Key adapted from Nakamura, 2001.1
- First dorsal fin sail-like, much higher than maximum body depth, highest at mid-fin (Fig. 1a); pelvic-fin rays very long, almost reaching to anus (Fig. 2a) Istiophorus platypterus
- First dorsal fin not sail-like, lower or only slightly higher than maximum body depth (Fig. 1b); pelvic-fin rays not as long, ending well forward of anus (Fig. 2b) 2
- Anterior part of first dorsal fin slightly higher than, or nearly equal to body depth (Fig. 3a); body laterally compressed, elliptical shape in cross-section (Fig. 4a) 3
- Anterior part of first dorsal fin lower than body depth (can be higher in individuals <50 kg) (Fig. 3b); body not compressed, robust and oval in cross-section (Fig. 4b) 4
- Bill short, less than 15% of lower jaw–fork length (Fig. 5a); anus located well anterior of first anal fin by a distance greater than length of first anal-fin base (Fig. 6a) Tetrapturus angustirostris
- Bill moderately long, more than 18% of lower jaw–fork length (Fig. 5b); anus slightly anterior of first anal fin by a distance of less than length of first anal-fin base (Fig. 6b) Kajikia audax
- Pectoral fins curved in shape, resembling a half sickle (Fig. 7a), rigid unable to be folded back against sides of body (more flexible in individuals <15 kg); dorsal-fin tip bluntly rounded (Fig. 8a); branchiostegal frill long, extending to near level with rear edge of operculum (Fig. 9a); second dorsal-fin slightly anterior to second anal-fin (Fig. 10a) Istiompax indica
- Pectoral fins strap like (Fig. 7b) flexible, able to be folded back against sides of body; dorsal fin pointed at tip (Fig. 8b); branchiostegal frill short, extending to well forward of rear edge of operculum (Fig. 9b); second dorsal-fin slightly posterior to second anal-fin (Fig. 10b) Makaira nigricans
Xiphiidae differ in having a bill flat-oval in cross section (vs. round); pelvic fins absent (vs. present); 1 median keel on caudal peduncle (vs. 2 keels) and a deep notch on both upper and lower profiles on caudal peduncle (vs. shallow notches).
Some large Belonidae may appear similar to small spearfish (Tetrapturus) when first dorsal fin is folded into groove. Belonidae differ in having both jaws prolonged (vs. upper jaw prolonged); dorsal and anal fins single (vs. double) and similar in size and shape (vs. first dorsal fin larger than first anal fin); pectoral fins not falcate (vs. falcate) (except in Ablennes) and pelvic fins located well posterior of pectoral fins (vs. nearly level with pectoral fins).
- Carpenter K, Niem V, Nakamura I. Istiophoridae. In: FAO Species Identification Guide for Fishery Purposes. The Living Marine Resources of the Western Central Pacific. Volume 6. Bony Fishes Part 4 (Labridae to Latimeriidae), Estuarine Crocodiles, Sea Turtles, Sea Snakes and Marine Mammals. Vol 6. Rome: FAO Library; 2001:3759-3764. http://www.fao.org/3/y0770e/y0770e00.htm.