- Precaudal tail much shorter than trunk
- Upper lobe of caudal fin broad and extremely elongated, about half of total length
- Subterminal notch on caudal fin
Key to the species of Alopiidae in South-east Asia (adapted from Compagno & Niem, 1998)
- Head nearly flat between eyes (Fig. 1a); eyes very large, directed upwards and clearly visible in dorsal view (Fig. 2a); a deep horizontal grove on each side of nape above gills (Fig. 3a); first dorsal-fin base closer to pelvic-fin bases than pectoral-fin bases (Fig. 4a); teeth large (Fig. 5a), <25 rows in each jawAlopias superciliosus (bigeye thresher)
- Head strongly arched between eyes (Fig. 1b); eyes smaller, not directed upwards or visible in dorsal view (Fig. 2b); no obvious groove on nape (Fig. 3b); first dorsal-fin base about midway between pectoral and pelvic-fin bases, or slightly closer to pectoral-fin bases (Fig. 4b); teeth smaller (Fig. 5b), 29 or more rows in each jawAlopias pelagicus (pelagic thresher)
Stegostomatidae are similar to Alopiidae in having a caudal fin about half the length of total body length. Stegostomatidae differ in having a barred or striped body markings (vs. no makings in Alopiidae); nasal barbels (vs. no barbels); transverse mouth in front of eyes (vs. mouth extending well behind eyes); small tricuspid teeth (vs. small blade like with erect to oblique cusps, cusplets small or absent); first dorsal fin located over pelvic-fin bases (vs. located within pectoral- and pelvic-fin interspace); second dorsal and anal fins large (vs. minute) and no ventral caudal fin lobe (vs. short but strong.