Scientists have solved a marine murder mystery in which a pregnant porbeagle shark was the missing victim and the open ocean was the crime scene.
Researcher Brooke Anderson and her team are studying pregnant porbeagle females and wanted to track their long migration. The movement of pregnant females has often eluded researchers. The team was looking for areas important for pup-bearing to protect them.
One female shark had a satellite tracking device that was supposed to stay attached for a year. During that time, it collects information about water temperature, depth, and location. After a year, it detaches from the shark and floats to the surface. Here, it transmits its data to scientists. But this tag began sending data far ahead of schedule.
Initially, the team assumed that something had malfunctioned, and the tag simply detached too early. But the data was bizarre. “I knew something had happened that I had never seen before,” said Anderson.
A mysterious rise in temperature
The last week of data showed that the temperature had increased and stayed higher for a few days. At the same time, the tag was almost constantly located a few hundred meters below the surface.
The team explored different scenarios but could only think of one logical explanation. “That would really only occur if the tag was now in the stomach of a predator that can warm its body temperature compared to the ocean.”
So something had eaten the pregnant porbeagle shark. Predator had suddenly become the prey.
“I couldn’t believe that an eight-foot-long porbeagle shark…would have gotten eaten by something,” said Anderson. An adult porbeagle shark is fierce and formidable, and becoming prey to another species was unprecedented.
The next mystery is what killed the porbeagle. They crossed orcas off the suspect list because they are warm-blooded — the tag’s temperatures, though warmer, were still too cool to come from inside an orca.
That means the only option was another species of shark, specifically a shortfin mako or a great white. Both can warm their bodies the appropriate amount. This, combined with the depth data, narrowed it down to one culprit –- a great white shark. Its dive pattern most closely matched the changes in depth recorded by the tracking device.
