What Is a Swordfish's Sword For? Slashing Prey Explained

How do swordfish use their sword to feed?

The best evidence comes from what swordfish eat. Squid and cuttlefish taken from the stomachs of swordfish often show clean slashes across their bodies, and one study found that most of the large fish prey had been cut in this way, while small prey had been swallowed whole. A swordfish appears to swing its head and sword from side to side through its prey, injuring or stunning it, and then swallow the disabled animal.

Scientists have also tested the sword itself. In a 2015 study, researchers loaded the bills of swordfish and blue marlin in the laboratory and modeled how each would perform when swung through the water. The blue marlin bill was stiffer and coped with loads from more directions, but the swordfish sword produced less stress and less drag when swept sideways. The authors concluded that the swordfish sword is suited to lateral swiping that incapacitates prey.

Swordfish hunt mostly in deep, dark water by day and near the surface at night, so very few wild swordfish have ever been watched while feeding. Much of what we know about how billfish use their bills comes from their relatives that hunt near the surface.

What is the sword made of?

The sword is not a separate weapon but a greatly lengthened upper jaw, formed mainly by the premaxillary bones. In cross-section it is a flattened oval with fairly sharp edges, which is why the swordfish is called the broadbill in some countries. Relative to the body, it is longer than the bill of any other billfish, and in adults it carries no teeth.

Near the base of the sword sits a structure discovered only in 2016: an oil-producing gland connected by tiny vessels to pores in the skin of the head. The oil spreads over the front of the head, and the researchers who described it suggested that it may reduce drag when the fish swims fast. This idea has not yet been tested on living swordfish, and the role of the sword itself in streamlining the fish is still unknown.

How does the sword grow?

Swordfish are not born with a sword. The larvae hatch at about 4 mm (0.16 in) long, with short jaws and a body covered in prickly scales. By the time a larva is about 12 mm (0.5 in) long, both jaws have grown long and are equal in length.

As the young fish keeps growing, the upper jaw grows much faster than the lower one, producing the long sword and the short, pointed lower jaw of the adult. Young swordfish lose their teeth and scales as they grow. The scales and the visible lateral line are gone by a length of about 1 m (3 ft), and adults have smooth skin and toothless jaws.

Do swordfish spear their prey? Myths and evidence

The picture of a swordfish skewering fish like a kebab is a popular myth. Stomach contents show slash wounds, not puncture wounds, and the flat shape of the sword is better suited to cutting sideways than to stabbing forward. A fish stuck on the end of a sword would also be awkward to swallow.

Another long-standing idea is that swordfish use their swords for defense. There is some evidence for this, because swordfish swords have been found buried in large sharks, but whether these strikes are defense, competition over food or simple aggression is still debated.

Swords in sharks, boats and submarines

The sword is strong enough to pierce tough skin, bone and wood. In September 2016, a pregnant blue shark 247 cm (8 ft) long was found stranded on the coast of Valencia, Spain, and died shortly afterward. A necropsy found an 18 cm (7 in) piece of swordfish sword that had passed through its head near the eye, injuring the brain. From the size of the fragment, scientists estimated that the swordfish was a young fish only about 1.1 m (3.6 ft) long. In 2022, a second blue shark with a swordfish sword tip in a head wound was found stranded at Hyères in southern France. Broken swords have also been found in the heads of dead or dying shortfin mako sharks, which are among the few predators of adult swordfish.

People have encountered the sword too. Museums hold pieces of the hulls of wooden sailing ships with swordfish swords embedded in them, in some cases after passing through several layers of planking. In July 1967, a swordfish struck the research submarine Alvin about 610 m (2,000 ft) below the surface and became stuck in its outer skin. Swordfish do not hunt people, and nearly all injuries to humans have involved hooked or harpooned fish fighting back, although a surfer was fatally struck off Indonesia in 2024.

How do sailfish and marlins use their bills?

The swordfish’s relatives also use their bills for hunting, but in different ways. Sailfish have been filmed hunting schools of sardines near the surface off Mexico. A sailfish slips its round, rough bill into the school and either taps an individual fish or slashes sideways, injuring several sardines in one strike before picking them off. Groups of sailfish take turns attacking, and their raised sails play a part too, as explained in Why do sailfish have a sail?

Marlins have similar round bills, and laboratory tests suggest that the stiffer blue marlin bill can strike prey from a wider range of directions. The swordfish, a solitary hunter of the deep, has the flattest and longest sword of all, built for powerful sideways swipes in the dark. Whether that sword also helps it swim fast is a separate question, covered in How fast can a swordfish swim?