Thresher sharks
Characteristic features:

Key adapted from Compagno, 2001.1

Alopiidae species
    1. Head nearly flat between eyes (Fig. 1a); eyes very large, directed upwards and clearly visible in dorsal view (Fig. 2a); a deep horizontal groove on each side of nape above gills (Fig. 3a); first dorsal-fin base closer to pelvic-fin bases than pectoral-fin bases (Fig. 4a); 25 rows of teeth in each jaw Alopias superciliosus
    2. 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); 29 or more rows of teeth in each jaw Alopias pelagicus
Alopias pelagicus Pelagic thresher
  1. Compagno, L, Carpenter, K, Niem, V. Alopiidae. In: FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Volume 2. Cephalopods, crustaceans, holothurians and sharks. Rome: FAO Library; 2001;2:1269-1273.