The swordfish, Xiphias gladius, is classified in a family by itself, the Xiphiidae. This family is part of the Scombroidea, a subfamily of marine fishes that includes tuna and mackerel.

Tuna and mackerel are two of the fastest-swimming creatures in the ocean. Swordfish rival them, as well as mako sharks, with their ability to reach speeds approaching 60 mph (96.5 kph) in short bursts. Their streamlined form and powerful build account for their speed, as well as the fact that their torpedo-shaped body is scaleless and smooth, thus decreasing surface drag. Swordfish have a tall, sickle-shaped dorsal fin that cuts through the water like a knife, and their long pectoral fins, set low on their sides, are held tightly against their body while swimming. Swordfish lack pelvic fins, and they have reduced second dorsal and anal fins which are set far back on the body, adding to the streamlining. It...