The Indo-Pacific sailfish (Istiophorus platypterus) is a sailfish native to the Indian and Pacific Oceans. It is dark blue on top, brown-blue laterally, silvery white underbelly; upper jaw elongated in the form of a spear; first dorsal fin greatly enlarged in the form of a sail, with many black cones, its front squared off, highest at its midpoint; pelvic fins very narrow, reaching almost to the anus; body covered with embedded scales, blunt at end; lateral line curved above pectoral fin, then straight to base of tail. They have a large and sharp bill, which they use for hunting. They feed on tuna and mackerel, some of the fastest fish in the Ocean. The Indo-pacific sailfish is related to the marlin.
It is theorized by marine biologists that the 'sail' (dorsal fin array) of the sailfish may serve the purpose of a cooling and heating system for this fish; this due to a network of a large number of blood vessels found in the sail and because of "sail-raising" behaviour exhibited by the sailfish at or near the surface waters after or before high-speed bursts.