Swordfish (Xiphias gladius), also known as broadbills in some countries, are large, highly migratory predatory fish characterized by a long, flat, pointed bill. They are a popular sport fish of the billfish category, though elusive. Swordfish are elongated, round-bodied, and lose all teeth and scales by adulthood. These fish are found widely in tropical and temperate parts of the Atlantic, Pacific, and Indian Oceans, and can typically be found from near the surface to a depth of , and exceptionally up to depths of 2,234 m. They commonly reach in length, and the maximum reported is in length and in weight.
They are the sole member of their family, Xiphiidae.
Taxonomy and etymology
The swordfish is named after its long pointed, flat bill, which resembles a sword. The species name, Xiphias gladius, derives from Greek (xiphias, "swordfish"), itself from (xiphos, "sword") and from Latin ("sword"). This makes it superficially similar to other billfish such as marlin, but upon examination, their physiology is quite different and they are members of different families.
Several extinct genera are known, such as a large sized Xiphiorhynchus and Aglyptorhynchus. Unlike modern taxa these have equally long lower jaws.
Description
They commonly reach in length, and the maximum reported is in length and in weight. The International Game Fish Association's all-tackle angling record for a swordfish was a specimen taken off Chile in 1953. Females are larger than males, and Pacific swordfish reach a greater size than northwest Atlantic and Mediterranean swordfish. They reach maturity at 4–5 years of age and the maximum age is believed to be at least 9 years. The oldest swordfish found in a recent study were a 16-year-old female and 12-year-old male. Swordfish ages are derived, with difficulty, from annual rings on fin rays rather than otoliths, since their otoliths are small in size.
Swordfish are ectothermic animals; however, along with some species of sharks, they have special organs next to their eyes to heat their eyes and brains. See heater cells for more information about these specialized modified muscle cells. Temperatures of above the surrounding water temperature have been measured. The heating of the eyes greatly improves their vision, and consequently improves their ability to catch prey. Of over 25,000 fish species, only 22 are known to have a mechanism to conserve heat. These include the swordfish, marlin, tuna, and some sharks.
Behavior and ecology
Movements and feeding
The popular belief of the "sword" being used as a spear is misleading. Their nose is more likely used to slash at its prey to injure the prey animal, to make for an easier catch. The use as an offensive spear in case of dangers against large sharks or animals is under review.
Mainly, the swordfish relies on its great speed and agility in the water to catch its prey. It is no doubt among the fastest fish, but the basis for the frequently quoted speed of is unreliable. Research on related marlin (Istiophorus platypterus) suggest a maximum value of is more likely.
Swordfish are not schooling fish. They swim alone or in very loose aggregations, separated by as much as from a neighboring swordfish. They are frequently found basking at the surface, airing their first dorsal fin. Boaters report this to be a beautiful sight, as is the powerful jumping for which the species is known. This jumping, also called breaching, may be an effort to dislodge pests, such as remoras or lampreys.
Swordfish prefer water temperatures between , but have the widest tolerance among billfish, and can be found from . This highly migratory species typically moves towards colder regions to feed during the summer. Swordfish feed daily, most often at night, when they rise to surface and near-surface waters in search of smaller fish. During the day, they commonly occur to depths of and have exceptionally been recorded as deep as . Adults feed on a wide range of pelagic fish, such as mackerel, barracudinas, silver hake, rockfish, herring, and lanternfishes, but they also take demersal fish, squid, and crustaceans. In the northwestern Atlantic, a survey based on the stomach content of 168 individuals found 82% had eaten squid and 53% had eaten fish, including gadids, scombrids, butterfish, bluefish, and sand lance. Large prey are typically slashed with the sword, while small are swallowed whole.