Most barnacles sit on hard surfaces, and filter small particles of food from the surrounding water. But Anelasma squalicola is an exception. It’s a parasitic barnacle that eats sharks, by fastening itself to their flanks and draining nutrients from their flesh.

Charles Darwin, history’s greatest barnacle fanboy, described Anelasma in his 1851 magnum opus, and suggested that it was most likely a parasite. He was right, but the creature is so rare that few scientists have been able to study it in detail. That changed when Henrik Glenner discovered a large group of velvet belly lantern sharks—small fish with glowing bellies—off the western coast of Norway. The sharks were infested with the parasitic barnacles.

Glenner’s team at the University