This past April, a trio of citizen scientists made a strange discovery on a beach in Libya. They spied the 14.5-foot body of a dead thresher shark sloshing in the tide and, upon closer inspection, saw something strange: what turned out to be a swordfish bill sticking out of a deep, penetrating wound between the creature’s head and dorsal fin.
A study detailing this instance of apparent undersea swashbuckling, published this month in the journal Ichthyological Research, is the latest confirmed report of swordfish stabbing sharks to death, reports Melissa Cristina Marquez for Forbes. The idea that swordfish might use their bills to impale their enemies or their prey used to be the conventional wisdom among fishers, whalers and even academics, writes Joshua Sokol for the New York Times, but “modern scientists were skeptical.”
The common explanation for a swordfish’s bill ending up buried in some other denizen of the sea was essentially that they were trying to swipe or stab smaller prey and missed, instead ramming into whales, sea turtles, boats and even submarines, per the Times. The paper’s authors are quick to note that they can’t rule out that this thresher shark’s death was the result of an unhappy accident, but, according to the Times, there have been at least six other documented cases of swordfish mortally wounding sharks elsewhere in the Mediterranean.
What makes the idea of these deep-sea stabbings being complete accidents a bit harder to fathom is their precision. In the case of this large thresher in Libya, the female shark was lanced straight through the heart, and a blue shark found in 2016 off the coast of Spain had been stabbed in the brain.
The researchers behind the current study came upon their subject when one of them encountered a video posted by one of the citizen scientists who first found the dead shark on the Libyan coast. The case of this dead thresher shark piqued the scientists' interest because an adult swordfish inflicted the wound.
“We knew of juvenile swordfish who attacked blue sharks in order to defend themselves, however in this case a rather harmless (at least, harmless for the swordfish) thresher shark was attacked by an adult swordfish,” Patrick L. Jambura, a shark researcher at the University of Vienna and the study’s lead author, tells Forbes.
Threshers aren’t known to prey on adult swordfish, so Jambura and his co-authors argue that the stabbing could have been a case of two deep sea predators fighting over a meal or territory.
It’s impossible to infer exactly what occurred in the inky fathoms, but Jambura thinks this “shows how aggressive swordfish” can be and that because the two fish were adults of similar size that we can “exclude a defensive behavior as the trigger for this attack,” he tells Forbes. “It either happened in the heat of the moment, when both species were hunting on the same prey resource (schooling fish or squid) or it might have been even a direct attack to get rid of a competitor.”
Jaime Penadés-Suay, a shark researcher at the University of Valencia, tells the Times, he doubts competition on its own would have been a good enough reason to make such a risky attack. Instead, he posits that the swordfish may have been trying to protect its territory in response to aggression from the shark.
Ultimately, Penadés-Suay tells the Times, this study highlights all we have yet to learn about swordfish, a species well known to diners but poorly understood by scientists. Per the Times, Penadés-Suay is partnering with a seafood company to measure the swords of a thousand individuals as well as the body size of their owners. He also sees a role for fishers and members of the public in expanding this area of research.
“Maybe a fisherman for 13 years has been catching sharks, and every year he finds this,” Penadés-Suay tells the Times. “We need everybody to be looking into this.”