Physical evidence of a blue shark (Prionace glauca) fatally injured by a swordfish (Xiphias gladius) in the north-western Mediterranean.

The blue shark Prionace glauca and the swordfish Xiphias gladius are two large apex predators in the Mediterranean Sea. The overlap of their ecological niche can lead to competition between the two species for the same food resources. Furthermore, their similar reproductive periods in the Mediterranean may intensify this competition. However, interactions between these animals can result in aggressive behaviors of the swordfish towards the blue shark. This study highlights a case of fatal interaction between one or more swordfish X. gladius and a female blue shark P. glauca found stranded on July 30, 2022, in Hyères (Var, France), exhibiting two external head injuries, one of which contained the distal fragment of a swordfish rostrum. The shark was observed alive for a period of three days before its death and displayed abnormal and repetitive swimming behavior. The hypothesis is that the impact of the rostrum on the shark's body may have induced internal injuries leading to potential hemorrhage having certainly been aggravated by repeated self-mutilations of the shark onshore. This study suggests that the swordfish may have potentially developed an adaptive defense or attack strategy during its coevolution with organisms perceived as competitors or threats, such as the blue shark in our case. Similar cases to this study warrant further investigation to confirm the existence of this adaptive swordfish strategy against sharks. [ABSTRACT FROM AUTHOR]