[5] The common name is derived from a distinctive, thresher-like tail or caudal fin which can be as long as the body of the shark itself.

This species is apparently found in the eastern Pacific off Baja California, and has previously been misidentified as the bigeye thresher.

The megamouth shark (Megachasma pelagios) was placed as the next-closest relative to these taxa, though the phylogenetic position of that species has yet to be resolved with confidence.

However, the position of the undescribed fourth species was only based on a single synapomorphy (derived group-defining character) in one specimen, so some uncertainty in its placement remains.

A thresher shark was seen on the live video feed from one of the ROVs monitoring BP's Macondo oil well blowout in the Gulf of Mexico.

A bigeye has also been found in the western Mediterranean, and so distribution may be wider than previously believed, or environmental factors may be forcing sharks to search for new territories.

[14] By far the largest of the three species is the common thresher, Alopias vulpinus, which may reach a length of 6.1 metres (20 ft) and a mass of over 500 kilograms (1,100 lb).

Lighting conditions and water clarity can affect how any one shark appears to an observer, but the color test is generally supported when other features are examined.

The thresher shark mainly feeds on schooling pelagic fish such as bluefish, juvenile tuna and mackerel, which they are known to follow into shallow waters, as well as squid and cuttlefish.

Fertilization and embryonic development occur internally; this ovoviviparous or live-bearing mode of reproduction results in a small litter (usually two to four) of large well-developed pups, up to 150 cm (59 in) at birth in thintail threshers.

[citation needed] Common thresher sharks are the target of a popular recreational fishery off Baja, Mexico.