Fish
Swordfish
Xiphias gladius · also called broadbill, broadbill swordfish
The swordfish is one of the ocean’s great predators — a fast, powerful, open-water hunter with a long bladed bill, a body built for speed, and a rare piece of biological engineering that lets it keep its eyes and brain warm while diving into the cold deep.
Overview
Swordfish live in the open ocean and deep shelf waters of every ocean and the Mediterranean, across a broad band of tropical to temperate latitudes. They are highly migratory, moving toward the poles in warm seasons to feed and back toward the equator to spawn, and they perform a daily vertical migration — deep by day, near the surface at night — that tracks the movement of squid and schooling fish.
The bill
The swordfish’s “sword” is a greatly extended, flattened upper jaw, smooth and rigid, that can be a third of the animal’s total length. Unlike the round bills of marlin and sailfish, it is shaped like a blade, and the swordfish uses it accordingly — slashing sideways through a bait ball to cut and stun prey rather than spearing individual fish. Adult swordfish have no teeth and no scales; they swallow their disabled prey whole.
The eye heater
Swordfish routinely hunt hundreds of metres down, where the water is cold and dim. To function there, they have evolved a heater organ — a mass of modified eye muscle that produces heat instead of contraction, warming the eyes and brain by 10–15 °C. Warm retinal tissue reacts to light far faster, sharpening the swordfish’s ability to detect and follow rapidly moving prey in the gloom. It is one of the clearest cases of regional endothermy (localised warm-bloodedness) in a fish.
Speed and body
Everything about the swordfish is tuned for fast pursuit: a stiff, torpedo-shaped body, a tall crescent tail on a narrow stalk, fins that reduce drag, and a possible drag-reducing oil layer secreted over the head. Estimates of its top speed vary widely, but it is firmly among the fastest fish in the sea.
Reproduction and status
Swordfish spawn in warm surface waters, releasing millions of tiny eggs that hatch into larvae with a normal-looking jaw; the bill grows as the fish matures. They grow fast and, unusually for such a large predator, live only about nine years.
The species is listed as Least Concern. Heavy longline fishing in the 1980s–90s depleted North Atlantic and other stocks, prompting quotas, gear rules, and consumer campaigns; most major stocks have since recovered to sustainable levels, though a few regional populations remain overfished, and bycatch of sharks, turtles, and seabirds on swordfish longlines is an ongoing concern.
Swordfish: common questions
Does a swordfish stab its prey with its bill?
Usually not. The "sword" is flat and blade-like, not round and pointed, and swordfish mainly use it to slash sideways through a dense school of fish or squid, injuring and stunning several at once, then turn back to eat the disabled prey. Stomach contents often contain fish cut cleanly in half. Swordfish have occasionally rammed boats, whales, and even submersibles, and their bills have been found broken off and embedded in the timbers of ships and the bodies of large sharks and whales, but this is rare and probably defensive or accidental.
How does a swordfish see in the deep, dark ocean?
It carries a built-in heater. A block of modified eye muscle next to each eye generates heat rather than movement, warming the eyes and the brain by 10 to 15 °C above the surrounding water. Warm retinas respond to light much faster, so the swordfish can track fast-moving prey in the cold, dim water hundreds of metres down where it spends its days. Only a few other fish — some tunas, the opah, and marlins — have anything similar.
How fast is a swordfish?
Very fast, though exact figures are disputed. Estimates of burst speed run from about 60 to 90 km/h, and possibly higher over very short distances. The swordfish's body is stiff and streamlined, it lacks scales and pelvic fins as an adult to reduce drag, and a gland at the base of the bill secretes oil that may create a lubricating layer over the head. Whether it truly outpaces the sailfish and marlin is unresolved.
Is swordfish safe to eat?
Swordfish is a popular food fish, but because it is a large, long-lived predator it accumulates methylmercury, and health agencies advise pregnant people and young children to avoid it and others to limit how often they eat it. On the sustainability side, North Atlantic and North Pacific swordfish stocks were overfished in the late 20th century but have largely rebuilt under quota management, while some other stocks remain a concern.