Observing a shark hunt in murky coastal waters with a notepad is virtually impossible, and many predation events occur completely out of sight. For decades, researchers relied on examining stomach contents, but this approach presents major hurdles. Stomachs are frequently empty, or the contents are so heavily digested that identification is limited to broad categories like fish or crab. For vulnerable species facing severe conservation pressures, killing an animal simply to inspect its stomach is completely unviable. However, scientists have successfully tested a subtle alternative: collecting a swab from the shark's cloaca and employing DNA metabarcoding to pinpoint the remains of recent meals. Despite potential squeamishness, this small swab yields a surprisingly detailed snapshot of marine predator menus.

Comparing Stomach Slurry and Cloacal Swabs

A recent study investigated 23 individual sharks across four coastal species collected in North Carolina waters: 10 bonnetheads (Sphyrna tiburo), 4 blacknose (Carcharhinus acronotus), 4 blacktip (C. limbatus), and 5 Atlantic sharpnose (Rhizoprionodon terraenovae) sharks. Researchers gathered both stomach slurry and cloacal swabs from each animal, analyzing them via DNA metabarcoding. This technique functions similarly to searching a genetic barcode database, extracting DNA from mixed samples and utilizing targeted genetic markers—in this case focusing on teleost fishes and crustaceans—to identify organisms. While stomach samples produced roughly 28% more DNA sequence reads due to less advanced digestion, having higher volume did not automatically translate to a superior dietary picture.

The Timeline of Digestion and Prey Diversity

Statistical analysis revealed no significant difference in the number and diversity of prey taxa detected between the two sampling methods, and the overall prey composition remained statistically similar. Traditional stomach-content analysis fared far worse; among 48 total sharks collected, 42% had empty stomachs, and nearly half lacked morphologically identifiable prey, restricting physical identification to just five prey taxa to the species level. Conversely, DNA metabarcoding identified 19 prey taxa from cloacal swabs and 25 from stomach slurry, largely resolved to species. Discrepancies between samples from the same animal likely stem from timing. Because digestion occurs gradually and DNA persists during transit through the digestive tract, the stomach captures immediate feeding events while the cloaca reflects meals consumed earlier, illustrating the complex timeline of digestion rather than technical unreliability.

Implications for Shark Conservation

While rarefaction curves did not reach a plateau, indicating that larger sample sizes are still needed to capture full dietary diversity, the implications for marine conservation are profound. Cloacal swabbing is exceptionally fast and entirely non-invasive compared to stomach dissection, unlocking vital dietary data from rare, protected, or high-value sharks without sacrificing them. Although metabarcoding does not quantify the exact biomass consumed or the energetic value of each meal, it uncovers hidden biodiversity that traditional methods routinely miss. As global shark populations decline, obtaining precise insights into foraging habits and dietary shifts will empower scientists to design vastly superior ecosystem-management and conservation strategies.