Deadly interaction between a swordfish Xiphias gladius and a bigeye thresher shark Alopias superciliosus

Interspecific aggressive behaviour has been reported for a number of fish species, including not only coral reef fishes that compete for territory (Myrberg Jr and Thresher 1974; Eurich et al. 2018), but also large pelagic fishes, such as billfishes (Istiophoridae and Xiphiidae), which are known to exhibit aggressive behaviours towards other species. The swordfish Xiphias gladius, which is a highly mobile, predatory fish is known to attack sharks (e.g. Starck 1960; Penadés-Suay et al. 2017, 2019; Romeo et al. 2020), whales (Machida 1970), sea turtles (Frazier et al. 1994), humans (e.g. Gooi et al. 2007; Georgiadou et al. 2010), and even inanimate objects, e.g. boats and submarines (Zarudski and Haedrich 1974; Romeo et al. 2017). The resulting impact can lead to a transverse fracture of the bill, with the distal segment often remaining embedded in the victim. Aggressive behaviour towards thresher sharks (Lamniformes, Alopiidae), however, was only suggested once before in a conference contribution (Vacchi et al. 1999). Here, we present the first verified illustrated documentation of a stranded bigeye thresher Alopias superciliosus that was fatally wounded by a swordfish and discuss this incidence within the context of a possible accidental collision or interspecific aggression.