Shark body scans reveal lighter frames in deeper waters

Shedding weight for the deep

Why would a walking epaulet shark pack forty times more mineral into its body frame than a giant Greenland shark?

Sharks are famous for having internal frames built from cartilage rather than rigid bone. Cartilage is the firm, rubbery tissue found in human ears and noses. Yet shark frames are not always soft. Many species harden parts of their cartilage with calcium minerals to stiffen jaws or strengthen fins. New scans show that shark skeleton mineralisation is not simply fixed by ancestry. Instead, it shifts dramatically depending on where and how a shark lives.

Scanning through the depths

To investigate this pattern, an international team scanned 99 individual sharks from 41 species, covering all nine living shark orders. They used quantitative computed tomography (qCT). This medical imaging method uses a calibration block to turn standard X-ray readings into exact measures of mineral density. This method matters because traditional flat X-rays cannot measure true three-dimensional mineral mass across whole animals.

By matching these scan values against ocean depth, the team discovered that overall mineral content fell significantly in deeper waters. In shallow shallows, reef dwellers like the epaulet shark reinforce their fin frames with minerals reaching nearly seven percent of their body volume. By contrast, the deep-dwelling Greenland shark registered tiny mineral levels near 0.17 parts per thousand. Heavy mineral deposits virtually disappear in the deep sea.

To test whether this trend was just an evolutionary accident, the authors applied phylogenetic generalised least squares (PGLS). This statistical test checks for links between traits while accounting for how closely species sit on the family tree. The test found no phylogenetic signal (a score of zero) for whole-body mineral content. Related sharks did not simply inherit identical density; their frames matched their ecological habitats.

Floating without heavy ballast

Carrying fewer heavy minerals helps deep-sea sharks stay neutrally buoyant without burning scarce calories. The Greenland shark takes this design even further. Most sharks rely on oily livers to float. Yet chemical tests showed the Greenland shark has a surprisingly low liver lipid fraction of around 38 percent. Instead, it stores over 20 percent fat inside its muscle tissue, compared to about four percent in typical sharks. Its entire body effectively acts as a buoyant float.

The study also examined ontogenetic changes, which describe how an animal develops from a young stage to an adult. In 11 out of 12 tested species, adult sharks built denser frames than young ones. This proves that mineralisation rises naturally with age.

Scans also turned up unexpected anatomy. Deep-sea frilled sharks do not harden their spine evenly. Minerals cluster near the head and tail, but fall abruptly along vertebrae 12 to 18. Why this midsection stays soft remains unknown.

Remaining questions

Living at depth correlates with lighter frames, but depth itself might not be the direct cause. Deep ocean zones also bring freezing temperatures, scarcer food, softer prey, and slower swimming habits. Any of these pressures could favour a lighter body.

Key limitations also remain. Sample sizes were small for several rare species, and adult Greenland shark values had to be estimated from young specimens. Maximum depth records also offer only rough proxies for daily habits. Biologists still do not understand the mechanics behind the patchy spine of frilled sharks, or how ancient fossil sharks handled their body weight millions of years ago.