Sailfish swim along with seemingly little effort, cutting through the ocean's waves and currents with ease. Equipped with strong aerodynamic and flexible bodies, and with special grooves into which they can retract their pelvic and dorsal fins, streamlining themselves even further, sailfish glide past other fish as though the others were standing still. Sailfish, in fact, are thought to be the world's fastest fish, capable of reaching speeds of more than 65 miles per hour.
The sailfish is an extraordinary creature, remarkable not only for its speed but for its appearance as well. It is named for the elaborate dorsal fin that runs nearly the full length of the body and can be raised and lowered like a sail on a boat. It is held upright when the fish is excited or threatened and is retracted when extra speed is needed. When fully extended, it is more than twice as tall as the fish's body.
Two species of sailfish are recognized: the Atlantic sailfish (Istiophorus albicans) and the Indo-Pacific sailfish (Istiophorus platypterus). However, genetic evidence distinguishing the two is lacking, and it is thought that the differences between them are primarily morphological and geographic. The Indo-Pacific sailfish, for example, typically is larger than its Atlantic counterpart, weighing as many as 220 pounds and growing to 11 feet in length, and favors the tropical and subtropical waters of the Indian and Pacific oceans. In contrast, the Atlantic species weighs up to about 125 pounds and reaches about 10 feet in length and is found throughout the tropical and subtropical Atlantic Ocean and less commonly in the Atlantic's temperate waters. Both species have been known to enter the Mediterranean Sea.
Sailfish come in a unique array of iridescent colors. For example, the Indo-Pacific sailfish typically has a metallic blue body with vertical stripes along the flanks, a silvery underside, and a dark blue dorsal fin. The Atlantic sailfish is commonly seen having dark blue upper parts with vertical striping along the length of the body, a silvery brown speckled underside, and a black-blue dorsal fin. In bright light, dark round spots can be seen on the dorsal fins, which themselves may acquire a purplish hue. When sailfish are excited, the vertical stripes on the body may flash yellow and even the body color itself may change.
Sailfish are aggressive predators, using their speed and quick maneuverability to corral their prey and their long, spear-like bill, an extension of the upper jaw, to injure prey. Their preferred prey are fish, such as anchovies and sardines, though they also eat crustaceans and cephalopods, including squid and octopus. Although Indo-Pacific sailfish are known to hunt alone, when fish are on the menu, the Indo-Pacific, like its Atlantic cousin, will hunt in groups, which may consist of as many as 20 individuals. The group works together to corral a shoal of fish, using their dorsal fins to encourage the fish to band together into a bait ball. Once the bait ball takes shape, the sailfish strike their prey from behind with their bills. The stunned fish, easily separated from the bait ball, are then eaten.
Sailfish are prized by sport fisherman, mainly for the skill and strength necessary to hook and catch one successfully. While sport fishing is not considered a major threat to the survival of either sailfish species, conservationists are concerned that their numbers could decline if they continue to be captured as by-catch in longlines and gillnets used for commercial tuna fishing. In addition, the Indo-Pacific sailfish is hunted commercially for its meat, a threat that does not apply to the Atlantic sailfish, since its meat is tough and not widely eaten. Fortunately, both species enjoy high reproductive output, and neither appears to be threatened with extinction currently.
Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.
This post also appears on the Britannica Blog.