Seals and blue sharks are approximately the same size, and both tend to feed on smaller forms of marine life. While it is more likely that a shark would take on a seal or another sea creature of similar size, Cape fur seals apparently turn the tables on occasion.
It is not known how often seals attack sharks.
Researchers believe that the seals may attack sharks to reduce competition in the ocean for food. The seals also benefit from ingesting the energy-rich organs of sharks.