same time, sharks adopted many lifestyles and forms like bottom dwellers, fast swimmers in the open sea, and even some of the smallest species in the deep sea.

The jaws of sharks are perfectly adapted to their diet and lifestyle. "The shape of the mandible can be used as an indicator of the shark's lifestyle, as it is related - among other factors - to its function. The jaw structure of an animal that eats hard food, for example, is different from that of an animal that filters plankton." explains co-author Christine Böhmer, professor and head of the Zoology and Functional Morphology of Vertebrates working group at CAU. Whether the jaw shape can be used to deduce the habitat and lifestyle in which the animals evolved was examined in the study using quantitative analyses of computed tomography scans and three-dimensional mandible models.

"We compared the 3D models of the mandibles of 90 shark species existing today using what is known as geometric morphometrics," says Böhmer, who is particularly familiar with this special technique for measuring skeletal parts. In this method, special software is used to place a large number of digital measuring points, so-called "landmarks", on the lower jaw in order to record its shape in its entirety. Subsequently, the jaw shape captured via "landmarks" can be compared to other shapes using statistical methods. "Traditionally, you would measure a bone by length, width and such. With landmark analysis, you have much more information because you can include more variables from the shape of an object, similar to facial recognition, for example," explains the paleobiologist, who is am member of the priority research area Kiel Life Science (KLS) at Kiel University.

Most variable jaw forms among deep sea sharks

Surprisingly, the results indicate that among highly species-rich groups such as requiem sharks, the jaws display low shape variations. This is interesting since requiem sharks are one of the most widely distributed group of sharks. Another interesting finding is that most variable jaws were found among species living in the deep sea. "Although sharks from the deep sea are not as extensively represented in the data as reef sharks, they display the most disparate forms seen in our analysis", explains first author Dr. Faviel A. López-Romero from the Department of Palaeontology at the University of Vienna.

Among many adaptations, sharks inhabiting the deep-sea exhibit various feeding strategies that range from taking big chunks out of whales to feeding on eggs or on cephalopods. For most of the species found in reefs and the large top predators in the open sea, the options seem more limited so that most mainly prey on fishes, and even other shark species. "Of course, many sharks in these environments feed on a large variety of prey with only few having adapted to a single, specific prey, such as the bonnethead shark, Sphyrna tiburo, which preys almost entirely on hard-shelled crabs, while shrimps and fish are only capture occasionally", Professor Jürgen Kriwet from the University of Vienna states, who was involved in this study.

The study also shows, says Christine Böhmer: "The deep sea is an important resource for biodiversity in sharks. New species emerge not only in shallow water close to the coast, as a classic hypothesis states, but also in the deep sea."

To Original publication Böhmer Group at Kiel University Research Group at University of Vienna