Who's tougher? Baby sharks or daddy sharks?

January 3, 2019, Florida Atlantic University

It's not just their teeth and jaws that people find intriguing. It's also their funky shapes and unique skeletal makeup that capture attention. Unlike humans and most land animals, sharks have mineralized cartilage skeletons instead of bones. This allows them to move at unbelievable speeds through the water. Since cartilage weighs less than bone and is less dense, sharks can bend, swim, and maneuver in the ocean much differently than their bony fish counterparts.

Because sharks vary in size and shape, there is great diversity in their morphology, physiology and how they swim. For example, the common thresher shark relies on its tail to stun prey when feeding, and the size of its vertebrae and their mechanics may explain why it depends on a strong and long tail that operates like a whip. To move this way requires low stiffness and toughness, or a lower resistance to deformation and ability to absorb energy, respectively.

A shark's vertebral column is governed by dynamic and complex interactions among tissue composition and morphology, and there are many differences in growth, mineralization and mechanical properties.

Scientists from Florida Atlantic University's Charles E. Schmidt College of Science and the National Marine Fisheries Service, National Oceanic and Atmospheric Administration (NOAA), predicted that the solid central part of the vertebrae in mature, older sharks would be stiffer and tougher. So they decided to put their theory to a test.

They examined cartilage mechanics from six species of sharks under biologically relevant conditions along the length of their bodies and over a range of ages. They looked at small, infant (young of year) sharks, immature sharks and mature sharks from each of six different species: the dusky shark (Carcharhinus obscurusp); the porbeagle (Lamna nasus); and charismatic sharks like the great white shark (Carcharodon carcharias); the shortfin mako (Isurus oxyrinchus) and the common thresher shark (Alopias vulpinus).

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